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Art's on the money, as always. And I never say always ;)
I usually suspect ECU last, when all the sensors check out but the car throws phantom error codes and runs strangely with problems that come and go. You can check for yourself what the ECU should be seeing from the O2 sensor, which will tell you if the mixture is off, or if the mixture is just fine. If it's just fine, that may eliminate the fuel control components like the temp sensor, FPR, O2 sensor, etc.
Given that it died suddenly and is time dependent, I'd check electrical components. Anything that has a moderate amount of power through it will heat up in a few minutes. The fuel pump relays might be to blame, I think the 240's with LH2.4 have Radio Suppression Relays too, which can fail once warmed up. The power stage amplifier might overheat and have a circuit fault show up after a few minutes of running.
I know the switch to the Crank position sensor reading on the flywheel happened near 89-90, but the FAQ knows for sure. I don't think the AMM can fry your ECU.
Does the car have at least half a tank of gas? Does it restart?
I would suggest that you familiarize yourself with the LH2.4 self-diagnostics. Then go start the car, wait for it to die.
Then, gently jiggle the wiring for the Crank Position sensor. See if it starts. If not, use the OBD1 test mode 3(?) - the self test mode. It will cycle IAC, RSR, fuel injectors, and the cold start injector.
If one of those doesn't cycle, that might find you a faulty component. Each one will click or buzz or "thock".
It's intimidating, but it's not impossible. Be ruthless and methodical and you'll find the problem. “Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth.”
Good Luck! You'll want a multimeter to test the O2 sensor and the ECU- I like analogs better than (inexpensive) digitals because you can watch the needle moving.
--
1990 740 Turbo, on its way to stock specs, maybe beyond
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