1991 Chevy Camaro 3.1 V6 Rough Idle

Hey guys. I’ve got a bit of a problem with my Camaro that’s somewhat intermitent. It’s most noticable on cold starts after sitting over night. The car wants to run very low RPM and generally stalls. If I give her some throttlw she’ll stay running and once I let go depending on how long I’ve kept throttle on it’ll generally level out, I also noticed alot of times if I just give it a little slip of throttle at the pedal and release, most of the time it doesn’t want to compensate because the RPMS will drop and alot of time it will stall again. Sometimes it catches itself, sometimes it won’t. Generallyt it seems to get better once it’s warmed up a bit but earlier it seemed like the idle wanted to bounce a bit and occasionally it would again stall out.

-We recently replaced spark plugs and wires for a tune up
-The MAP, IAC and TPS sensors were replaced last year.
-I checked the coolant temp sensor for the computer, the resistance seemed to be reading normally.

  • We recently also cleaned the points for the distributor cap since there was some corrosion.
    I tested Fuel Pressure on the test port on the rail and pretty instantly got 40 PSI of fuel pressure. I ran the car with the tester and it never really dropped below 35 PSI.
  • I let the car off for a few minutes with the tester hooked up and the pressure remained constant. I’m thinking my injectors aren’t leaking with that, although it’s possible that one or more may not work properly.

There are no Check Engine lights on at any point when this happens. I did put it in diagnostic mode to check for codes as well and got nothing.

Some other potential candidates that I can think off the top of my head
-Mechanical timing advance is off
-compression related problems
-fuel injector(s) not working.

If anyone has any other thoughts based on the symptoms or past experience of what to look for, please let me know.

Edit: I forgot to mention she does smell like she’s running fairly rich. I think the best analogy would be it smells like good ol bumper boats

A new distributor cap and rotor is cheap and worth a try.

I’d suggest pulling spark plugs and seeing if all of them are showing rich… or just one or 2. One or 2, I’d replace the injectors for at least those cylinders running rich.

I’d run a compression test while I had the plugs out just because the plugs are out. I don’t expect you to find a problem but it is worth doing because it might. Also check the vacuum at idle. There are a lot of things that vacuum readings can tell you.

I’d take a look at the PCV valve and EGR valves to see if they are stuck open or clogged.


didn’t even really think about it when I pulled the plugs but yeah I’d say all are running very rich. The question is, it injector related or something else? I’ve seen a couple places saying that these injectors are notorious for failure.

Based on that statement and those plugs… I’d say that 29 year old girl needs some new injectors!

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I’ll go ahead and order a set, certainly can’t hurt anything to throw some in as a good tune up. I am going to get some resistance measurements and see how they’re firing anyway.

I’ll make a prediction… the injectors are firing… a bit TOO much firing. Not fully closing, I’d guess.

I think if you tried to draw a vacuum through the engine side of the injector, they would all leak.

I’m thinking you’re probably right. We’re going to pull the injectors out of the cyllinders to try and monitor how they fire…being careful not to blow anything up.

Doesn’t this have TBI? Makes it cheaper to replace the injector(s).
Edit - nevermind, multi-point. My '95 Suburban was behind the times!

Actually have a new development, the vacuum line under the plenum that goes to the fuel regualtor was destroyed. that is probably our issue…going to replace the injectors anyway. got a set of Bosch OEMS for about 70$.

fixin the vacuum lines did not fix the problem but we have been clued into something that might be a bit more tragic. the oil smells pretty rich with fuel.

That’s not surprising, old engines running rich have lots of blow by, and I bet you haven’t had it out on the road very often to get the oil up to temperature.

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That is true.

I think I know now who I should donate my ancient spark plug cleaning machine to.

Low compression can cause rich running. Compression should have been checked with the plugs out.

Rather than yank the plugs out again (at this point anyway) I would suggest connecting a vacuum gauge to an intake manifold vacuum port to see if there is any abnormality. That’s quick and easy. Depending upon the gauge results then a compression check might be warranted.

I unfortunatley do not have a vacuum gauge, on my long list of tools to get. I do however have a crappy Harbor Freight compression tester. I ran 4 out of the 6 cyllinders. all of which held at about 117. Normally I’d be a tad concerned although not horrifically at those numbers. However, I know I got similar numbers on a healthy 09 Toyota Yaris so I know my tester doesn’t read full pressure From what I’ve read the way those testers are hooked up, several people have experienced that low reading before. I’m thinking my actual values are probably closer to 137 if not a bit higher. Thankfully compression does not look to be our problem. We’re definitely leaning more towards those ancient injectors and possibly needing a new CAT and O2 sensor given how rich it’s been running. That CAT’s gotta be getting close to clogged if not already there. The test was also done on a cold engine, I imagine had I done the test after it warmed up, my results probably would’ve improved.

The high-to-low compression test readings are more important than the actual number. That can be affected by many things… inaccurate gauge, as you point out being one. I’d be curious about the 2 cylinders you didn’t read. Too tight to get the gauge hooked up? The back 2 on the passenger’s side, maybe?

If those 2 plugs looked OK, maybe you can omit those and feel safe they are OK.

all 6 plugs were coated. actually just after running the new plugs for a bit they’re already blackened up. and yup, you got it, one of the passenger plugs is a bear to get to so after we read 4 all being the same, yes the other 2 could be different but we suspected the others would be the same.

A 137 is bad also; much less the 117. You might consider running a wet compression test. That’s where a small squirt of oil is added to each cylinder before it is tested. If the number goes up considerably that would point to a piston ring problem.

It sounds to me like the engine is worn out. A reasonably healthy engine should have 180 PSI on all cylinders.

I was reading on a forum that the 3.1 good compression is between 145-160. That was only one source. I’ll probably do a wet test on it just to double check. I’m definitely leaning more towards injectors considering I’ve had numberous people on this forum and a ton of otheres mention these injectors definitely lead to problems, especially hold old they are. It could be a combo of both as well, injectors and failing compression.