Problem: Car idles irregularly and runs like it is misfiring while driving.

Background info: In July replaced spark plugs and wires. Car completely died, took it to Chevy dealership, need 4 new injectors. Replaced injectors, car ran fine for a while. Now car idles terribly and sometimes while stopped at a light feels like it is going to stall (car is an automatic) When I start the car up cold it runs great until it reaches operating temp, then starts to act weird. My friend says the injectors sound fine and there is perfect fuel pressure. So I am at a loss here and really losing faith in owning a corvette. Someone please help, even a suggestion or something I could do to test the car would be great. Chevy had it for 3 days and 2 other shops had it for 2 days each and couldn't give me an answer. The forum has helped big time before, I am hoping it can again. Thanks in advance!!!



P.S. I just had a thought...Back when all the problems started in July, I had just adjusted the timing which I later heard was bad. I had the dealership check the timing and they said it was fine. What I am wondering is, even if the timing looks fine but it is slightly off from where it used to be, can the computer be slightly over advancing/retarding the timing once it is warmed up? If so, is there a way to have the computer adjusted to run properly?

 

What does it idle at?
Does it back fire at all?
Do you have a vacuum leak?(sounds Like it)
Check the vaccum near the distributor drivers side a 3 way and the hose leading to the evap loss can at drivers side wheel well.Let us know.

 

I am thinking it is idling at 800 or so, havent drivin it in a while so cant be sure. It hasent back fired but sometimes it feels like it wants to. I'll check the vac tomorrow. Thanks!

 

I'm trying to understand the ". . . sometimes it feels like it wants to." part.

Anyway, without doing a diagnostic, all you're going to get is a lot of suggestions and possibilities from other members - which you can interpret as "guesses". No one can be expected to accurately diagnose the cause based on what you've written so far.

The cause could be one - or a combination - of several different things. Without having to resort to pulling out your hair over this, buying a lot of parts you really don't need and spending countless hours checking one part after another, just invest in a data logger, like Diacom or ScanTool. Hook it up and read the numbers. Some sensor(s) or part is out of range; not operating within its design parameters.

Remember, it's all about the numbers.

Keep up posted.

Jake

 

First off, you'll want to make sure it is going closed loop ..
You can do this using the field mode in the ECM..
I *believe* you start the car with the diag terminal grounded.
Quick flash means closed slow flash means open... I think..
Dont have my books with me but someone should chime in..
It's possible you are in open loop and the O2 isnt being read
by the ECM or you have bad data going in if in closed loop..
Reason I say this is if you had 4 injectors replaced, its possible
you fouled the heck out of the 02 prior... my two cents .. HTH
This is cheap and quick way to start..

 

Pardon me for saying this but-3 shops couldn't diagnosis it! First Don't go back to them. Another thing is you replaced 4 injectors-why not all 8? seems like if 4 were trashed the other 4 might have been on there last legs too-how many miles?

about possible vacumn leak-get yourself a propane cannister-small one like for BBQ, get a torch nozzle for it DO NOT LIGHT IT-just turn it on and go over the vac lines-if the RPMs change you more than likely have found a vac leak.

On the 02 sensor-mine has thrown a code when I've misted it.

How long ago was the fuel filter changed?

So many things it could be-you really need someone to scan it to get an accurate picture. Good luck.

 

Check out the cap and rotor if you haven't done so. I've had points on the cap that fell off so the car was only running on seven cyl. I've also had the rotor screw work loose so the rotor wasn't on all the way and was flopping around. It can't hurt to throw a new rotor on it's probably $10.

 

[QUOTE=C490Vette] When I start the car up cold it runs great until it reaches operating temp, then starts to act weird. [QUOTE]

Cold engine running fine==> Open Loop ==> O2 sensors ignored
Operating Temp runs crappy ==> Closed Loop ==> O2 sensors in play

I would suspect your O2 sensor(s) are faulty

Have you ever noticed your Check Engine light coming on?????

and I quote:

FAILURE SYMPTOMS
A faulty Oxygen Sensor can produce poor fuel economy - gas mileage may be reduced up to 30%, and other driveability problems associated with overly rich fuel mixtures. The “Check Engine” light will be on intermittently.

 

Im dealing with the same problem. new cap/rotor/etc found a miss on #5, and the imedance is different on that injector then the rest so i'm assuming it's bad. I cleaned my TB very thoroughly, pulled out the IAC, all gaskets and covers, and went at it with a toothbrush and TB cleaner. This made a HUGE difference, and it now idles fine, but still has a miss which I attribute to the injector - I'll be replacing all of mine soon, not sure which brand (im going to start a thread on this) ill go with.

Are you injectors the silver/grey multecs? Accroding to Rich from Cruzin Performance they are junk, and he won't even bother cleaning/testing them - he recommends they all be replaced. You may have a yet another bad injector - have you ruled that out? Try pulling your throttle body off and seeing if meybe it just needs a good cleaning. It doesn't seem like it could cause such a problem but it does. Make sure to clean the IAC valve and all related plumbing.

 

Don't you see what's happening here?

It's exactly like to told you it would. Whenever someone posts about a problem similar to yours, all sorts of suggestions get posted; everything from the back of the car (fuel pump) to the front (MAF) and every stop in between.

So unless you follow my advice and get a data logger, you're going to be running around like a chicken with its head cut-off checking one thing after another and it's gonna drive you crazy.

Do yourself a BIG favor and invest in a data logger; you'll find that you'll be using it over and over and it'll pay for itself in no time.

All I can do is give the best advice I can; you can lead a horse to water but . . .

Jake

 

I guess no one should post then?
I think we are just trying to help the guy out.
Maybe he wants some ideas in case he goes back to
the same folks that just put 4 injectors in ...
With no check eng light, yes it is cheap and quick
to enter field mode and get an idea if he is running
closed or open loop. Whats the big deal? You dont
need a scanner for that..

That's all .. No need for the heat

 

Wow! Lots of good suggestions. Your symptoms match mine exactly ('91 vert auto), changed everything I could think of, cleaned IAC, TB, then read on this Forum (jfb and Sam Lam) that a simple resistance check of the injectors can tell you if they're bad. They should be 16-17 ohms, lower readings indicate shorting and they will need replacement. Five of 8 were shorted on my engine, bought Accels from Summit and the car has run great ever since. My $0.02. Good luck!

 

I agree w/having a full diagnostic done by a pro before you do anything else.-What I CAN say is I had the same driveability problem as you. I also replaced 2 Multecs and had the rest cleaned.
In the end, I pulled them out AGAIN, trashed 'em and stuck 8 new injectors in and haven't had a hiccup since. Multecs are junk.

 

this is a good suggestion...i use datamaster to determine malfunctions first.. i built my own rs-232 PC interface to work with it and it has saved me a lot of molly so far.. i believe i also have the an interface for sale

 

Check your PM

 

So what's the latest with your problem(s)?

Do you have any sensor numbers to show us so we can zero in on the cause?

Jake

 

I had the nearly the SAME THING in my 90 L98 a few months ago. Suddenly it wouldn't idle well, then not at all. This is how I fixed it:

I suspected an IAC problem so I used a washer to hold the throttle slightly open. This caused the car to idle smoothly, albeit at too high an rpm. When I removed the washer after several minutes it idled like new again.

Although I'm making an educated guess as to what happened, I know under certain conditions the computer can forget the settings for the IAC. I think this resets them. I had to swap my ECM ($140), but that was after I got it idling again. Error codes showed my ECM wasn't talking to the CCM. I think whatever zapped the ECM also made it forget the IAC setting. This is in the manual, pages 6E3-C2-5 and 6.

Keep the Faith