82 Prom Operation Question

Dirtbuster1

Well-known member
Joined
May 13, 2008
Messages
649
Location
Georgia
Can someone tell me how a prom functions with the ECM. I have run about every test in the service manual to no avail. My IAC motors will still not funtion properly. I have replaced the ECM, wiring harness, IAC motors, & fuses. The VSS seems to be ok based on the service manual. While testing the IAC motors with a test light, the sequence seems to be different than last time. I am getting 2 dim flashes, 1 brighter flash, and 1 that is a constant bright light. This seems to be a problem based on the IAC motor test. It says to replace the IAC motors or ECM. I have done both, but I have not changed the prom. ECM failure will not give a CEL light, how about the prom?

The IAC motors only functions off of the VSS & the nuetral safety switch. Does anyone have an explanation on what I should do except trash the CFI.

Help
 
I've never heard of anyone ever checking an IAC motor with a test light. How about some details on how you're doing it so we can make some sense out of it.
If there was a problem with the PROM, I would expect the ECM software to be able to detect it. The old ROMs were "hardwired" internally, versus later EEPROMs, so there shouldn't be any "flipped bits" in the PROM. The software usually has a checksum operation in it to determine that the bits of the PROM are as they are supposed to be. If a bit changed for any reason, the checksum would be different, and the software would know that there is a failure in the PROM chip.
You lost me on your comment regarding the IAC, VSS, and the neutral switch. The software is what determines what the IAC motors are going to do.
 
The test with the test light is straight out of the GM Service Manual. There are 4 connections on each of the IAC connectors that go to the ECM. I know that there is 2 wires that supply the High side coil of the IAC motor, & 2 wires that supply the LOW side coil in the IAC motor. I am assuming that 1 coil runs the stepper motor in, & the other backs it out based on the VSS reading. Other than that I don't know. I have had several ECM's fail with no CEL light. I am assuming that it is possible for the prom to fail the same way. With the CFI only being made 3 years, there is very little support or knowledge base on this system. I see why most people take these systems off. I am trying to keep everything as original, but its getting harder by the day.:smash::smash::cussing:

Thanks for the help!
 
Just a little info about the IAC system (from someone who is somewhat familiar with the electrical aspects of them, and fortunately hasn't had any problems requiring on-car IAC debugging. LOL). The IAC motors are stepper type motors with two sets of windings in them, hence the four terminals. The ECM (via an internal H-bridge driver IC) sets a pattern of squarewave signals on these two windings (the H-bridge circuitry allows the polarity of the winding wires to also swap as necessary), and the motor will step forward or backward depending on the pattern of the signals. It's easy to see this pattern with an oscilloscope, but, as you said, a test light will give you an indication of whether the polarities are switching.
 
You sound well versed on the operation of the motors, but what drives the stepper motors. I suspect the prom does this, but it's only a guess. I'm still wondering if the removal of my air pump has caused any of my problems with the ECM. I would hate to have to reinstall all of this, to find that was the problem.

I am open to any suggestions.

Thanks
 
You sound well versed on the operation of the motors, but what drives the stepper motors. I suspect the prom does this, but it's only a guess. I'm still wondering if the removal of my air pump has caused any of my problems with the ECM. I would hate to have to reinstall all of this, to find that was the problem.

I am open to any suggestions.

Thanks

The PROM is just the calibration memory chip that tells the microprocessor chip what rear axle ratio you have, what the A/F ratio should be at WOT, when to lock up the torque converter, etc. Essentially, the software has all the calculating and processing capability, but the PROM just supplies the information that the software needs to customize the operation of the drivetrain.
The IAC motors are controlled by an integrated circuit chip, sometimes known as the SMD IC (stepper motor driver IC). The microprocessor will send a digital pulse train to the SMD, and it will automatically output the correct waveform/polarities to the dual windings in each IAC motor to make them turn the correct direction.
I doubt the air pump removal had any affect. The ECM needs feedback sensors to know if something is there or not, and I doubt if that ECM was that sophisticated.
 
You sound well versed on the operation of the motors, but what drives the stepper motors. I suspect the prom does this, but it's only a guess. I'm still wondering if the removal of my air pump has caused any of my problems with the ECM. I would hate to have to reinstall all of this, to find that was the problem.

