JustinL

I'm running out of ideas to track down my problem with a hot idle. The engine is a 968 race engine. I rebuilt the engine recently and the problem existed before and after the rebuild. I'm lucky to have a trove of spares I can use for testing.

Symptoms Car starts and idles beautifully until it starts to warm up. Once it's warm it will stumble periodically and eventually stumble so bad it will stall. When it's fully warm and acting it's worst it will need throttle to stay running. It seems to drive pretty well and engine temperatures are a constant 88°C. AFR ratios are good until after/during stumbles.

What I've tried

I swapped MAFs, that didn't fix it.

I swapped in the megasquirt and ran the fueling with megasquirt and it still has the same symptoms. This eliminates fueling as an issue.

I swapped DMEs and the problem persists.

I swapped DME relays and the problem persists.

I put in a new coil, no change.

I cleaned the MAF connector and the TPS connector, no change.

I unplugged the TPS, no change.

The next suspects

I'm going to check the coolant temp sensor at the DME for proper resistance.

Is there any wiring at the alternator or starter that could cause this kind of problem? My starter wiring is a bit questionable, but the starter works very consistently.

It seems to be something ignition related or power related, so if anyone has any insight, that would be very helpful.

mytrplseven

I've noticed higher than normal idle speed (1200) after installing a new TPS and Rogue_Ant suggested it was the idle circuitry between the TPS and the DME. I plan to check the DME connector for the proper readings to verify this as a possiblity. I've had stumbling acceleration issues that I've been chasing for a long time and when the new TPS was installed, I could go to WOT and it really hauls. But with partial throttle I still have an issue and now this. I'm also going to go back and re-check all the vacuum fittings for leaks. Good luck to both of us.

JustinL

Tonight I checked the temp sensor at the DME harness. It seems fine. I tried running the car with it plugged in and unplugged and it still died. I put on a new distributor cap that I had as a spare, that didn't help. I ran the car with the ISV unplugged and my foot on the throttle trying to keep it at 1000RPM and same thing happens. It idles for a while and then just stumbles and dies.

What else could briefly kill ignition as it warms up? Do those ignition relays ever go bad?