Knock like sounds from rear end..... - 986 Forum - The Community for Porsche Boxster & Cayman Owners

blinkwatt · 04-20-2009, 11:25 AM

I ran home the other day and left the car running in the driveway. My driveway has a steep incline. After running back out to the car to put stuff in the rear trunk I heard a knock like sound. I think it was either coming from the cats or tranny. As soon as I brought the car on a flat surface the sound went away and I had to really try and listen in to even hear a faint trace of it. My car was idling on the incline for about 2 minutes before I even heard it and this was the first time I've heard something like this from my car. I've had the rear end backed up on ramps and had the car running for 10-15 minutes and haven't heard anything unusual before.

Anyone have a clue what it could have been? My car hasn't leaked anything from it and as been running strong and good lately.

Lil bastard · 04-20-2009, 11:36 AM

Not the best idea to idle it on an incline.

All the oil collects at the back of the engine and heads and doesn't flow naturally back to the sump. Also, it's possible that you were starving the oil pickup tube. It doesn't take long for oil starvation to cause some issues with bearings, etc.

blinkwatt · 04-20-2009, 12:38 PM

Alright good to know. Thanks!

ChefCarnivore · 04-20-2009, 02:23 PM

I thought that a dry sump engine by its very nature was not able to starve the engine because the oil is constantly flowing down to the engine from the oil tank.

JAAY · 04-20-2009, 07:33 PM

It maybe time to change your tranny fluid I had the same symptoms

Lil bastard · 04-20-2009, 07:38 PM

Quote:

Originally Posted by ChefCarnivore

I thought that a dry sump engine by its very nature was not able to starve the engine because the oil is constantly flowing down to the engine from the oil tank.

Right you are, but the Boxster is not a dry sump engine.

Porsche very carefully describes the M96 engine as having an "integrated dry sump". Much more a marketing description than anything else.

It lacks an external tank, and scavenge pumps typical of a true dry sump system. It relies on gravity to return the oil to the sump where it is picked up by the pickup tube and fed to the pressure pump to recirculate through the engine. It is in fact much closer to a wet sump system than a dry sump one.

The Boxster has had issues with oil starvation, especially with the early models in high g maneuvering. Updates to the baffling of the sump have lessened these, but not entirely, especially if you were to operate it at idle on an incline where the oil level can flow away from the sump and pickup, exascerbated by the low rpms of the oil pump itself.