Phillip May

I've just started having this problem about a week ago. I took my car to autozone to have check engine light checked out and it said I had random misfire and my o2 sensor "downstream" was going out. The guy said that the o2 sensor was most likely the cause of the misfire, also my IAC was going out. I decided to change my spark plugs and change my fuel filter and i would handle the o2sensor and IAC at a later time. Once i changed my spark plugs and fuel filter a few days later my car shut down on me while i was driving as if i turned the key back. I was able to immeadiatley turn it back on and continue, but it shut down again after a mile or less. I finally got it back to my house and called to see what the problem was. I was told that it could be the ICM or the fuel pump or filter. I changed the ICM today and took the car out for a spin to see how it would drive and after about 10-15min the car shut down again and wouldn't start back up. The engine would turn over but not start. After a while the battery died and I had to get someone to tow me back home.
I noticed that this only happens when the engine gets warm. From a cold start the car will run fine until the engine gets warm... then thats when it shuts down. Before i could get it to start again, but now the last 2 times that its shut down it wouldn't start back up. I'm getting really frustrated because this is my only car and I have to get to work and back. Any help would be greatly appreciated...thanks in advance!!!!

Chet1185

I'd recommend testing the opti. when my last one was bad the car was shutting off like I was turning the key off, then would start after a little while, then die again. the issues you are having sound like opti failure to me.


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timnfxr

I recommend checking the fuel pressure with key on/egine off. It should be ~41-47 psi, and should hold after the prime cycle (within a few lbs).

Injuneer

Scan it for codes. Then come back here with the exact numerical codes. Autozone didn't do you any favor with their analysis.

shoebox

Intermittents are hard to track down, but you have to diagnose while the trouble is there. Check for spark and fuel pressure when it quits. It will give you a direction to proceed, rather than guessing and throwing parts at it.

slick1hunting

just curious how they determined you iac is going out. When my iac went bad it would stick at high idle(like a vacuume leak but higher rpm) and sometimes die because of rpm dropping so low(only stay running if I feathered throttle). My iac counts were also very messed up. Turning the ac on will usually show it working but if it struggles a little may point at other things too.


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Phillip May

When i had it checked at autozone the code that came up said there was a problem with the IAC. Which i know is true because the car tends to idle very low (less than 1k rpm ) like around 800 rpm

Phillip May

I'm going to have my ICM checked again. When i took my original ICM to autozone to have it tested, they tested it about 4 or 5 times and it passed. But i just realized that my new ICM that i bought i didn't put any thermal grease on it before i installed it so i hope i didn't already burn it out that quick.

shoebox

The heat transfer compound is essential. I hope it is not damaged, too.

Injuneer

Is your car auto or manual trans? (Adding a "signature" would help people help you.) Stock program or aftermarket reprogram? Stock idle speed - engine fully warmed up - for the A4 is 550rpm in gear and 650rpm in neutral. For the M6 it's 800rpm. Even allowing for the typical stock tach reading of 100-200rpm above actual, you shouldn't see an idle speed over 1,000rpm.

If by "downstream" O2 sensor, they meant the after-cat sensor, there is no way it can cause a misfire. It's there to monitor the efficiency of the catalytic converter. Next time, get the actual alpha-numeric code in the format Pxxx. The verbal descriptions are not adequate.

There are four codes related to the idle speed - P0506 = idle control system low engine speed (88rpmm below target); P0507 = idle control system high engine speed (100rpm above targer); P1508 = idle air control valve circuit low; and P1509 = idle air control valve circuit high.

Get it scanned again - when the engine shuts down, there's a chance a code was involved. One of the fatal flaws, loss of low resolution pulse form the cam position sensor in the Opti, for example, shuts down the fuel system, and does NOT turn on the SES light.

Phillip May

Ok yea i understand now. Tomorrow I will go back and have it scanned again and i will ask for a printout of the codes that pop up. I didn't realize that about the idle speed. I always thought that 1,000rpm was usually the norm for an LT1. And yes my car is automatic and it is all stock other than KN air filter. I will post again tomorrow with what the codes are when i take it back to autozone...

Injuneer

An OBD-2 code scanner is really dirt cheap.... and an essential tool for anyone who owns a 96/97 LT1.