My 1.8 fuel injected only pops when deceleration. Any ideas ? Thanks, Jeff

Might be an exhaust leak. I had one at the head, the copper gasket was leaking and it made all sorts of racket at decel.

I think that you might be right. I asked one other Porsche mechanic and he thought this was the most obvious culprit. Thanks, Jeff

My 1.8 does this also. As best I can tell it's benign, my mechanic could never get rid of it.

Paul

There is a decel (deceleration) valve that allows extra air into the motor for emissions and to suppress backfiring on deceleration. Suggest testing this as part of your troubleshooting process.

likely unburnt fuel popping in the muffler.

my 1.8 does it. for a reason. but only when its still cold or coldish, not after its fully warmed up. ........ got rid of the decel valve a long time ago.
the decel valve is the original solution for burning up fuel during deceleration.
brought the engine down to idle in a two step slower and burnt that fuel during slowdown. without decel valve it can tend to dump some fuel.

if its just popping in the muffler its harmless.
ie if its not real loud genuine backfires but just popping.

some here will say the car needs the decel.
i wouldn't argue with them.
but i do know this. the D jets and L jets down here from that era have no decel valves (the VWs and mercs from late 60s and early 70s). the USA cars did have them ------my view primarily for emissions.

Agreed
Most common cause of popping

Anneal the gaskets (glowing red) and let the cool without water before re- installation

(No quenching)