backfire when accelerate!
 

Car is 996tt. If I want to accelerate while the car is cruising at a 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?

Maybe you have injected too much fuel into your engine. Buy a exhaust gas temperature (thermocupule) gauge, This will tell you the air/fuel combination of the engine. The adecuate exhaust temperatures is 1,400 or 14:1 air fuel ratio. A number below that, like 13.1 means that the mixtures is too rich. Temperatures over 1,400 degrees or more than 14.1 means that the mixture is lean. Lean condition elevates exhaust temperatures. Rich condition lower exhaust temperatures.

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

That sounds to air/fuel mixture problems.

Dealer just changed the MAF sensor and a few diverter valves because of a slight vacuum leak.

The mechanic is unable to find the cause of the rear end clunk sound 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?

What about the c.v. joints?

they said they are ok too.

Mechanic said the subframe and sway bar are tight. Also, the engine and transmission mounts are ok.

Mech said to me today that the car has a slight backfire when accelerating from a low speed. He has not yet been able to locate the cause of the backfire. Any views?

Consdier fuel injectors if you have them being clogged, or a clogged fuel filter, same thing happens to me when i ran her down to nothing and some scale came through the lines.