This 1994 Civic EX has me stumped.

CEL light stays on solid, indicating an ECU problem. Car will turn over, but not fire.

If I remove the ECU (unmount, unplug), then replace it, the car fires up and runs fine.

Two hours later the car will still start and run fine. Four hours later the car will start and run fine.

After sitting overnight, ...it won't start! So I repeat the ECU unmount / unplug, and it starts right up.

Unhooking the battery does not work. Unplugging the ECU does not work. Unmounting the ECU does not work. Only unmounting and unplugging works.

Does anyone have any idea is this happening ?! This is driving me insane.

Any help appreciated.

 

Check the contact points in the plugs themselves.

 

Moisture/condensation effects the capacitors on the ECU circuit board where they will work intermittently before they eventually totally fail.

Especially capacitor C14 on the circuit board that effects the fuel pump priming.

I think your removing and reinstalling the ECU creates enough vibration that disturbs the caps enough that they work. Next time just try tapping on the ECU with a wrench to see if the vibrations activate the cap rather than totally removing the ECU.

I have a damaged P28 that I'm going to bring to the electronics repair shop and get C14 and the other caps replaced. I don't know if it's totally cost effective but I'm more curious than anything to figure it out.

Good luck.

 

Swap in known good ECU and then go from there.

 

Check the pins in the connectors too.
Make sure they are all in the same position and not sitting back further in the plug.