Greetings,

My 1988 Camaro (stock) with around 260,000 kms is missing at idle. It accelerates good and runs down the road fine. But at idle, the engine will stumble (you can see the tach drop), but it does not completely stall out.

When it first did it, I replaced the IAC and the problem went away. The problem returned so I checked the IAC and noticed that the threads on it had broke off (it had broken in half!). So I replaced it and the problem went away for a while.

The car then had a different problem, which was diagnosed as the magnet on the reluctor shaft being cracked, so I replaced it and the Ignition Pickup Module (in the distributor). The car ran fine again for a long time.

But then this idling problem returned, so I:
Replaced the ESC module - no effect. Tried setting the minimum air idle to 450, 500, and 600 - no effect. Set the timing to 6 deg and 0 deg (no sticker under the hood) - no effect. Set the TPS and checked the range for over 4 volts at WOT - no effect. Checked the fuel pressure (40 pounds running) and it drops within 15 seconds to 30 pounds when I shut the car off, but holds at 20 pounds for over 20 minutes. The injectors measured between 17.4 and 17.7 ohms across all 8. I replaced my factory MAF sensor around 2 years ago.

Since I couldn't figure out what to do next, I took the car to my mechanic. He said that it does not seem to be running excessively lean or rich at any given time, and was unable to find anything wrong with it. The EGR tested ok. No vacuum leaks were found. No codes are being set. He cleaned some carbon build-up off the throttle body plates hoping it would help. He said that the idling problem could also be caused by a build up of deposits on my valves, and recommended I pickup some fuel additives. He said to run double the recommended dosage at each fill up for three tanks, and use Premium fuel from now on.

So I filled up, put in the additives in, and noticed that the car was idling way too high. Hoping that the problem was due to a dirty throttle plates I was setting the minimum air idle and noticed that the IAC had become loose!! So I tightened it, set minimum air, and the TPS. The car ran like a dream!...for two days.

So, thinking that perhaps the IAC had become loose again, I checked it - but it was fine. Then I thought maybe it had plugged up with carbon when the throttle body was cleaned - so I replaced it. No difference.

Not sure if it's related, but:
If I put the engine under heavy load, the car 'shakes' or 'vibrates'. It stops doing it if I push a little bit harder on the gas peddle, or if it downshifts. It only does it when the engine is under heavy load (like going up a hill and just before it downshifts).

As well, around 2 years ago I started going through fuel pumps and pressure regulators. I noticed that the material in the regulators was actually melting! I went to a dealer, trying to figure out what was going on when I was told that they had quite a number of cars in with similar problems. He mentioned they had all been buying gas from the same supplier that I had switched to, and were advising customers to stay away from that brand. After switching, and again changing my pump and regulator, I haven't had any problems since.

I am at a loss to figure this one out.

Please Help!

 

Have the injector ever been cleaned in the life of the vehicle?

 

I've owned it for 5 years. I've run injector cleaner through it around 3 times.

I can't speak for the prior owner, but it only had 88,000 km when I got it. Now I've got 260,000 km.

 

Vader,

I've never pulled them out and had them cleaned. I've only run cleaner through them.

I'd also like to make this more confusing in that the last time I set minimum idle air, I was able to adjust the throttle plates to fully closed (the screw was not even touching the stop plate), and the car was still running a little high on the idle.

I'm not sure what would cause that?

Could what-ever is causing that cause the stumble at idle too?

Don

 

Ok. I solved it (but I think just temporarily)?

#8 plug was almost completely covered with ash deposit. Haynes says it can be caused by oil / fuel additives / oil entering the combustion chamber. I've certainly crammed enough of that through. Those plugs were pretty new - all of them were fine but #8.

I've changed all 8 plugs, and my oil and filter. I'll watch it and see what happens.

I'm pretty sure that all of this started when I changed fuel brands. Sunoco no longer sells in Ontario - so I had to change to Petro Canada. Since then, I've had drivability issues, but nothing that I could pin-point directly to fuel. I think I'll try Esso.

 

Clean and fresh gasoline, minimal additives (only if and when there is a problem) and a working evap system can help prevent that. High load at colder engine temperatures can also create deposits. Keep a light foot on the pedal until its warmed up, then don't be afraid to make it work.