c489

I have a 1992 lt1 c4. bought it not knowing a thing about vettes. it had a bad missfire. changed plugs then wires, no fix. changed opti, coil. icm. no fix. brough it to chevy too have them check it out they tell me #7 injector is shot. ok i bought all rebuilt from fuel injector connection. put them in car runs like new. misfire gone. changed the fuel filter and water pump changed the coolant temp sensor. now after it warms up and i drive it around for a while when i pull up to a light or park it and keep it running the rpms like to jump up and down from 500 to 1500 then down to 200. then it will rev up to about 1000 then just shut off. each time is different like it will just randomly turn off without the rpms moving. as if i turned the key. asked everyone i knew about it. some tell me check the vacuum some say check under the valve covers. it seems more electrical to me that anything. Any input is helpful Thanks

93Rubie

Could be a lot of things. Could be the parts you shot gunned at it.

Is your CEL on? Have you done basic fuel pressure checks? Do you have good solid consistent spark with a spark tester at idle?

You might feel if the EGR is closed. The diagram should be down at idle.

FYI, vacuum leaks typically will make a speed density system (MAP, this is what you have) idle high not low and jump all over unless the car is running SUPER lean.

If you don't have a clue as to what I'm talking about, go to the internet. Its full of good info, Scannner Danner on youtube is who I recommend. Unlike a LOT of the internet HE actually knows what he is talking about.

You have three choices:
1. Throw parts at it.
2.Learn about the car and how it works, buy a few tools, diagnose car.
3. Pay someone else to do #2.

c489

Thanks so much for answering, no check engine light but when it shuts off i get infl res? as you can probably tell im new to forums. Im definitely going to learn more about it because alot of what your talking about i have no idea about. Ill do some research and post my results. Thanks again

antfarmer2

Did you use thermal grease under the ICM?

c489

yes i cleaned both surfaces then applied grease to both

antfarmer2

Clean or replace the IAC?

c489

No i haven't touched the iac

antfarmer2

I would clean and inspect it and your throttle body.

c489

Well i jist pulled off the egr after reaseaching about it. Its hard as hell to push down. It holds a vaccum but in not sure if the vacumm from the car is strong enough to pull it down. Also im not sure if its closing all the way. Any thoughts?<br/>

c489

Justin pulled off the throttle body and iac. The iac measurement is 7/16 it's filthy but idk how to tell if it's good or bad, the throttle body looked fine

antfarmer2

Have you ever removed the TB, taken off the IAC, sprayed the pintle GENTLY with brake cleaner, wiped it clean, taken off the IAC housing, clean it and the passages, reassembled WITH NEW GASKETS and reset it? FORGET THE FSM. Use the scanner and set for 20 counts on IAC.Don't much care for cigars in the engine bay of a gas car myself but those are more subjective than interrogating the ECM. Get the car hot to stable operating temp after you have cleaned the IAC as mentioned above. Everything but the motor off. Get the IAC reading. It should be about 20 counts. If it is higher, you turn the adjustment screw in. That would let more air in. Rev and read again when it stabilizes. Repeat till you get 20. If it is less, you turn the screw out. And here is the important part. If you get a 0 count, it means the ECM is saying it has all the air and possibly more than it needs. That could signify a leak

Stolen from Aklim

c489

I'm sorry I'm kindof New with car lingo, what's an FSM and what kind of scanner am I using and where am I plugging it in? Your probably laughing at what I'm asking but I honestly don't know and 20 counts of what? I have the throttle body off right now and the iac out its real dirty tomorrow I'm going to clean them both and reassemble

antfarmer2

FSM factory service manual. Scanner plugs under drivers side pad. Right now clean it good and new gaskets. Look on ebay for the fsm best thing for you two book red cover.

c489

OK cool, now what kind of scanner am I looking for

antfarmer2

Yours is a 92 so you will need a ob1

c489

OK thank you, I'm going to do all of this tomorrow and I'll let you know what the outcome is

antfarmer2

Hope it is not your opti

c489

well egr was changed took out iac and soaked it for hours put everything together car ran perfect didnt shut off bog down idle surge nothing... yesturday im driving it shuts off at two stop lights in a row. today it shut off in my drieway. So i bought this car used ofcourse and under the hood theres a sticker that says hypertech chips... never paid mind to it i was always so worried about the missfire i fixed, so just now i pulled apart the computer and theres a hypertech chip in there... how much of this could be my problem. also i put and msd opti on it a while ago and im not sure if your famiiar with those but they have a manual timing adjustment screw. how much could the chip be screwing with the opti timing adjustmen?? im clueless about this stuff

antfarmer2

Do not know about the chip did you clean out good where the iac was might have some crap still in there that broke loose.

c489

yeah cleaned if for while. then had someone sit in the drives seat and turn the key while i held the iac plugged in and it moved back and forth with the key turning