backfire only on downshift?

[mkiigtidubber]

i have a 91 8v gti that i bough 3 days ago and everythings good but it pops/backfires when i decelerate in gear. it never does this while accelerating or when clutch is in. is this an electrical problem? its reallllly annoying
thx

[Oddball]

If I remember, popping is caused by fuel burning in the exhaust manifold. This happens on the overrun, or deceleration. I think it happens due to the overrun, the engine turns faster than the fuel is being supplied. Less fuel cools things down. This slows down the burning of the fuel which means the exhaust valves open before the fuel is completely burnt. Thus you get unburnt/still burning fuel in the manifold.

I think running too rich is what creates the circumstances, but I don't think it's a specific problem. Apart from being annoying and scaring old people to death that is.

[mkiigtidubber]

alright, thanks ill take it to vw and see if i can get a quote on how/if they can fix it without a swap. at least i know whats happening now

[RichieL]

Hi, What did they say? I've got a 1991 Mk2 8v PB too and with the same problem.... you're right, it is really annoying! Thanks

[Smutts]

My old 2.0 liter Renault Fuego used to do this all the time. "They all do it Sir!" was for once actually true. All had cracked exhaust manifolds, this draws in air on the overrun, "ponkponkponkponkityponk". etc.
Is there a damper dashpot that stops the throttle shutting too fast, cracked manifold, emmissions control weirdness? Actually no idea.:p