I am open to any suggestions.

Thanks

The PROM is just the calibration memory chip that tells the microprocessor chip what rear axle ratio you have, what the A/F ratio should be at WOT, when to lock up the torque converter, etc. Essentially, the software has all the calculating and processing capability, but the PROM just supplies the information that the software needs to customize the operation of the drivetrain.
The IAC motors are controlled by an integrated circuit chip, sometimes known as the SMD IC (stepper motor driver IC). The microprocessor will send a digital pulse train to the SMD, and it will automatically output the correct waveform/polarities to the dual windings in each IAC motor to make them turn the correct direction.
I doubt the air pump removal had any affect. The ECM needs feedback sensors to know if something is there or not, and I doubt if that ECM was that sophisticated.

Where is this SMD IC chip located.?1st I heard about this, unless it's built into the ECM.
 
Yes, the stepper motor driver is on the ECM board but it would be very unlikely for it to go bad.

When you ground the a&b terminals on your ALDL, do the stepper motors cycle? Do they ever reset? You know that when you pull the battery leads off that after hooking back up you have to drive above 35mph to reset the IACs?

What makes you think your IACs are not working properly? Did you synch the TBs and set proper minimum air (idle) ? The 2 IACs are parallel wired and the ECM always wants to return them to a low position, around #10 (runs from 0 -255) and if it can't do that because of too high minimum air it will start to cycle and you get that dreadful hunting idle.

The only feedback that the ECM uses for IAC operation is the tach signal.

Oh and yes, if the prom is damaged it will trip a code, just like 69427 said, there's a checksum in the prom.

The prom is a calibration indeed and it has base spark, fuel, rpm maps in it. The ECM uses these as a base calibration during open loop and when in closed loop it uses these and the O2 sensor input to alter the fuel and spark maps to fine tune. For fuel there are 2 correctors, the INT(eger) and BLM(block learn multiplier).

The INT is a short term (almost real time) correction for the injector open time, it's a matrix cell (load/rpm) with values anywhere between 0-255. The "normal" value is around 128 (no correction) or thereabouts where a lower number means a shorter injector opening time (adjusting for slightly rich mixture) and a higher number a longer open time (adjust for lean). The influence is quite large when compared to the same system for the BLM. Meaning, a correction of X points for the INT has a bigger influence of inj. open time than the BLM


BLM is what is referred to when people talk about an ECM "learning". This is a long time correction factor. It too is a matrix like the int with cells corresponding with the load/rpm map. The load/rpm matrix gives a (usually 4x4 cells so 16) correction factor to the fuel map in the prom for the current engine conditions (load & rpm). The BLM is stored in memory (RAM, will be wiped when you pull the 12v!). If the BLM has values of 128 there's no correction, lower values adjust for rich and higher for lean, just like the INT.

That's basically how the fuel mapping is done and how the PROM fits into this all. The actual calculations are done with formulas stored in the ROM (software) and it use the PROM and INT/BLM values to calculate current spark & fuel demands. Same with the IACs.

Here's how the PROM values are used (the actual values are HEXadecimal of course but if you get the dec. value, this is how it will calculate the actual value, N = the PROM value)
[tr][td]Unit Type[/td][td]Calculation[/td][td]Units[/td][/tr][tr][td]Temperature, Coolant[/td][td]N x (191/255)-40[/td][td]degrees C[/td][/tr]
[tr][td]Speed[/td][td]N x 1 0-255[/td][td]MPH[/td][/tr][tr][td]RPM[/td][td]N x 25[/td][td]RPM[/td][/tr][tr][td]RPM, low range[/td][td]N x 12.5[/td][td]RPM[/td][/tr][tr][td]WOT AFR[/td][td]N x (100/256)[/td][td]AFR[/td][/tr][tr][td]PE (power enrich)[/td][td]N x 10[/td][td]---[/td][/tr][tr][td]VE (volumetric efficiency)[/td][td]N x (256/100)[/td][td]0-100%[/td][/tr][tr][td]Knock retard[/td][td]N x (45/256)[/td][td]0-45 degrees[/td][/tr][tr][td]Knock attack Rate[/td][td]N x 0.0225[/td][td]milliseconds[/td][/tr][tr][td]Knock recovery Rate[/td][td]N x (500/256)[/td][td]milliseconds[/td][/tr][tr][td]Spark Advance[/td][td]N x (90/256)[/td][td]0-90 degrees[/td][/tr][tr][td]Spark Retard[/td][td]N x (45/90)[/td][td]0-45 degrees[/td][/tr]
 
