Rough Idle - Shaking Passengers Seat

Hello Fellow Rennlister's, I have a 2000 996 with 66K miles, which I bought a year ago. The engine runs strong, does not smoke, nor hesitates. But, it does have a rather rough idle; not a bouncing idle (it always finds idle. warm ~700rpm), just a high frequency vibration which one can feel and also see, as it shakes the passenger seat. About a month ago, after a track day, the CEL came on (not flashing). Took the car to three mechanics (two local Indy and to the Porsche dealer). They all agreed that the issue was valve chain tensioner for bank#1. The angle values for the camshafts pointed to this. The engine was opened and the wear parts of the camshaft chain together with the chain tensioner assembly were replaced for bank#1. While doing the repair, they found the cam timing on bank#1 to be out of adjustment, so they re-timed cams on bank#1. Then, they also found the cam timing on bank#2 to be out of adjustment, so they also re-timed cams on bank#2. I drove the car for a week and noticed that the rough idle did not go away. Took the car for a re-check, but found no errors on the log and the angle values for the camshafts look fine. Everything looks in order, until you see the shaking seat� Anybody has experience with this issue? The car is fine, it is just annoying to see and feel the rattle while sitting in traffic. Also, I am hesitant at tracking it again, and then have to pay +12hrs of labor for another camshaft issue. Any ideas?

Is one of your plugs or one coil, playing up? It doesn't seem so as you say that it pulls great and whatever. But maybe the other 5 are helping it at higher revs. My tick over on cold start is 900rpms not 700rpm. A spark is a spark is a spark, but could one weak spark cause a rough idle? I don't know. Just a thought. When did you change plugs lately? I found an amazing difference and a chain slap that I always heard on start up has completely gone since I changed the plugs.

Some have commented on the shaking passenger seat was related to bad engine mounts.

Thanks for your comments. I have been researching for this type of issue, and the mounts do come frequently as a cause, but mine have been checked and seem fine... As for the sparkplugs, I do not know when were they last changed, so I am seeing a weekend project for me in the very near future. Doesn't hurt to try.

It's almost definitely the motor mounts. I had a similar issue on my '99. The important parts of them are made of rubber and hydraulic fluid, neither of which does great after many years, especially considering what they are subjected to. They are a very easy DIY and I don't recall the price, but I'm thinking it was a few hundred bucks to order a replacement pair off of Pelican. The only other thought I've read on rough idle that isn't the common things you may think is that the throttle body may need cleaning (also an easy DIY). Good luck...but if I were a betting man (and I am) I'd order new mounts and replace them. You only need a floor jack, and a few hand tools, none of which are unique or specific. Search for write ups here or on Renntech and you will get what you need from an info standpoint. DS

Meant to add, you referenced they were checked, how did they "check" them? Unless they jacked up the engine and removed them, and then were somehow able to put them on a bench and stress test them, I doubt they were thoroughly tested. DS

I'd bet on engine mounts as well. DS is right, its doubtful that the mounts were properly tested. They also seem to go bad right around the mileage of your car.

I just did my engine mounts after I did the plugs and sleeves. (For no particular reason only PM). https://rennlist.com/forums/996-foru...ounts-diy.html That's a good link with good photos before and after remarks.

First time poster, long time car guy . I have the exact same issue, just changed out all the plugs and coil-packs. When I was under the engine I noticed there was a ton of sweating around the AOS return line. After that I read up on it a ton, looks to me like I might need to do an AOS fix. When running if I take off the oil cap, the engine stutters (lots of suction). Also have a little of oil burn on startup which is what clued me onto this. When these things go it causes a vacuum leak condition which would explain the crappy idle Take a look and maybe you might be suffering from the same problem.

where is the actual OAS . Does anyone have a photo of where it is situated in the engine compartment? PLus ...what are the tell tale signs that it needs replacing?

I had passenger seat shake that didn't completely go away with engine mount change. It still vibrates a little bit but the car is comfy at idle.

Quotes source post: 10853822

Ya don't put too much fate into engine mounts, i replaced mine, and the vibrations in the seat are still there. My car didn't shift any better either as most report after changing them. Did fix the sagging tail pipes though.

Quotes source post: 10853759

Its at the back of the engine and nearly impossible to get at: Signs of it going are: 1. Rough idle (warm or cold) 2. Smoke on startup/smell of burning oil 3. Slight Hesitation 4. More then normal vacuum in crank-case (i.e. difficult to open oil filler cap due to it being sucked down, and at idle there should be no vacuum in crank-case) 5. With oil filler removed sound of vacuum and an even worse idle I'm taking my car into the local Porsche dealer to confirm the issue before I do the work, I'll let you guys know what they say.

HA! and so the plot thickens... Crowd opinion points to engine mounts, which makes a lot of sense. I agree with Invius, they were "checked", but not really tested... Ignition with spark plugs and coils, could be easily checked by removing and visually inspecting the plugs. I will do this regardless since I do not know when these were changed for the last time. Now for the AOS. I have removed the oil cap, and do hear the idle go rougher and the slight hissing (some vacuum there). Question: should there be NO vacuum at all on the case, or is a small amount normal? I was able to remove the cap with the engine on, so it is not a lot of vacuum, but there is some. Thanks again for the comments and suggestions. I will keep posting progress as I go.

On every car I've ever owned there is no vacuum at idle (slight amount of back pressure) coming from the oil filler cap, when you rev the engine (1000rpm+) you start to get vacuum, and if you have pressure at that rpm and higher you have a bigger piston ring issue.

Engine mounts (Wevo's) cured the passenger seat vibration on mine.

Just started the car cold and it ticked over at 900rpm. I removed the oil cap with ease and it dropped to 700rpm but then it was ticking rather chunkey. When the engine was over 180F degress, it ticks over very smooth at a smidgers over 600rpm. I had just put a new air filter in and removed the serpentine belt and all pulley wheels were smooth to turn with no play what so ever in any.

So got word back from Porsche, I love being right, AOS is going and they said: "For now its okay, however it will get worse and it will need to be replaced. Start planning for this as it is not a small repair" I'm going to drop the engine and fix this along with few other maintenance pieces (i.e. valve cover gasket leak and Inspect IMS bearing, clutch, RMS), the previous owner said he had replaced them and had receipts to prove it, but I like piece of mind

After all your very comments, I took on trying to narrow down my options by more closely looking at my symptoms. My throttle body was clean as a whistle, I did not had have even a trace of smoke coming out of my muffler, and even though I do have some vacuum at idle on my oil cap, it is minimal (no problem at all removing the cap). So, AOS diagnose relegated to not very probable. I went over the service documentation and found that the sparkplugs had been recently changed. Also, the car has a strong and even pull throughout the RPM range. So, ignition and fuel delivery was also relegated. Nevertheless, sparkplug change is next in line for my DIY. This left the engine mounts. And after looking at the issue more closely, I also noticed a rough gear change between 1st and 2nd when under torque. So, bought the replacements at Pelican and installed the yesterday: WHAT A CHANGE! The shaking seat is gone, the car feels much smoother, and the gear changes have also improved. Easy and straightforward DIY project. Highly recommend it to any 996 with 65K plus in the odo.

WP_20131123_006.jpg picture of busted mount.