When I first start the car in the morning it runs fine, but 15 minutes later, when it is at normal operating temperature the car runs like crap. You can slowly here the car misfiring but as you drive it more it becomes worst. It has a MSD 6Al box I put back the stock ignition and it runs even worst. I changed the coil, distributor, and fuel pump. Check the spark plugs and wires and they are all fine. But the problem still exist. You have to let the engine cool down for a day, then you can drive it with nothing wrong, but as soon the the engine heats up it starts misfiring. Any ideas whats wrong?


1988 Iroc-Z
5.7tpi
turbo 350

 

Have you thought about the spark module? It can heatup, and start to screw up. May be worth a try?

 

no codes?

 

Pull one spark plug wire off at a time when it acts up
to find the offending cylinder. Could be a sticking valve.
Or something.

 

clogged converter ??? not sure but mabye a problem

 

Hmmm, clogged conveter, good point! In the same line, I had problem with the autom. tranny = torque converter (hidraulic clutch) too hard for the poor engine. For discard a hard tranny problem check/compare in P/N vs. D with engine hot running bad. Some difference?.



Denis V.

 

Clogged cat won't cause one (or any particular) cylinder to misfire.

Did you replace the distributor cap with the new distributor? Check for cracks or carbon tracks.

 

Thanks for the help but the car still misfires when it become hot. There are no codes, the converter is drill out and its a knew cap and rotor. Where is the spark module located, if thats the module under the rotor its already changed. I practically change the whole ignition system and it still misses. Any other ideas what could be wrong because i'm all out. And I got races next weekend too. Thanks again.



1988 Iroc-Z
5.7tpi
turbo 350

 

Did you do that yet?

You could also have an injector coil going open on you.

 

I'll go do that when I go home. But what do you exactly mean the coil is going up in one of the injectors. And if one of my injectors is indeed doing this, I assume that it needs to be replaced. I'm pretty sure the stock injector size is 19lb/hr. If I upgrade to 22lb/hr will my car run like crap or do I have to modify the computer buy burning a new chip. Thanks.

 

He means the injector may be going bad, intermittent coil opening when hot, no fuel, when its cold, coil works and fuel is given from injector.

I had the same problem, rough idle when hot, say 200*F. I did find an intermittent injector. I bought a 24 lb Accell set from Summit for $230 USD. You have to remove the fuel rail to replace all of them. Use the high impedance.

The car does run better, the existing chip is fine. 350 CID came with 22 lb/hr stock, 305 CID came with 19 lb/hr.

 

My car is doing the same thing....idles fine until closed loop. I myself have replaced everything minus the IAC and Map sensor....both of which check out to be fine. I did check my injectors this morning....only TWO of the 8 were in spec..lol Is that my problem? I guess the heat from the manifold is heating up the coil in the injectors? Would that cause my rich condition Im having as well?? (BLM stuck at 108 in closed loop....) Sorry, didnt mean to steal the thread....just found somebody with the same problem as myself.....

 

test your fuel pressure. That could be a likely cause. IF there isn't enough gas, then your car will run like a yugo.

 

I have checked the fuel pressure...seems ok. I also replaced the filter last weekend....Im thinking its the injectors heating up and leaking or not spraying like they should....ALSO, other than a crappy idle, the car still seems to have all kinds of power....drives fine on the road, and under WOT.

 

does it "seem ok" Or does deliver a constant 40-45 PSI when it starts to run like crap? Leaky injectors would make your car huff black smoke. Id swap the mass air flow sensor out with a known good one, and test the relay

 

The fuel pressure is a constant 47psi...and doesnt bleed off fast when the car is shut off. The pump itself is only about 2yrs old....Since my car is a 91, I dont have a MAF sensor....my car is a Speed Density setup....using a MAP sensor to plot the fuel tables...Anybody else have any suggestions????