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Tapping Sound at Idle


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Hi Guys,

Model: 1999 Carrera 996 "C2" 3.4L. Automatic (Tiptronic).

I have been having a tapping sound from the right rear corner of the car, coming from the engine.

It seems to tap every revolution of the engine. I can try to measure the "beats per minute", if that helps?

I was suspecting the hydraulic lifters for the longest time, but I realised that:

A ) Same single sound, so it would have to be only one sigle lifter (not sure if that is likely).

B ) Only happens at idle, base rpm, goes away during the slightest throttle opening.

C ) Tapping sound regardless of oil pressure and temperature (tested at full cold start and at full engine temp).

D ) Only at NO LOAD condition. Idle rpm in Park and Neutral, there is sound. In Drive or Reverse, with brakes on, at same rpm as idle, no sound.

I think I have ruled out hydraulic lifter problem or worn cam lobe from above observations. (please correct me if I'm wrong at any point).

Could it be chain slap?

Timing Chain tensioner problem?

Broken / worn down chain guide?

Thanks in advance.

Edited by 996noob
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Could be slack in one of the timing chains. The tensioners are hydraulic, so at idle when the engine has the least oil pressure, it's possible the chain isn't tight enough, and is "slapping" a bit. However, whenever I've had the tensioners out of these things, even static, they have a pretty significant amount of pressure, so perhaps if one of the tensioners went bad that could cause this. Might consider wear of the ramps themselves as well, though I've town down M96 engines with close to 100k on them and the ramps looked almost new.

Unfortunately, the tensioner at the right rear is for the 4-6 bank, and that one is the least fun to get to, as its buried under the A/C compressor. If you want to entertain pulling a tensioner, it is IMPERATIVE that you put the engine at TDC, and since your motor is an early VarioCam motor, you don't HAVE to lock the cams with the timing tools (though it's still a good idea).

It IS possible to have one bad lifter, and these people have had to put lifters into these motors before. Obviously, if one comes out, it'd be wise to replace all, at least on the bank.

Have you looked at the oil? Might not be a bad idea to drain the oil and cut open the filter and just make sure it's not making metal.....

Edited by Cloudsurfer
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The engine is at 100k KM, or around 60+k Miles.

That's a good idea to check the oil for metallic tell tale signs during the next oil change, I'll remember to do that.

I will be changing my AOS soon, so most probably will have the engine out, which will make it easier to do all the checks you suggested.

Since the engine will be out, what else should I get done other than change Lifters, check (or change?) Tensioners, and check Ramps?

Could be slack in one of the timing chains. The tensioners are hydraulic, so at idle when the engine has the least oil pressure, it's possible the chain isn't tight enough, and is "slapping" a bit. However, whenever I've had the tensioners out of these things, even static, they have a pretty significant amount of pressure, so perhaps if one of the tensioners went bad that could cause this. Might consider wear of the ramps themselves as well, though I've town down M96 engines with close to 100k on them and the ramps looked almost new.

Unfortunately, the tensioner at the right rear is for the 4-6 bank, and that one is the least fun to get to, as its buried under the A/C compressor. If you want to entertain pulling a tensioner, it is IMPERATIVE that you put the engine at TDC, and since your motor is an early VarioCam motor, you don't HAVE to lock the cams with the timing tools (though it's still a good idea).

It IS possible to have one bad lifter, and these people have had to put lifters into these motors before. Obviously, if one comes out, it'd be wise to replace all, at least on the bank.

Have you looked at the oil? Might not be a bad idea to drain the oil and cut open the filter and just make sure it's not making metal.....

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Check your spark plugs first. They often will work they way out.

I had same noise you are talking about -exactly.

I had one plug that I removed with my fingers, it was already coming out. 2 others were barely finger tight.

Replaced them all and now no noise.

Edited by 15psi
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