Dan1990

I just brought a 94 m6 z28 today off a 70yr old. The car is in immaculate condition. Nothing I have three concerns 1st I have a idle surge. When I am idling it revs from 1k to 1.5k rpms. What can I check? I dont want to just replace things and it doesn't fix the problem. 2nd I have an air bag light. The man told me he hit a lightpole a year ago and thats when the light came on. I hate having dash lights, how can I get rid of it. Lastly, the exhuast sounds louder then I expect. It has a kow rumble at idle. Is that normal or can I have an exhuast leak?

Dan1990

Can anybody help me?

Injuneer

Check the vacuum lines for leaks. Look in particular at the vacuum line to the power brake booster. Most of the original lines, particulary the hard plastic stuff, can be cracked and leaking in a vehicle this old.

Scan if for codes. Check the fuel pressure.

To diagnose the airbag light, you will have to have it checked by a shop with a Tech-1 or Tech-2 or a high end scanner. That will provide a code indicating the source of the problem.

As far as the exhaust, anything is possible. You're going to have to get under it. If you know what stock looks like, you'll be able to tell. If you don't know what stock looks like, here's a diagram. If its not stock, take a picture and we'll try to tell you what it is.

Courtesy of Shoebox:

http://shbox.com/1/exhaust_system.jpg

Sorry to see someone sell their car because they are "old".... I'm holding on to mine. Glad you got a good deal.

Finally, please don't push a thread back to the top without waiting at least 24 hours for a reply.

Dan1990

Also, just notice yesterday when I am cruising and I accelerate the car hesitates and bucks. I have to war of the gas and it smooths out. Once while in 6th and under 2k. The cat did the same but so bad, it felt like it was going to stall out, and had to put it in 5th.

Injuneer

Check the rubber elbow between the MAF sensor and the throttle body for tears and leaks.

When you try and accelerate in 6th gear at low RPM, any weakness in the ignition system is going to show up. How many miles on the car? While it may be immaculate, the front end appears to have been repainted, with all the black parts painted over - headlight openings, hood grilles, Z28 emblems. When were parts like the spark plugs, plug wires, O2 sensors, etc last changed?

Hard to follow this part "The cat did the same but so bad".... did you mean to type "car"?

Dan1990

The car has 94k miles. The front was repainted. I'm going to look into the elbow and see if I see any tear and yes I meant car not cat. Damn swype autocorrect. The repainting was due to a small accident into a light pole.

Vader

If the engine does not surge in open loop (cold) and then does surge in closed loop, it will be the opti. The bearing is bad and the high res ring at low rpm is wobbling and producing a false rpm signal, which then makes the ECM command the IAC valve. At anything above about 2500rpm the engine will run fairly normally as it switches to the low res ring at that speed ( or close to it I think).
Low resolution is used during closed loop.
So if it's fine cold, and bad hot, I'd suspect the opti.

Injuneer

The PCM has to operate on the low resolution pulse signal. That's the only way it can tell where the crank is in the firing order. The high res signal is used to keep track of engine speed between the pulses of the low res signal, for increased ignition and injector timing accuracy, and reduced spark scatter. Engine can't run on the high res signal alone.

Vader

For sure - understood. My point is that when the PCM switches to using the high res signal for rpm in closed loop, that's when the problem will occur. I've observed the rpm signal with a scan tool on a faulty opti, and the signal rpm wavered hugely, more than the tach on the dash or the engine could respond to. That's what led me to the diagnosis.
So if the OP has a steady idle cold, and surges hot, it's the opti, and it's the high res signal messed up from the bad bearing.

Dan1990

Today driving on the highway in 5th gear. I accelerate being lazy I didnt downshift at all. I was during 65mph at 2krpm and when I gas it it went to3.5krpm and then the car just started to hesitate and the car completely cuts off on me. And yes when the car is warm it gets worst but when cold you barely hear the idle surge.

Dan1990

Found out the problem I have a leaking oring in the injector and a crack on the elbow between the throttle body and maf. Where can I get this elbow? Can I but the slp intake and it comes with one?