Marck,

That is alot of info to digest. The Service Manual says that the IAC motors are controlled by the VSS. Not to dispute you, but it says that with the car in park or nuetral it controls one way, when the car starts moving it bases it changes on speed. I check the VSS yesterday. If I rotate the wheel the voltage starts to change. I am assumming that verifies that it is working. The IAC motors do cylce when in the diagnostic mode, but they will not reset after driving the car above 45. I am clueless what to do!:bonkers: I know for a fact there are not any open circuits on the wiring harness. All repairs to the harness have been repaired, and all blown fuses have been replaced. I even added in additonal grounds from the intake grounds to the chasis ground to eliminate that as being a problem. Reading the Service Manual on 1 test, it says the the purge canister valve might cause a problem. I do not think this would affect the IAC motors. Also the SM states on 1 page that open loop gives a 1 per second flash, and closed loop gives a 2 per second flash. On another page it states the exact opposite. Which is correct? I know that the CFI is not the problem itself, but the ECM controlling is responsible somehow.

I think I will go remove the prom today to see if the ECM will throw a prom code. I have also been considering going back to the 82 ECM to see if that would help. I am cluless on what else to do.

Please give me any and all suggestions possible. I will try anything to put this behind me.

Thanks

Herb
 
One more question. Most of my problems started after I installed my headers. Could the O2 sensor be getting a false signal with only 1 side of the engine heating it? The O2 sensor is located in the same place as the original location. Would a heated O2 sensor fix this problem? How would I need to install this?
 
The VSS doesn't control anything, it's the vehicle speed sensor and in your case it consists of an optical sender/sensor in the back of the speedo and a little interpreter board further down on the cable (it's yellow and somewhere near the dr. side a pillar)

This sends signals to the ECM which uses those to asses vehicle speed and adjust certain things, one of which the IAC with that info.

With the car in P or N it controls the iacs to control the idle, in any gear it controls it for idle but also adjusting for engine load. The AC clutch also influences the IAC position (as such the idle) With the car in gear and running the IAC opens with the throttle.

How do you know the IACs have not reset? They don't reset to a fixed position, they start off again around pos 10 (I think it was anywhere from 7 - 15 but not sure, been a long time since I tinkered with these things) and then adjusts them for a proper idle.

If the purge valve (purge solenoid) is always open it will give a source of additional air and fuel (fuel from the canister) and it will mess up the idle. What you need to do is disconnect ALL vacuum accessories from the manifold (remove the hoses from the F shaped vac port), you can leave the brake booster vac on, or clamp the line with some vice grips. Also remove the PCV line (the purge solenoid comes in via the PCV line also) or clamp it off too. Remove the EGR solenoid vac line and the one for the thermac (when removing the air cleaner you DID plug off the thermac vac switch line right?) You should end up with all the ports on the TBs plugged except the dr. side one for the MAP and no vac. accessories. Only then can you work on your idle problems to see where the problem is coming from. When you have a vacuum leak you will get all sorts of weird symptoms. Also, what kind of hose are you running from the TB to the MAP? You should use vac tubing and preferably plastic tubing like that from festo (dunno if available over there, commonly used in pressure/vacuum systems) if you have a soft line that collapses under vac. you can get a pulsing map signal and some real funky idle issues.

How do you know for a fact the IACs are not working properly, they either work or don't work at all. They are stepper motors like mentioned above, not much to go wrong with them. They can gum up but that's about it.

