Tapping causing insanity!

[Trevace]

Hi,

1st Thread, and sorry, its another tapping question.

I know there are 1 million threads on this topic (because I've read them all, some more than twice!). I just can't find one which describes the exactly the tap I have! And i know ultimately, I'm probably not going to get a definitive answer here, its going to need a professional and probably a boroscope to diagnose.

The car is a 1998/99 996 3.4 C4. Just over 100K miles. All standard except exhaust. I did find in the service history that its running an aftermarket ECU map. 75K of main dealer service history with the remaining in the main at specialists. Runs great, no drop in performance I can feel. I bought it last summer and have never topped up the oil which shows full on the in car electronic gauge and mid way between upper and lower limit on the dipstick. Oil pressure looks all good and does what you'd expect with engine temp/ revs. But its tapping!

I don't drive it often at all, its garaged and I take it out for a (usually spirited) drive when I can. I didn't notice the noise when I bought the car, or for many drives. But then I noticed it and had the usual new 996 owner panic. There is a reason i hadn't heard the noise and that is that it only does it under specific circumstances.

• Only when the engine is hot is the noise constant. Although, sometimes sounds like a standard sticky tappet on start up which fades/ stops in seconds.
• Only at idle with AC off
• Disappears if the AC is on (which might be why I didn't hear it to start with, or it wasn't there). Turning the AC on increases the revs slightly, enough that one you've pressed the throttle once the sound goes.

I'm a petrol head and although I wouldn't ever do any mechanical work on this car, I've worked on plenty of cars in the past. I'd describe myself as fairly mechanically competent. If it wasn't for everything I've read on forums I wouldn't loose any sleep over this and would assume it was a tappet. But because of what I've read, I'm fairly sure its piston slap and I'm going to need an engine rebuild.

Before I bought the car, my research made me think that the 3.4 versions were less prone to bore scoring? But I'm keen to hear from anyone who has had similar (particularly if its a good news story).

Many thanks for taking the time to read this. Hopefully you can put my mind at ease until the car gets serviced and an expert opinion in spring.

Trev

 

[C2dweller]

We look after a few cars which have this same complaint. It isn’t borescoring, which is everyone’s worst fear - we checked this with our camera.
We haven’t got to the bottom of it I’m afraid.

My suspicion is that it’s probably little end bearing play (top of Conrod). The cars that have this don’t seem to get worse quickly either.

I also don’t think it’s tappet noise - the frequency is wrong.

It’s also something that I have seen on low mileage and high mileage cars.

Kind Regards Lee Jones

 

[75pno]

I have this tap tapping , my mechanic is pretty sure its very slight wear on cam bearing.
All runs fine , I’m not loosing any sleep !
99 C4 3.4 78k miles

75PNO

 

[SteveAndrews]

I have a 1998 996 C2 tiptronic on about 124k miles and have what sounds like the same tapping noise at idle. Only when hot and it disappears as soon as a gear is engaged.
Like you I have researched the matter and the most likely culprit appears to me to be a bit of "snatch" in the mechanism somewhere. Hartec have written quite a bit about this (couldn't immediately find it to quote) and think that it is something that should just be lived with rather than spending a lot of money trying to get things rebuilt which may not even cure it.
I was at a 996 technical seminar at club HQ a couple of years ago and raised this with the experts there and was told that Porsche had told dealers not to try and fix it, as it didn't cause any problems.
Even if your car is not a tiptronic what you are experiencing sounds very similar - hot car - at idle only and noise disappears as soon as any load is put on it.
I have personally driven mine about 24k miles since noticing the noise and, touch wood, no problems relating to it. Bit of smoke sometimes but only on start up and no oil consumption and no blackening of the exhaust either side.
Obviously your car's problem may be something more serious but I would be very reluctant to start spending a lot of money which ultimately may not fix it.