Help me figure this one out.

C

clutchdust

Millionaire Playboy
Joined
Mar 27, 2008
Messages
1,726
Location
In transition
I have an idea but I want to put the scenario out there undiluted and see if you guys think of something I haven't thought of.
Car: 1998 Pontiac Firebird, 3.8L v-6 auto

History: When I started looking at it, the car had been overheated. I replaced several failed PLASTIC coolant fittings just to get the cooling system sealed, only to find that it had a blown head gasket. Pulled both heads, had them magnafluxed and machined, new head gaskets, reassembled.

Current symptoms:
Throws P0303--cylinder #3 misfire
distinct miss at idle, less noticeable while driving.
hunting idle.
runs a little warm: idles around 210-ish but will get up to 220-230 under load. Drops down to 180-190 moving with no load.
tapping sound. Sound almost goes away under acceleration and gets more pronounced when no load, returning to idle. So if I rev the engine from idle to about 2-3k then let it return to idle I get noticeable tap then tap almost disappears, then tap gets more noisy until back to idle, then noticeable tap. Make sense?
The tap is definitely a tap. Doesn't sound like a knock. I've heard rod knocks in the past and this just doesn't sound like it. Doesn't really behave like it too.

What I've done so far:
Aside from the head gaskets, I switched #1 and #3 injectors and I switched #3 and #6 plug wires (uses a common coil) to determine if either the coil (I know, long shot) or injector were bad. Problem stayed on #3 cylinder.

I have a good idea at this point of what I think the problem is but I really want to see if any of you guys come to a similar conclusion.
 
Listen with a stechoscope probe to each injector, make sure their bodies all sound the same in the same spot....

push down the #3 wire to make sure of connection....

check plug??

:crutches:
 
mrvette said:
Listen with a stechoscope probe to each injector, make sure their bodies all sound the same in the same spot....

push down the #3 wire to make sure of connection....

check plug??

:crutches:
Click to expand...
Wires are good. Listened to the injectors. Besides, switching them should have moved the fault, unless the problem is actually in the wiring. Guess that's still a possibility. Will need to check that also, but putting the scope on the injector I hear a click like it's firing.

SmokinBBC said:
misfire, tapping noise that changes with engine speed......could be a sticking valve
Click to expand...
Close to what I'm thinking. Didn't really think about valves sticking. I did roll it over by hand and didn't notice a valve sticking, but I also wasn't looking for it either. Will do tomorrow.

Keep 'em coming
 
some genius chose a very good location for the coils as well as the ignition module: the module is getting baked by the heat from the header and wet from the leaking rubber gasket at the hood surround....
the ignition module is $160 at AdvancedAuto, i had to buy one a few years back.... it was annoying , it misfired but idled ok..... i changed coils first, then the module which finally fixed it.....

these V6 3.8s are pretty good engines, i wouldnt expect a mechanical issue like a bent and stuck valve .. do you know the car's and engine's history ?
 
Tapping noise could be transmission. my 97 makes makes a tapping noise in the trans that goes away above idle. Been like this since 30k miles .... As for your misfire check your plug wires, mine occasionally trashes the 6th plug wire for unknown reason and about 6 inches from the plug boot the wire gets swelled and brittle.

Your temps seem way off though, the only time I see elevated temps is when stopped ... the 3.8 is a pain to get all the air out of the cooling system and mine will tap when it's not purged. If yours is like mine there is a bleeder screw on the thermostat housing.
 
Last edited:
Don't you just love the position of that coil pack??? Assholes. Can't even run the engine with the valve covers off, well, at least with the left cover off. And that's where I suspect my problem is.
I'm certain the problem is in the engine, not the tranny. This tap is all engine, really sounds valve related. My thinking is collapsed lifter and I'm going to prove/disprove that today. Very difficult to check since the rockers are fixed height.
As for the ignition module, what are you thinking? For one, it's Vegas, so rain/water damage is not a likely cause. Heat, more so. Besides, doesn't the engine "batch" fire a pair of cylinders at a time? I switched the two wires on the #3 coil pack (#3 and #6) with no difference in run/drive characteristics. So does the ignition fire cylinders individually? If the ignition module was failing, wouldn't it be two cylinders failing?

Oh, the other thing that I think indicates a valve issue is compression. Before I pulled the heads, I did a compression test. I could only get 30psi on #3 cylinder. At the time, I simply chalked that up to the blown head gasket. But that didn't really jive with what I saw with the heads off that showed obvious signs that #1 had been leaking and no real signs #3 was leaking. By the way, #1 barely pulled 120psi.
 
Last edited:
Heat on that ignition module is nearly as bad as water. And it's location is no help. It fires 2 cylinders at a time, one while the exhaust valve is opening and the other on compression. And yes, it's pretty dry there, but antifreeze gets into the electrickery stuff and helps it exactly zero.

And your little tick could be the spark leaking someplace it's not supposed to be.
 
Well, here's the latest. Pulled the covers off, all the rockers, plugs, etc. I rolled the motor over by hand and couldn't find a single pushrod not giving full lift. Put my fingers on every one and could not determine a single one bad. The #2 exhaust and #4 intake both had a very small click I could feel with finger pressure. Feels like the plunger gives maybe 1/32"-1/16" before I hit firm spring pressure. Not sure what this means.
I rotated it with the rockers back in place and checked each rocker/spring to see how much lift I was getting. They all looked identical. I know eye-balling is not particularly accurate, but I put a tape measure on the head to see how much lift I was getting. With just the grading on a tape, it looked like about 1/4", maybe a tick more, on every valve.
Now here's the part that worries me most. I can't get a compression gauge on most of the cylinders, I'm sure Karsten can back me up on this, it's virtually impossible with the exhaust manifolds in place. But just plugging the spark plug holes up with my finger I got NOTICEABLY less compression on #4 cylinder.
This kind of jives with what I saw on the plugs. #1, 2 and 5 looked beautiful. #3 looked gummed up, like fuel was being injected but not burned, rather turned to gum just off the heat absorbed by the head/plug. #6 and #4 had a noticeable black coat of carbon. Surprising since all these plugs were just replaced last week. #3 and #6 I can explain because I found that the coil that fires those two cylinders was bad. #4 though, worries me a bunch. Why would that be so fouled already? Only thing I can think is that with reduced compression, that cylinder is running richer than the compression warrants.
The other thing is with the plugs back in it was significantly easier to roll the motor over on #4 compression stroke than any of the other cylinders. This concerns me. I did pretty much any thing you could think of to check compression without actually putting the gauge on. Just damn near impossible without more disassembly.
I inspected all the cylinders when I had the heads off and they cleaned up beautifully. The heads showed absolutely no indication of a problem. The only thing I can figure is maybe the valve seat is carboned up and not sealing well.
So I replaced the #3/6 coil pack and put everything back together, filled it with water and the thing runs a ton better. Still has a distinct tick but for whatever reason, not as bad as the other day. Also has a bit of a miss, kind of like it's running on 5-1/2 cylinders. Engine temp never went over 180*
I talked to the machine shop guy and he tells me it's not too uncommon to get a tap/clatter from the front of the engine due to a bad timing chain. I can't prove any of this. I put my 'scope on the front cover, all down the rocker covers, oil pan, even the transmission with the car up on ramps and could not determine any place exactly where the tap is coming from.
The guy is really happy at how much better it's running, but I'm not satisfied yet. I would like to know what the hell that tap is and why I don't have compression on #4.
Any thoughts are appreciated.
 
You must log in or register to reply here.

Latest posts

Back
Top