91 Z28, LB9 automatic. starts OK, runs and idles fine until it's warmed up. Then it wants to stall and runs terrible. Been doing this off and on for at least 2 years. It will run fine for a month then it acts up again.

According to the mechanic it had 2 bad cyls. 5 and 6. Compression is fine, but when it's running like crap pulling the plug wires on 5 and 6 makes no difference.
Has spark. He said that indicates vacuum leak and to replace intake gaskets. Already did that last summer. Didn't help. He also said possible EGR.. that was replaced with no changed also.
He's going to check the injectors and also the brake booster.

Any other ideas?

 

when exactly does it stall out, at a light? or coastting? what do you mean by warm? driven for a good half hour in traffic?

 

It's at a light or stopped for any reason you have to mess with the gas to get it to move again. As in one foot on brake and the other giving it gas to keep the RPMs up. It still runs bad when it's moving and is way down on power.

As for warmed up, start the car.. let it sit and idle for 10-15 minutes and it starts doing it. Before then you can get in and drive it and it is perfectly fine.
Coolant temps are normal.

Also tried using a different ( 90-92 LB9 ) PROM, and ECM with no effect. The annoying thing is that it can run and drive fine with no issues for a month or more, then it starts acting up. I reset and adjusted the IAC at one point, this seemed to make the car run great for a about half a day then it was back to the same thing. The CTS was replaced with simmilar results. TPS was adjusted, same deal.

The car is basically all stock, with a Flowmaster cat back. Has the single cat exhaust. Unfortunately if the problem can't be figured out it may be put up for sale since the car can't be enjoyed. It's a convertable and used just for driving on nice days.

 

"Unfortunately if the problem can't be figured out it may be put up for sale since the car can't be enjoyed. It's a convertable and used just for driving on nice days."

Ouch man I feel you pain my dad has a vert iroc as well and had those same problems in the past,

-Have you ever given the car a full tune up? changed the distributor, coil,plugs,plug wires?

-Whats your idel set at?

-Lets say you've been driving the car for a good 30 min and the car is hot, when the car is turned off will it start again? or do you have to wait for it to cool down?

Jay, I understand this is fustrating I've pushed the same car you have for many blocks In till my dad said %uck this put it away and dropped in a 350. If I were you I would not let the car go there absolutly head turners those verts, be pateint

 

It's not my car, it's a friends.
Tune up has been done. Idle is set within factory specs. Turn the car off when it's acting up it'll start back up with a lot of effort and still have the same issues until it's sat for awhile.

That's what I said.. drop in a new 350 with a different wire harness and call it a day

 

Those late vert third gens are hot! Don't let your friend get rid of it. Tell him to be patient. Check the injectors? maybe try a new TPS. fuel filter? There are many things that could be causing it to act a fool.

 

Did you check the coolant temp. sensor because it sounds like it may be bad ,If so it keeps on telling the computer the engine is cold so it will keep on dumping in gas to warm the engine up whether its hot or cold.

 

CTS was changed and made no difference. It reads the correct temps when scanned. TPS was changed. Not sure on the fuel filter. Ther mechanic is looking at the injectors, and going to switch them around and see if the problem changes cylinders.
Unfortunately the car is like 200 miles from me at the moment. If the mechanic doesn't figure it out in the next few days I might end up with it here for me and my brother in law to figure out.

It's a nice car, but it's supposed to just be a nice day summer crusier. She's already got a 69 Camaro project.. really doesn't need another car giving her issues and sucking up $$$$$

 

Hey man I hope for the best for yea, it's a shame these gremlins show up now and then

 

Well her mechanic is out of ideas. He says the injectors are fine. He tested the brake booster for vaccum leak, said it checks out.

Personally I think it's electrical as the car had a small electrical fire and the harness was either repaired or replaced.. I don't know which. It looks very nicely done and you would never know it was repaired. Under the hood is quite clean. Her mechanic says the symptoms indicate it's not electrical.. but I am not convinced. fire = bad

I'm trying to get her to bring it up here and leave it so I can check it out a lot closer.

 

ignition module.. inside the distributor.. when they go bad.. they do it intermittenly.. u'll test it for spark and it will have spark one minute and none the next, then because the module is going bad the timing will shoot all over causing it to run like crap.. i had the same problem.. trust me ignition module.. dont sell her yet.. i didnt figure it out until the damn thing went all the way dead haha