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Plymouth Colt - Trouble Starting Engine When Cold

My Car: 1989 Plymouth Colt Hatchback (similar to Mitsubushi Mirage); 4 cyl. 1.5 liter fuel injection; 93,000 miles.

Problem: Check engine light comes on & main relay clicks on & off when starting the engine cold. Holding down the gas & keeping the engine RPM’s up keeps it from stalling out until the engine warms up. After that, everything is fine.

The problem isn’t in the main relay, because we’ve tried replacing that, and it’s okay. Either the engine control computer or something else is sending the main relay a signal to shut down. We know that the engine control computer has gotten wet, and there has been some corrosion there. That’s clearly not good, and we’re now protecting the computer from moisture, but is that really the problem?

My mechanic thinks that the processor in the computer that controls only the cold starting sequence is no good, and that the only thing to do is get a new computer (if that’s even possible). Is it possible that an engine sensor of some kind is not working properly when it’s cold, and is sending either the main relay or the computer a signal to shut down the engine?

Has he checked the coolant temp sensor?