Open loop gives a CEL flash (engine running w/ A&B grounded!!!) every second or faster, this is the O2 sensor not having warmed up yet, it's the ECM probing for an off "center" voltage. This only happens when the chemical potential changes due to exh. gas O2 content (o2 permeating through the permeable zirconium, which is only so if the thing is warmed up). Probing for a voltage .46 (I think that was it, working off top of my head here) means the sensor is still "cold" when warm, the voltage will change anywhere from close to 0 to 0.9V, usually between .25 and .75
When the voltage changes, the ECM checks run time and coolant temp and when parameters are ok it goes into closed loop mode and adjusts fuel mixture (INT!!!) and timing (knock sensor) via this feedback system. When in closed loop the CEL flashes whenever the AF mixture passes 14.7:1 (stoichiometric) or lambda = 1 (so from rich -> lean and lean -> rich) It's called a cross count. These are much slower and the CEL will flash every couple of seconds. You know that when this happens the system is in closed loop.

WHAT ECM do you have? An 84 model? You DO have the one for the auto tranny right and the butt ended IACs? Not the sharply pointed ones for the manual? The manual ECM reacts different to idle down and idle settings this because of the manual tranny.

So, fast CEL = open loop, slow CEL = closed loop.

I wrote an article a long time ago about how the system works, I'll see if I can dig it up.
 
I found my old writings on someone elses website :)

The pulsing of the injectors is controlled by the ECM this is known as Pulse Width Modulation (PWM). The ECM (
Electronic Control Module) is a computer, wich is mounted in the drivers side storage compartment in front of
the battery.

The ECM is the "brain" of the fuel injection system.

The '82 ECM is the 1225550, the 84 manual is the 1226430 and the 84 auto is the 1226026.


The ECM is devided into three major sections:


The ROM (READ ONLY MEMORY.)
The PROM (PROGRAMMABLE READ ONLY MEMORY) also known simply "the chip".

There's also a CALPAK, wich is the calibration for the type of engine. Some ECM's have been used in bothe V8 and V6 engines, the '7747 is such an ECM. These have different calpaks (can be swapped out).


The RAM (RANDOM ACCESS MEMORY).

The ROM contains the basic set of instructions for the ECM to follow. The programming in this chip cannot be
erased or changed. The ROM is NON Volatile.

The PROM works along with the ROM to fine-tune the functions of fuel and timing control. The PROM can be
replaced by a "power" chip (also a prom) or an EPROM (or even EEPROM) from wich the data can be erased and
changed. The PROM is NON Volatile.

The RAM has three primary functions:

To act as scratchpad (temporary memory) in case of a mathematical calculation.
To store BLM (Block Learn Multiplier) information when the engine is off or the ECM is in open loop.
To store fault (trouble) codes. These codes are stored for 50 starts or whenever the power (orange wire on
juction block for '82 vette) is disconnected. Unlike the ROM and PROM the RAM is volatile.

The ECM controls the injection system, it fuctions in either one of 3 different modes:

* Shutdown Mode..
When the ignition is off for 2 or more seconds. Only the RAM (trouble codes) and the BLM remain powered.

* Startup Mode.
When engine is cranked. Timing is locked at initial starup and air/fuel ratio is enriched. This mode is on for
about two seconds after starting to ensure the engine will continue to run.

* Running mode
devided in:

- CLOSED LOOP.
The red arrow doesn't indicate a signal, it's an event. Due to the more/less pulsing of the injectors a
rich/lean condition will be observed and the ECM will compensate for this.

In closed loop mode the ECM has 2 differend modes;

- Enleanment Mode
When the throttle is closed.

- Enrichment Mode.

At wide open throttle.

High engine load.

High engine RPM.

Overheating engine. (the cold fuel is a great contributor to engine cooling)

The ECM is normally in enleanment mode, it constantly lowers fuel addition. When the oxygen sensor detects a
lean condition enrichment mdode is entered untill the oxygen sensor doesn't sense a lean condition anymore.
Than enleanment is entered again. This (theoretic) gaurantees an "ideal" air/fuel ratio.

* OPEN LOOP.

- Fuel cutoff Mode.
When the throttle is closed and the MAP sensor notices a negative load (deceleration) untill about 1500 RPM..

- Clear flood Mode.
When throttle is depressed more than 80% while cranking. Injectors are turned off or air/fuel ratio is lowered
to 20:1.

