iim1awh

So I am battling a couple of issues with my 99 996 C2.

For the record I am a Porsche newb and have only owned this 996 for a few months now. Car has 65K on it. The only change I have made to the car since I purchased it was installing a set of Fister 3.6 modded cans. It would seem unlikely that the new cans are causing the vibration issue.

1) I am starting to notice a engine vibration at around 1000 and then again at about 3400 RPM. It is most noticeable when the car is up to temp vrs when its cold. It was subtle at first but seems to be getting more noticeable.

2) There is a rattle sound coming from what i think is the front of the 6 spd gear box at idle it is not constant but comes and goes every 20 to 30 seconds.
I cannot hear it when I rev up the engine as everything seems to get washed out by the fisters.

I tried searching the forum but did not come up with anything so I figured I would ask the experts.

Thanks,

Andy

fpb111

Does the car have a light weight flywheel? Does the rattle change when the clutch is pushed in? Also check the engine mounts and the trans mount. New engine mounts could make a big difference if the current ones have leaked/gone "flat"

iim1awh

I am not sure about the LW flywheel is there an easy way to tell? The rattle is not altered by pushing in the clutch. I climbed under the can and it sounds like it is coming from the very front of the tranny vrs the rear.

I will search the forum for additional info on the mounts and see what i can do to inspect them. First thing that came to mind was IMS failure but from everything I have read if these go there is little to no warning. The tranny noise has me most concerned.

Andy

iim1awh

Finally droug the wife out to the garage at 10:30 tonight to help me try and figure this out.
My original conclusion was somewhat flawed in relation the the gearbox rattle I am hearing.

The following observation was made after the car had had a chance to completly cool down.
1. The gearbox rattle is much quiter when cool vrs hot
2. Derprssing the clutch does does significantly reduce but not completly get rid af the rattle.

I have read several posts that a LW flywheel will exibit more gear rattle than the dual mass version.
I guess my next question is

Are LW flywheels factory ot aftermarket? I have the receipt for when the clutch replacemnt was done. the only mention of anything flywheel realted is "Machine Flywheel $90"

How common is gear rattle and what is considered normal vrs excessive.


thanks,

Andy

thepenguin99

From what I have read the flywheel on these cars is either in good shape when inspected and reused or replaced. I would be suspect of it being machined (resurfaced). That said, check the mounts first.

KrazyK

I think this may be your clue. Any Porsche dealer knows better. Some Indy's may not. It is not recommended that the flywheel be resurfaced. There is a simple pass/fail test for the DM function and surface check. Regardless of mileage, the flywheel is 14 years old and should have been replaced (@$450) not "machined" when the clutch job was done.

I would try and find out a little more about the shop and exactly what work was done. I suppose there are many other possibilities as to the cause but the clutch job and flywheel work is a "red flag" to me.

fpb111

LWF are not stock on these cars.
If you still hear it with the clutch depressed it is not from a LWF. That rattle is from gear clatter caused by not having the vibration damping effect of the Dual Mass Flywheel (DMF).

If the rattle is present when the clutch is depressed with the trans in gear & the car stationary it is most likely not something in the trans itself. Because in that situation nothing in the trans is turning.
I would suspect the DMF in that case, also possibly the throw out bearing.

TSB 911 (996) 8/02 1360 Dual Mass Flywheel Check procedure

http://www.renntech.org/forums/index...-procedurepdf/

Can the dual Mass Flywheel be twisted approx. 15mm in each direction (0.590”), left/right, and does it return to approximately its initial position? If so it is good.

Can the DMF be twisted beyond dimension stated above without a noticeable increase in the spring force? Or is it fixed (no rotation?) if so bad

fpb111

Some animation of one type of DMF and U-tube of good vs. bad DMF

iim1awh

Is gear clatter typically constant? The nosie I have hearing seems to fade in and out every 10-20 seconds.

I am planning on putting the car up on jack stands tonight and redoing the following check
If the rattle is present when the clutch is depressed with the trans in gear & the car stationary it is most likely not something in the trans itself. Because in that situation nothing in the trans is turning.
I would suspect the DMF in that case, also possibly the throw out bearing.

I am also planning on draining the gear box and inspect/replace the oil.

I will report back on my findings. Should I assume that the car is non drivable at this point?

Thanks,

Andy

fpb111

Non drivable? That is a tough call.
You don't say if you hear it when the car is in gear underway. I would fix it sooner rather then drive it if that is the case.
If it is a "light" rattle only when the car is stationary at idle it I would probably drive it while making plans to fix it.

I would recommend using the Porsche trans oil. As with any lube there are many opinions on this. I have used non stock gear oil and found that the two I used caused very stiff shifting when the trans was cold. Mobil Delvac synthetic 75w90 and Ultron 75-90 YMMV

The most common gear clatter in a 911 is caused by variations in engine power pulses when the trans is not under load at idle.
In the past Porsche used rubber centered clutch disks. The rubber would start to break down and cause problems. The trick fix was to use a spring centered disk as a replacement. This caused noticeable harmless, but to some irritating, gear clatter at idle.

It would fade and get louder as the speed of the internal bits of the trans change in relation to the input shaft (engine) speed.

iim1awh

fpb111

First I what to say thanks for your responses. I am feeling a little better about things, Still need to pull the tranny i think and take a look at the flywheel at a minimum. once the car is up in the air and I can get under it I will have a better chance hearing any gear rattle above Idle. Currently I can only hear it at idle. I will attempt to catch a audio clip of it with my phone if possible.

As for locating approved gear oil that appers to be more or a problem then originally thought.
Can not get it locally so I am going to have to order some in. Other than OEM porsche oil are there other porsche approved brands?


Andy

trapperdog

My car had a rattling sound coming from the tunnel area behind the shifter. I took out center console and noted the shift cables would vibrate a little against the metal chassis of the car at certain rpm. I stuffed some foam rubber between the cables to isolate the vibration from the car and the noise went away.

KrazyK

Glad you have decided to check the flywheel as suggested. As for trans oil, nothing beats the Porsche oil. Best price (they ship fast) is probably here at this well known Porsche dealer:

http://www.suncoastparts.com/product..._Code=996Trans

Please keep us posted.

iim1awh

Put the car up in the air last night so I could take a closer look at things.
I have verified the following.
1. The gear rattle is only present at idle with the clutch disengaged, engage the clutch and there is no more rattle.
2. There is no sign of the rattle at RPM's higher than 1K.
3. While it was up there I drained the gearbox and found no signs of any foreign matter in the oil, it was a greyish black in color and the typical gearbox oil type order ot it.

I refilled the gear box with Redline 75W90 NS GL-5 just to get me buy until I drop the tranny in the next few weeks. I will order the good Porsche stuff for the re-install.

Car still drives and ****s perfectly; the rattle is still there at idle. The vibration seems to come and go. It presented itself during last night’s test drive but not during this morning’s drive to work. My plan is to still drop the tranny and take a look at the DMF to see that’s going on there as soon as I can carve out the time and make some arrangements to call in some favors.

Andy

Macster

To state the obvious then the noise is the normal tranny rattle these cars suffer from. Some more than others. My 02 Boxster with over 268K miles hardly makes any noise at all and the 03 Turbo even before it obtained its current 109K miles would once in a while have the tranny rattling. But a guick dip of the clutch pedal quiets the rattle.

The modification you made to the exhaust could account for some increase in the rattle as the engine now idles less smooth than it did before. About the only way I know of disproving this theory would be to fit the stock exhaust, but I could certainly understand if you elected to not do this.