Hi all, new to the 996 forum but not to Porsche. I'm experienced with air cooled 911's and own a Mezger 996TT but just now learning of the M96 maladies first hand. I recently bought a 2002 996 Carrera Tiptronic Coupe for use as a daily driver. The car had 111k miles, a west coast car with good service records and no signs of a hard life. I've put two thousand miles on it over the last three months of ownership and my M96 baptism by fire has already begun.

Recently the car has been making a moderately loud rattle at idle when at operating temperature. The sound seems to be loudest at the right bank timing case area. It does not make this noise on start up or when cold. When the idle rpm's are raised even slightly to 1k or so the noise immediately disappears. The car runs fine and sounds normal while driving.

I've read the many posts on this forum and watched the Jake Raby YouTube videos on bore scoring, etc. Being new to the car I wanted to take a close look at the oil so I dropped the sump and cut open the oil filter. The inside of the engine looks surprisingly clean and there are no signs of metal or any other debris in the filter. I also took this opportunity to scope the cylinders and found a hint of early onset bore scoring but nothing I would think would cause noticeable piston slap. The reason I don't suspect bore scoring as the cause of the rattle is that I've only driven the car 2k miles and it did not make the noise when I bought it. It's not logical to think the very light bore scoring progressed to piston slap in such a short time. The car has also used zero oil in 2k miles and both tailpipes are very clean. It does not smoke at startup or at any time.

The frequency of the rattle seems inconsistent with piston slap. Because of the location I'm leaning toward the timing chain or valve train. If this is the case, what's the path forward? Tensioner? Chain guide? Either way it seems like I'm headed for a motor drop to pull it apart for inspection... I have also become aware of the low idle oil pressure issue with these engines and I'm wondering if the oil thins enough when hot to maybe cause a slack timing chain issue at idle? When rpm goes up, noise stops. This could either be from oil pressure increasing with rpm or from loads changing on the components. I'm also thinking of trying the updated oil pressure relief plunger and spring to see if that helps with idle oil pressure.

I really like the driving dynamics of the 996 but my entry into what I believed to be over-hyped M96 failure hell has been swift! I'm reluctant to spend the time and money required to correctly rebuild the M96 as it will easily exceed the purchase price and value of the car. Unfortunately the more I learn about this series the more I regret buying one. So fellow Rennlisters please take a moment to listen to the attached sound clip and let me know what you think.

 

Sounds like something loose external to the engine. Try removing the serpentine belt and running the engine briefly to see if the sound disappears and check all your pulleys for looseness or crunchy bearings. Also, check your exhaust system thoroughly, possibly a loose element banging around inside a catalytic converter, or a muffler mount come loose from the side of the engine or at the rear where they cross over. Best of luck.

 

I agree with the above and add possibly broken-off header bolts causing blow-by of a single cilinder. Although I admit the rythm/frequency of the rattle seems too high given it's idling.

 

If that’s internal, it’s bad… I hope it’s one of the other things mentioned above for your/the engine’s sake! Good luck figuring it out sooner rather than later 👍

 

May want to check service records and look for water pump being replaced and when.

Sounds like pump impeller hitting block.

 

All of the above are good suggestions. I would add consider replacing the two easy to access tensioners.

 

All good suggestions, I will pull the serpentine belt and check. The water pump has been replaced at some point. Will report back….

 

Weazer called it. Loose element inside cat appears to be the cause of my death rattle. After warming up the car I put the *** end up in the air and got under there with a stethoscope. On first inspection the sound appeared to be coming from the right timing case area which happens to be right in front of the right cat. With stethoscope it’s clearly loudest on the cat itself. With the car idling I tapped different areas of the exhaust and noted changes in the sound. After a few not so gentile blows to the cat the sound went away completely.

I didn’t mention in my initial post that a check engine light came on about the time the rattle started. My initial overreaction was “misfire due to bore scoring”. The Code turned out to be P0430: Catalyst System Efficiency Below Threshold Bank 2. So there’s some corroborating evidence to support the cat theory.

So short term I’m debating replacing the right cat with a used one or possibly an aftermarket exhaust system. This car is a daily driver so I’m not interested in a loud or droning exhaust. Does anyone here have a stock exhaust cluttering up the shop they want to get rid of?

Long term I’m going to do some soul searching about the risks of long term M96 ownership. I’ve learned a lot recently about the questionable durability of these engines particularly as they age. I may decide to punch out and quit while I’m ahead on this car. Kind of disappointing because I really like the car.

 

If you go aftermarket, make sure they are CARB approved, I believe there is a set of Cats in Sacto Craigslist for the 996.

Nope, I checked and no ad currently, sometime they repost.

Magnaflows has CARB approved replacements right side 444067 left 444066