Hi everyone!
I don't usually ask for help on forums, but I have this car that has been kicking my butt for a while.
Saab 93 1.8, belt driven.
This is what happens. Upon startup, it will run fine for about 2s, and then the idle will bounce up and down AND, most importantly severly backfire into the intake... This will only last for about 3s and then you have to wait another 8h for it to act up again... so you can see that I have about 3s every day to diagnose and catch anything suspicious.
Generally speaking, I focused on the causes of the backfiring. Obviously no codes present...
I don't have any experience with backfiring, but I checked the usual.
I'll explain what I've noticed when the symptoms occur.
Fuel psi fine, doesn't bounce when backfiring.
Car V stable. Thought it might be a faulty alternator causing problems.
No fuel leaking from injectors. Even took them out and the spray pattern was fine. Left the rail out in the open and no fuel leaked.
No problems with the COP assembly (verified with scope current and secondary patterns). Even changed the whole assembly, changed nothing.
Cam/crank sensors fine when looking at scope.
Timing also fine. I even suspected a stretched timing belt (which doesn't happen as with a chain, but I'm out of ideas here).
Knock sensor was disconnected during startup. Changed nothing. Even disconnected it whilst the backfiring was happening. No change.
So far, every single thing that could relate to a backfire has been checked. Ignition, timing, knock, lean condition (smoke test), leaky fuel into the manifold, everything.
Even the ECU was replaced with a known good (pays off to have a parts changer boss sometimes.
Even the crank sensor was replaced.
So I suspected something mechanical...
The thing is, I can even crank the engine over with the spark/fuel disconnected for a good while, and it will STILL backfire when I put everything together. Thus, eliminating low fuel psi, valves not seating correctly on first startup, belt tensioner stretching over night, etc.
I've noticed that the throttle body changes it's position erratically during the backfiring, but swapped it with a used known good and nothing changed.
This is so frustrating, not in the least because I have 3s to catch anything.
Is there anything else I could check? I've run out of ideas! haha.
Thank you.
