91 starts up and runs fine, then starts missing when it warms up.

[BLACK90ZR1]

My 91 has 12,000 miles on it, and lately it has begun to miss after it warms up. It runs great until it warms up, then it starts running bad. Does anyone have any ideas what might be wrong, or where to start looking for the problem.
Fenwick

Update 1/24/10 : I ran the car today for about 30 miles to get it hot, and got the following resistance readings on the injector coils:

#1 1.7 Ohms
#2 13.8 Ohms
#3 7.0 Ohms
#4 13.5 Ohms
#5 14.0 Ohms
#6 13.6 Ohms
#7 10.7 Ohms
#8 7.7 Ohms
I appears that #1, #3, and #8 are bad. I'm supprised at how well the car ran with them. I'm ordering a new set now.
I did not check the Secondaries, because I'm going to change them anyway.

[tccrab]

Originally Posted by BLACK90ZR1

My 91 has 12,000 miles on it, and lately it has begun to miss after it warms up. It runs great until it warms up, then it starts running bad. Does anyone have any ideas what might be wrong, or where to start looking for the problem.
Fenwick

List of usual suspects:
- fuel injectors
- coil packs
- spark plugs
- spark plug wires
- fuel filter and or fuel pump issue

If injectors are original Multec's it's nearly certain that they're responsible for the misfire.

Throwing any codes?

TomC
'Crabs

[BLACK90ZR1]

No Codes. Would bad injectors cause it to run bad when it is hot, and run good when it is cold? The coils may go bad when they get hot, but I don't know about the injectors. I would like to check everything that I can before I pull the plenum.

[HIZNHRZ]

Originally Posted by BLACK90ZR1

No Codes. Would bad injectors cause it to run bad when it is hot, and run good when it is cold? The coils may go bad when they get hot, but I don't know about the injectors. I would like to check everything that I can before I pull the plenum.

Running fine when cold and then running like crap after it gets hot are the exact symptoms I experienced when my injectors went bad. After replacing the injectors, all was well again.

[ScottZ95ZR1]

Originally Posted by HIZNHRZ

Running fine when cold and then running like crap after it gets hot are the exact symptoms I experienced when my injectors went bad. After replacing the injectors, all was well again.

I also just had the exact same symptoms with my '91.

Started and ran fine for a short period of time but when hot would develop a terrible miss that was accompanied by one heck of a knocking flywheel.

This article by Marc Haibeck explains that the MULTEC injectors solenoid coils are more likely to exhibit shorts when they are hot.

http://www.zr1specialist.com/HAT%20W...in%20Place.pdf

Jon at Fuel Injector Connection offers a set of 16 injectors that look like OEM.

He offers others but I know you've had your car judged so that might be of concern to you. Here's a link to FIC: http://fuelinjectorconnection.com/sh...ewCat&catId=68

I was absolutely amazed at how much better my car ran after the original injectors were replaced.

Best of luck!

[BLACK90ZR1]

Thanks a lot for the responses. I have been suspecting the injectors, but I wanted to make sure. Your responses confirm my suspicions, and I'll get ready to change the injectors out.