Car runs fine when cold, then dies when warm

[Tran Thach]

I am the original owner of an 85 Turbo, CA Emissions, 5-speed, digital. I love this car, but for the last month, it’s been inoperable. The car starts and runs fine when cold, until it warms up, about 10 minutes, then suddenly dies. When I try to start it up again, it kicks over, then either immediately dies again, or sometimes sputters. I can slowly rev it up, but you can hear that something’s wrong. The engine stumbles, backfires, vacuum goes way down, etc. I can keep it going at about 1500 rpms, however, it dies quickly back at idle.

 

I have checked/replaced the following:

 

All vaccum lines. I have all new silicon tubing. No leaks. Perfect seals throughout the intake assembly.

 

ECU Codes are all fine, but even so, I have checked/replaced many components: Crank Angle Sensor, Air Flow Meter, fuel regulator & temp sensor, CHTS, Ignition Coil and power transistor & Throttle Valve switch. I ran the AAC test according to the FSM and either the test is wrong or I’m doing something wrong, since it failed. I checked voltage going to the AAC, and that was fine, and I checked continuity on the valve. I double-checked the continuity from the AAC to the ECU harness. Even though everything checked out, I still replaced the entire AAC assembly, since I know that this is a critical component. No change, so I am confident the AAC is working. When I disconnect the AAC connector when the car is running, it stumbles and dies, so I know it’s working.

 

I double-checked the EFI relay, even leaving it connected to a power source for 15 minutes to see if the relay faltered, but it’s good. I pulled the Air Regulator and it closed like it should when hooked up to 12 volts. I thought I had the problem solved when the Oxygen sensor failed the diagnostic, and I know this is a critical component. I replaced it with a new Bosch (fun getting it off!), test runs fine, but the problem remained the same.

 

I hooked up a fuel pressure gauge and it always has a healthy 30 to 40 psi. Pressure goes up when vacuum hose is disconnected from the FPR, as it should. Also, the timing is spot-on. I have rewired all suspect electrical wires and replaced connectors that have cracked. I know it shouldn’t make a difference, but I even rewired the Fuel Injectors back to the way it was before the recall.

 

So, by now I figured that maybe I have a faulty ECU. Steve Berman (300zxpartsforyou) had some tested ECUs for my car for year. I replaced it and same problem. I looked carefully at a ECU chart that was posted, and found that the stock ECU was listed as an ’84, even though the manufacture date on my car is 1/85. So I asked Steve if he had a working ’84 ECU, and he did. I tried that, and it still did the exact same thing. Very disappointing, but at least I have, what appears to be, 3 working ECUs.

 

I am open to anybody’s suggestions, and will be forever grateful if someone comes up with something that leads to a solution. I figure that since the car runs so well when it’s cold, I don’t need to check for compression, but perhaps when things heat up, something may be expanding. Do you think I should run a compression check anyway? I would have to run the engine until it’s hot, then quickly remove all plugs and test before it cools down.

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Bariatric Surgery

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