Miss after 5 min, runs great over 3000 RPM

[rollinsperf]

I'm working on Ali's 91 ZR1 that he brought to the shop after it sat for 3 years.
Runs fine for the 1st 5 minutes then runs like spark scatter in a distributor or a rev limiter until it hits 3000 and then it runs fine again to the red line.
Turns out he had FIC injectors put in 3 years ago, car ran fine for 2 weeks then same problem. He got mad at the car and parked it.
Replaced fuel, checked fuel pressure, ohm'd injectors, replaced them anyway with new units from FIC (the guys were were very helpful), replaced the coils with Delco units, replaced plug, ohm'd plug wires. I've had the plenum off so many times, I've got Jerry's Gaskets on speed dial.
I have no codes, and with all the new parts, the car runs fantastic, until it heats up, and then it runs fantastic over 3000 RPM. The problem occurs with the power switch on or off.
The only modification is a Prom for a 92 added after the 1st injector swap.
Typically our shop can fix anything, but this has us stumped.
Any suggestions?

[98indypacer]

Originally Posted by rollinsperf

I'm working on Ali's 91 ZR1 that he brought to the shop after it sat for 3 years.
Runs fine for the 1st 5 minutes then runs like spark scatter in a distributor or a rev limiter until it hits 3000 and then it runs fine again to the red line.
Turns out he had FIC injectors put in 3 years ago, car ran fine for 2 weeks then same problem. He got mad at the car and parked it.
Replaced fuel, checked fuel pressure, ohm'd injectors, replaced them anyway with new units from FIC (the guys were were very helpful), replaced the coils with Delco units, replaced plug, ohm'd plug wires. I've had the plenum off so many times, I've got Jerry's Gaskets on speed dial.
I have no codes, and with all the new parts, the car runs fantastic, until it heats up, and then it runs fantastic over 3000 RPM. The problem occurs with the power switch on or off.
The only modification is a Prom for a 92 added after the 1st injector swap.
Typically our shop can fix anything, but this has us stumped.
Any suggestions?

It might be the spark plug wires need dielectric grease on the inside of the boot. I had the same problem. See this tip from Marc Haibeck.

http://www.zr1specialist.com/HAT%20Web/facts%20and%20failures/spark%20plug%20carbon%20track.htm

[Dynomite]

Originally Posted by 98indypacer

It might be the spark plug wires need dielectric grease on the inside of the boot. I had the same problem. See this tip from Marc Haibeck. http://www.zr1specialist.com/HAT%20W...on%20track.htm

Here is the same link (left clickable).

Marc Haibeck arcing spark Tip

Did you replace plug wires?

If you continue to find a need to pull the plenum often, eliminate TB coolant at the Injector Housing

See Item #4 LT5 Eliminated Systems

[batchman]

You checked FP with a gauge visible while driving? My guess would have been primary fuel pump failing when warm.

HTH,
- Jeff

[A26B]

You may have a weak or bad primary fuel pump. When cold, both primary & secondary fuel pumps will run, until engine reaches 176degF (about 5 min from cold start), then the ECM turns the secondary pump off. After engine temp exceeds 176degF, the secondary pump is not activated until 3000~3200 rpm, when the ECM switches to open loop.

To check the primary fuel pump alone, pull the fuse on the secondary fuel pump. Do the normal key on, engine off fuel pressure check (48~55psi). Then, key off & watch leak-down. Follow up with fuel pressure check while the engine is running (3~10 psi less).

[Funracer]

Originally Posted by rollinsperf

I'm working on Ali's 91 ZR1 that he brought to the shop after it sat for 3 years.
Runs fine for the 1st 5 minutes then runs like spark scatter in a distributor or a rev limiter until it hits 3000 and then it runs fine again to the red line.
Turns out he had FIC injectors put in 3 years ago, car ran fine for 2 weeks then same problem. He got mad at the car and parked it.
Replaced fuel, checked fuel pressure, ohm'd injectors, replaced them anyway with new units from FIC (the guys were were very helpful), replaced the coils with Delco units, replaced plug, ohm'd plug wires. I've had the plenum off so many times, I've got Jerry's Gaskets on speed dial.
I have no codes, and with all the new parts, the car runs fantastic, until it heats up, and then it runs fantastic over 3000 RPM. The problem occurs with the power switch on or off.
The only modification is a Prom for a 92 added after the 1st injector swap.
Typically our shop can fix anything, but this has us stumped.
Any suggestions?

I have a 91 as well. Had these same EXACT symptoms for many years.
Dielectric grease in the plug boots solved the issue.
Costs nearly nothing in parts, labor or time. Give it a try.

It worked for me

[rollinsperf]

I wasn't part of the plug change, or ohming the wires, but I can certainly dielectric grease the boots easily enough. The wires were replaced with the 1st set of injectors 3 years ago but only have 30 miles on them
I didn't know the fuel pumps run in tandem until the engine warms up, I'll take it for a spin with a gauge taped to the windshield.
Thanks for the info, I'll post again after a thrashing.

[Kevin]

so how much are we getting paid to help with diagnosis?

[rollinsperf]

Just as much as I am since I'm doing this in my time off. But if you're ever in Sarasota, I'll buy you a cup of coffee

[Kevin]

Originally Posted by rollinsperf

Just as much as I am since I'm doing this in my time off. But if you're ever in Sarasota, I'll buy you a cup of coffee

I was just kidding around with ya.