backfiring on acceleration!

[superfans]

Car is 996tt. Sometimes if I want to accelerate while the car is cruising at low speed (around 20 to 30 mph), I'll hear a knocking sound at the back of the car with a little hesitation. My mechanic said it may be a backfire. What do you think? Have you guys experienced this before and if so how to fix it?

[GT3 Rob]

might be caused by same problem as this..

www.pistonheads.co.uk/gassing/topic.asp?f=48&h=0&t=188214

[jasonc]

You are in the right gear aren't you when you accelerate? At what RPM does this happen?

[superfans]

Trans is tip. Gear is 3rd and sometimes 4th. Rpm would be around 1800 to 2000.

[superfans]

Actually, it is more like a "thump" at the back (with a slight hesitation) when accelerate from cruising speed?

[jasonc]

I think you just need to chage down if you want to accelerage hard from that speed. If you are in second does it happen?

[superfans]

sometimes it does happen even in 2nd gear.

[superfans]

Mechanic is unable to find the cause of the rear end clunk sound which happens when accelerate quickly during low speed cruising (it doesn't clunk all the time when accelerate). Mechanic said everything (mounts, suspension and sway bar) is tight and he is still checking to see if there is anything else which could cause the clunk.

I've seen some web saying that the noise could be from a faulty transfer box or torque converter, any views?

[turbomania]

I heard of this probelm before from bob the technitian at RGA - he just fixed a 996tt with a similar problem. he may be able to help point you in the right direction. his number is 0207 793 1447..say hi to him from alex

>> Edited by turbomania on Tuesday 5th July 14:02

[superfans]

Alex, have you heard anything from Bob before as to what could be the cause of this problem? It would be great if you could give me some hints.

[superfans]

To give a better description of the problem. The sound is more like a metallic "thunk" from the rear around the engine area (it seems to come from underneath the engine, maybe from the exhaust). It doesn't happen all the time. Sometimes it happens when the car accelerate (not WOT) at a cruising speed of around 30 to 40 mph (at 3rd gear and sometimes 4th gear, probably around 2000 rpm). The thunk sound does not seem to be from the transmission coz it is from the rear end of the car. Mech thinks that it could be a backfire but unable to locate the problem. I am not sure if this "thunk" sound is actually describe by other people as "ping" in the engine (spark shock). Since I have been using high level octane gas so the sound shouldn't be the sound of a detonation (plus it doesn't happen all the time). Any comments or views?

[superfans]

Please! Anyone? Mech said to me today that it is a backfire (when the car accelerate from a low speed). He has not yet been able to locate the cause of the backfire. Any views?