AE86 4age engine not starts crank ok backfires to air intake

[jondee86]

The first explanation that comes close to fitting the situation that you
describe, is that you have one cylinder that is leaking compression. For
example, a bent or burned exhaust valve, damaged piston/ring or leaking
head gasket. This would not make smoke out the exhaust.

If the leak is small it could lower the compression on that cylinder enough
to cause it not to fire at low rpm's. But at higher rpm there is not enough
time for the compression to drop so low, and the cylinder will fire. It will
not be as strong as the other three cylinders, and I would still expect that
there would be some vibration.

The second situation would be if oil or coolant was leaking into the
cylinder. Only this would usually make smoke out the exhaust, and you
would have noticed.

I think a compression test should be your next move. If there is a
compression leak enough to stop one cylinder from firing, it will be obvious
when you test.

Cheers... jondee86