- Limp Home Mode.
When there is major trouble with any of the critical sensors/actuators.

The ECM monitors different sensors and controls the injectors and actuators to respond to changes/demands.

The different sensors and actuators are:

The Oxygen sensor

The oxygen sensor is the only sensor on the car that creates it's own voltage and has therefore only one wire
coming from it. It's actually a chemical voltage generator, based on the potential difference between the
oxygen rich condition in outside air and the lean condition in exhaust gas.. When it's at it's operating
temperature of about 600ºFahrenheit it will produce a Voltage ranging from 100 to 900 mV. At operating
temperatures the sensor will respond to changes in the oxygen content in the exhaust gasses. It creates a
potential difference between the "clean" oxygen rich outside air and the oxygen poor exhaust gas. The voltage
potential is developed due to chemical imbalance between the two oxygen concentrations. When the voltage is
low (oxygen levels in exhaust high) the ECM will assume the engine is running lean and thus enter the enrichment
mode. Lean conditions usually produce voltages lower than 450mV, rich will be over 450mV. As you can see there
is a very critical value for "ideal" air-fuel ratio. This is done to ensure that the engine never runs very
lean/rich. This way a nearly ideal air fuel ratio is always obtained, the engine never gets the chance to
run off this point by far without the ECM noticing it.



Due to the nature of the oxygen sensor it has a hard time distinguishing between a perfect and a rich running
engine, when all oxygen is burnt the mixture can be ideal or rich. The oxygen sensor doesn't recognise surplus
(unburnt) fuel in the exhaust. Therefore the ECM will always assume the engine is running rich unless it
notices a lean condition.




The Tach Signal
The Tach signal is used to synchronise and sometimes (WOT) sequence the injectors. At very high engine RPM the
ECM always enters enrichment mode.




The TPS is actually a three wire potentiometer. It's got a 5V reference sent to it
by the ECM. The TPS is grounded through the ECM. The 5V is lowered to a different voltage through the
potentiometer (read variable resistor) When at no throttle the resistance is lowest and the voltage is also
lowest (normalle set to 0.525V ±0.025) When at full throttle the resistance between the 5V and the ground is
highest (theoretical infinity) and the potential difference (voltage) will be 5V.


The Idle Air Control valve or IAC (stepper motor)


The IAC is a motorised plunger that regulates the idle speed of the
engine by regulating the incoming airflow. The IAC has 256 positions. At normal idle it's somwhere around
position 20. When full throttle is applied the IAC valve completely opens.




The EST retards timing when the knock sensor (a sort of microphone) notices
detonation. Normal spark timing is controlled by the EST, commanded by the ECM.


The EGR or Exhaust Gas Recirculation

EGR is a method to decrease emissions by venting exhaust gas into the intake
(about 7% of the intake volume is rendered "useless" due to this). It also decreases detonation due to the
uncombustable (inert) exhaust gas. The EGR position is ECM controlled via the EGR position control solenoid.
This is a solenoid that regulates vacuum to the EGR valve thus opening/closing it.




The AIR system or Air Injection Reactor

The A.I.R system consists of an AIR pump or just SMOG pump, that pumps fresh air into the headers/catalytic converter.
When the engine is in closed loop air flows into the headers (and cylinder head exhaust ports) to speed up O2
sensor warmup. When the engine is running in closed loop air is pumped into the catalytic converter to speed up
the chemical reactions taking place there. Also during deceleration the air is vented to the atmosphere to reduce
detonation in the exhaust. The route the air travels is regulated by a control solenoid (that is ECM controlled)




The TCC or Torque Converter Clutch (lockup converter)

The TCC is engaged when the car is travelling at about 30mph at
relatively light load and in a high gear and when a relatively steady signal is observed from the TPS (steady
throttle) This is done to maximize fuel efficiency.




AC compressor clutch


When the Air Conditioning compressor clutch is on the ECM increases the RPM at idle to account for the increased
load.




