I just purchased a 1993 Impreza 5speed, 2wd, EJ18s engine with 200k miles. It passed smog in California on the first try.

 

Compression was about 140psi on one side but only 100psi on the other bank.

I don't know if that's ominous or not.

 

I cleaned and gapped all the spark plugs and checked all the ignition wires.

 

The problem is it has a massive flat spot right off idle after starting the car. I can press the accelerator about 3/4 of the way down and the engine doesn't rev past idle until I actually floor it (then it revs normally). It stumbles for about 3 seconds then drives normally thereafter. This problem doesn't reappear at all afterwards and car drives fine at highway speed. The problem only appears when I turn the ignition off then restart so happens regardless of engine temperature. This is reminiscent of a carb engine with a bad accelerator pump but since I have MPFI I don't see how it can happen. :-\

Never see any warning lights on the dash either.

 

Anybody have a clue, what to do?

the difference in compression may mean the timing has jumped. this may also explain the sluggishness.

 

the t-belt was due at 180k miles.

 

you can pull the cam timing covers to check the timing marks.

 

the engine is non-interference, so no damage done.

Any check engine light? Probably not if it passed emissions (will fail if any current codes are set or if the light doesn't work), but it might be worth it to pull the codes anyway. You can search here and get directions for that. There might be a How to in the USRM section.

 

Compression was about 140psi on one side but only 100psi on the other bank.

Look like cold engine numbers. Retry with a warm engine and see what you get.

Also do wet and dry numbers

Thanks for the suggs.

 

Problem went away by itself after a week.

 

It appears to have been residual moisture somewhere in the engine after I washed it.