Wondering if anyone has any experience or insight they can share on this issue.

2004 C4S 47k on the chassis and 10k on Porsche reman engine.

I had the car to the dealer today and spent some time using search here in an attempt to understand what might be the cause of a fairly consistent vibration experienced on deceleration.

This happens only when the car exceeds 5200 rpm and I completely let off the throttle and the car is under deceleration with compression braking only. As the rpm's come down to about 4800 rpm there is a vibration that can be felt and heard until the rpms drop below 4500 rpm and then it is completely gone. This does not happen upon acceleration.

I can replicate this in 2nd, 3rd, and 4th gear and it is rpm dependent and not speed dependent. Haven't tried 5th or 6th because, well, doing this in 4th already puts me in the triple digit speed range.

The tech took it on a road test with me and confirmed the symptom but has zero clue as to what it could be. He did put it on the lift and removed the belly pan and the only thing we discovered was a slightly cracked drive shaft flex disc but the tech is unsure as to if this has anything to do with the vibration.

He also ran full diagnostics and everything came up clean. I wanted it checked out now as the engine still has a few months warranty but the tech can't say whether or not something in the engine would be the cause. His best guess was "something drive train related".

Condition is the same with either my summer tire and wheel package or my winter tire and wheel package.

If anyone has any experience with this or suggestions as to how to further diagnose I would greatly appreciate it.

Thanks
John

 

Before my front differential failed in my C4S, the symptoms of failure were felt and heard on deceleration. That being said, it happened at much lower RPMs than you mentioned. I'm not sure if your issue has anything to do with the front diff, but it's worth at least checking it out.

 

How's the condition of the engine mounts and transmission mounts? Just a WAG.
Rev dependent seems to point to engine and mounts, as opposed to the drive train.

I had a similar experience on my C2 except it's a loud rattle, as opposed to vibration that can be felt. It turned out to be one of the muffler heat shields touching making rattling noise only at around 5k (resonance) but it occurred both directions when the rev goes up or down trough that rpm though.

 

If your sure about this, you can rule out anything from the back of the trans to the front of the car, including the guibo disc to the front differential.

Seems it could be anything from near clutch area, trans and engine. If it never happened before the reman engine was installed, may be related to the work they did? Dealers/shops have been known to not tighten/torque bolts properly. Maybe took 10,000 miles to work loose? Hopefully its not a problem with the new engine.

While its still within warranty period, I would make them deal with it. Maybe start by checking all visible fasteners? Does it vibrate while stopped, neutral, clutch in, clutch out?

 

No vibration at idle or upon acceleration throughout the entire rev range. Can replicate it by paying attention to the rpm range 99% of time. Did now notice that the more aggressive the acceleration the more aggressive the vibration on deceleration. Taking it gradually to 5200+ rpms results in a barely, if at all, noticeable vibration. I will try a coast down in neutral or with the clutch in as well.

And my seat of the pants senses that it is from the right rear of the car. And never did it with the old engine.

I'm going to go back to the dealer and make sure they document a repair order just in case it is something in the engine. At this point I agreed to just pay attention to the car and see if it appears to get worse.

Thanks

 

Im sorry. To be clear, I meant you for sure cant feel/hear anything while stopped and revving.

 

Correct. Although I am not a fan of revving an engine in neutral, I know kids think it sounds cool, at the shop we did hold it steady at 4k, 5k, and 6k rpms and no vibration or unusual sounds. Even tried to hold the revs steady in the range of vibration on decal but the vibration didn't persist. It has to be decelerating with compression braking only for the symptom to be present.

Any chance it could be the flywheel? I do recall the tech sort of scratching his head saying he remembers the engine install and the flywheel appeared as new, excellent surface and passed the spring back test with flying colors. I agree with his cautious approach in that he doesn't want to start throwing thousands of dollars at repairs unless he is clear on a diagnosis.

thanks again

 

My guess would be motor mounts first, then flywheel next.

When you blip the throttle, is there excessive movement with your exhaust tips??

 

Good suggestion, just had the wife blip the throttle (think she sort of enjoyed it and I certainly enjoyed the sound of the PSE), in my opinion the movement is excessive and loose. Left tip dips and right tip rises and right tip seems to bounce when coming of throttle. My C2 is in the body shop and I have new mounts in that one, I'll compare when I get the C2 back.

I am hoping that's it as even with a bad hand I can DIY that one.

Thanks everyone, appreciate the responses.

 

Hopefully they changed the motor mounts when the engine was replaced but you never know. Maybe check the invoice to be sure. Great time to upgrade the mounts if you need too.

 

Not sure what is causing the vibration -- you have some good suggestions as to what it may be -- but about that "slightly cracked drive shaft flex disc".

This should be almost replaced.

Seriously the slightly cracked drive shaft flex disc should be replaced regardless whether the tech can attribute the symptom to this or not.

 

Using search here I did find the flex disc separately although Porsche wants you to purchase the entire drive shaft. Any experience with just replacing the disc, the tech is willing to do so provided the part I provide is the appropriate one. He is with the crowd that doesn't understand Porsche's spec to replace the entire shaft.

Nope, unfortunately the dealer believes that unless the mounts are leaking then they are just fine forever. Many of us on here know better. It was a major bone of contention with the new service writer as I clearly instructed him to call me when the new engine arrived as I wanted to opt for some other items like throw out arm upgrade, release bearing, and new motor mounts. Unfortunately when he did call it was to tell me to pick up the car. When I asked why he didn't call in advance he simply said that he knew enough to tell if any other work was needed and didn't need my input. We get along okay these days, but we had a rough patch for quite a while.

 

I keep thinking that it's either motor mount or tranny mount. Also, the fact that it happens on deceleration only - and I'm speculating only when the car is moving - tells me the weight shift dynamics have something to do with it, which make motor and tranny mounts the primary suspect culprits to me.

 

Might be a good time to change all 3, motor mounts, trans, and guibo disc. Couldn't hurt. I noticed a difference after changing my badly cracked guibo but I didn't have the RPM specific problem.

 

Thanks Keith. Will definitely do the motor mounts and the guibo disc if I can source the disc only. Any suggestions as to where to source the disc or how you handled getting a replacement for yours?

I did notice the tech inspecting the trans mount pretty closely but he said it looked good as new. He told me it can be replaced but it is a press fit so again he cautioned about throwing dollars at it through process of elimination.

I am thinking doing the motor mounts first to see if it corrects the problem and in any event will follow the advice to replace the guibo as well.

Thanks again