The Auxiliary cooling fan

The Cooling Fan is engaged when the ECM detects a high signal from the coolant temperature switch. On the Cross-
Fire system the auxiliary fan is NOT ECM controlled




CTS or Coolant Temperature sender


The Coolant Temperature sensor on the manifold sends a signal to the ECM. The one on the drivers side cylinder
head goes to the temperature gauge, the passenger side one controls the auxiliary fan.




The MAP sensor or Manifold Absolute Pressure sensor

The MAP monitors the manifold pressure and is an indicator for engine
oad. Also the ECM calculates the quantity of incoming air from the signal of this sensor.
 
Marck,

I just hooked up WINALDL to gather some info. I noticed that the O2 sensor is getting lower voltage at a high idle. At a lower idle it will give off a higher voltage. I did some playing around with the IAC ports. I put some tape over them to restrict air flow. When the engine idled down from 1600 to 825RPM the engine started hunting. I am assuming this has something to do with the O2 temp. It went from open to closed loop when this happened. I never saw the O2 voltage get above .7 volts. Looks like this could be causing the IAC motors to go in & out or something else.

What is MWAF1 reading? this is one of the few things that vary.
What is PROMIDA ( 15 ) or PROMIDB ( 71 ) ?
Most of the data points changed when I turned the engine off while collecting data. I now think that everything might be working ok, but getting bad info from somewhere. Possibly the O2 sensor.

See if any of this makes sense.

Thanks

Herb
 
O2 should't go above .7V, close to .46 is around stoichiometric.

Where is your o2 sensor located? running headers? no cat con??? need more info.

MWAF1? I don't know for sure, but most likely AF ratio. What are the numbers there? anywhere from 10 to 17 or so??? You can post the WinALDL dump.

PROMID is the PROM ID, just a set of numbers identifying the prom. Nothing to really worry about. Don't know what the PROMIDB is, probably another (second) identifier

1225550 (82)
1225550
Description: Same ROM as 1225570
Application: 1982 5.0 V8 CFI "7" LU5
1982 5.7 V8 CFI "8" L83
MEMPAK IDs: "7": TO, ALX, ASY
"8": TS, ARY, ARZ, BMU, BPB
Program ID: 3 ($03)
Pgm ID Addr: $3002
Scan ID Addr: $33F8:$33F9
PROM type: N82S181
PROM offset: $3000


1226026 (84 automatic)
Application: 1983-84 2.0 L4 TBI "P" LQ5
1983 5.0 V8 CFI "S" LU5
1983-84 5.7 V8 CFI "8" L83 w/ AT (exc. Canada)
MEMPAK IDs: "P": AMR, CDS, CDY, …
"S": ALL, ALM, CHH
"8": CCC, CDR
Program ID: 15 ($0F)
Pgm ID Addr: $3002
Scan ID Addr: $33F8:$33F9
PROM type: N82S181
PROM offset: $3000


1226430 (84 manual)
Application: 1983-84 5.7 V8 CFI "8" L83 w/ MT
1983-84 5.7 V8 CFI "8" L83 w/ AT (Canada)
MEMPAK IDs: CDF, CDK, CFB, DBP, DBR
Program ID: 22 ($16)
Pgm ID Addr: $3002
Scan ID Addr: $33F8:$33F9
PROM type: N82S181
PROM offset: $3000

Looks like you are running the 84 auto ECM (1226026)
 
O2 should't go above .7V, close to .46 is around stoichiometric.

Where is your o2 sensor located? running headers? no cat con??? need more info.

Looks like you are running the 84 auto ECM (1226026)

O2 sensor is located 29" from the end of the collector tube. Heddman Elite HTC headers, no cat, dual exhaust. The ECM is 1226026 with factory prom for AT. All of this started after installing the headers when I think back. I thought it might have been the Air Pump to start with. The Air pump is also removed. I will attach good data from June, and todays data for comparison.
How do I attach the files?


Thanks

Herb
 
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Uhm, LOL that's pretty much useless. You should be able to output it in a table format with columns and rows.

You should really mvoe your sensor to the collector, it may not get warm enough where it is. Stock it's near the cat con because there it reads both sides and more importantly the cat con is a tremendous source of heat.

The air pump on a normal system pumps O2 into the shorty headers (during warmup), speeding up sensor heating.

You can put the O2 in the collector and use a heated 3 wire model. Just run it off a relay switched by the oil pressure switch. A heated O2 not only warms up faster, it also operates at a more steady temperature.
 
Uhm, LOL that's pretty much useless. You should be able to output it in a table format with columns and rows.

You should really mvoe your sensor to the collector, it may not get warm enough where it is. Stock it's near the cat con because there it reads both sides and more importantly the cat con is a tremendous source of heat.

The air pump on a normal system pumps O2 into the shorty headers (during warmup), speeding up sensor heating.

You can put the O2 in the collector and use a heated 3 wire model. Just run it off a relay switched by the oil pressure switch. A heated O2 not only warms up faster, it also operates at a more steady temperature.

I can not get the files small enough to attach. Do you think that we are on to part of the problem? Do you know a part number for the 3 wire sensor?
 
a 3 wire sensor is pretty generic, you can get them almost anywhere. I think this really is your problem.

Having the system go into open/closed loop intermittently will give you some real idle problems. The high idle (you have a high idle right?) will most likely be a part of it as the IACs only reset @ 35mph in closed loop (forgot to add that the prev. time)

It may also still be a vac. issue.

Also, having the O2 in the collector gives another benefit, you won't have a false lean signal in case a collector gasket blows out. An even better way to prevent this is by welding a close fitting tube inside the collector, or you could even press/tap one in place. If you have a 3"collector and no reducer after it you can use a short section of 3" pipe, cut a slit from end to end removing a thing strip, then using some band clamps reduce the OD by clamping it down and tap it inside the collector so that it extends past the flange. This way the gasket and rest of the exhaust slip over it. In case of scraping the ground or another reason that might blow out the gasket, it won't. It will remain sealed up.

I had a scraping/blowing gaskets problem with my truck (it's kind of low :D) and this soled it once and for all.
 
a 3 wire sensor is pretty generic, you can get them almost anywhere. I think this really is your problem.

Having the system go into open/closed loop intermittently will give you some real idle problems. The high idle (you have a high idle right?) will most likely be a part of it as the IACs only reset @ 35mph in closed loop (forgot to add that the prev. time)

It may also still be a vac. issue.

Also, having the O2 in the collector gives another benefit, you won't have a false lean signal in case a collector gasket blows out. An even better way to prevent this is by welding a close fitting tube inside the collector, or you could even press/tap one in place. If you have a 3"collector and no reducer after it you can use a short section of 3" pipe, cut a slit from end to end removing a thing strip, then using some band clamps reduce the OD by clamping it down and tap it inside the collector so that it extends past the flange. This way the gasket and rest of the exhaust slip over it. In case of scraping the ground or another reason that might blow out the gasket, it won't. It will remain sealed up.

I had a scraping/blowing gaskets problem with my truck (it's kind of low :D) and this soled it once and for all.


I never thought about open/closed loop at 35MPH. I will go and see if I can find a 3 wire sensor. The Hedman Elite have the slip joint instead of gaskets at the collector. This makes life a lot easier, instead of fighting gasket leaks. At least I have a direction to go in now. I will call and see if I can get my O2 sensor moved today, & hope for the best.

Thanks for the help. Don't go to far as you know how my luck is!:lol:

Herb
 
Ok, Marck I was with you guys until the part about reseting (synchronizing ) the IACs at 35mph in closed loop. Trouble understanding the logic in that, as well as wondering how the ECM accomplishes it? Don't you have to drive the motors to a stop to sync them up? How is that done trekking down the highway at 35mph at partial throttle while executing the IAC throttle following routine?

One other question comes to mind. With the addition of headers and the elimination of the CAT, how much did that effect the VE of the engine? Does the BLM have enough range to handle the shift? Even if it does, the engine will be operating in a biased condition effectively reducing its ability to handle other variable factors. I guess one could purchase a mail order calibration prom, but you guys already know what I think of them. :D This is an interesting discussion, Marck. Not only are you an expert mechanic and fabricator, you know your way around these old EFI systems. :thumbs:

Herb, Good luck, with the witch hunt. BTW, since bone stock is not a consideration, why not upgrade to newer technology??


Bullshark
 
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