harsh noise on deacceleration

[hcoles]

Mike,
I found the 950 301 401 00 in PET. I'll ask my mechanic if anyone has put one on my model or if he has seen one installed. Thanks,
-Henry

[Jandrews]

I'll tell you what... my RS motor had an "elusive vibration" for months and months. I drove it on the track, and was still outrunning SC's and Carrera's. It was running at approximately 80% power, but still ran pretty darn good. It turned out that I had a spark plug that was firing, but producing just a weak spark. This was very hard to diagnose, until I finally swapped in some new plugs. What took me so long to swap plugs, is that I was getting fire in every cylinder, so I thought ignition was good. I tested EVERYTHING.

I will say this, it is amazing how well these cars run on 5 cylinders (not good by any means, just not horrible like you would expect). And also, they don't seem to "miss" hard the way other engines do. They just generally degrade on power, which is what makes things very difficult to diagnose.


JA

[MBEngineering]

HI if the chain tensioner has collapsed, you will have a bad rattle at app 2500-3500 rpm and then above it will go quiet again,

regards mike

[Rob 930]

Any possibility of a rod knock from a bearing? Have you examined the oil? Is oil pressure okay on decel? Has it changed over time (and gotten lower, for instance) since you first ran the engine?

[hcoles]

Nobody so far has mentioned rod knock.

In the last couple of months a very slight "ticking" noise has developed...sort of like the ticking I had before when discovered some valve guides were worn large. Could be a valve with slight need of adjustment. This slight ticking doesn't seem to be related or the same sound at all. Oil pressure seems same as after the rebuild 1bar at hot idle and 4bar or a bit more when running.

Probably will do some more listening with the stethoscope and compare car A and car B with same type of engine and recent top end rebuild also.

[hcoles]

Here are a couple of pictures showing what my exhaust valves and headers/exchangers look like..these are the worst positions.
I think I have about 8k miles on the top end rebuild at this point.
I don't know if this helps identify the vibration issue....the colors on the valves look very "white" but that may mean it is running on the lean side when putting around town which is probably ok. The build up in the header is a bit disconcerting.

On most of the exhaust valves there is just a small trail of black downstream..I guess this is where a small amout of oil is burning off...

[WERK I]

Are you running an aftermarket chip on your ECU?

[hcoles]

Currently running what I think is the stock chip that came in a box with the car. I put this stock chip in the car just after I got it about 4 years ago. Do you think the car is running lean?

The one interesting comment...above was that I can get the noise/vibration when coasting against the engine compression...at this point the injectors are supposed to be off or at least not putting out much.

[WERK I]

Don't know if you're running lean, but it appears your exhaust gas temperatures appear to have been hot, very hot. Is your exhasut valve really that white? Wouldn't worry about the exhaust header buildup. It really doesn't look that bad. You could have had that before your top end rebuild from worn valve guides.
The noise still could be anywhere. The chain tensioner collapse, rod knock, transmission bearing, pinion bearing, etc. Btw, do you have a Limited Slip Differential in your car?
An experienced Porche wrench could easily spot the area just by the sound and feel alone. These cars have a personality all their own.

[BReyes]

Sound bite?

So what is the coroboration at this point.

Don't drive the car, or it is fine? Earlier you mentioned, ft shifting and that is no problem. Why were you short shifting?

[hcoles]

short shifting to avoid back firing at shift from 3rd to 4th e.g. on straights at Laguna and Thunderhill - shift at 6 instead of 6.3k

Porsche wrenches - Don Wise and head mechanic at Jerry Woods have listened and confirm the noise vibration but no clear root cause ideas...

the exhaust valves are pretty much the white that you see...would like to hear if this is really an issue...are my exhaust valves much whiter than what is standard...car seems to run ok

I guess I'll try recaping:
- noticable noise/vibration most distictive at 2600-2700rpm

- most noticable when coasting down against the compression of the engine in 2nd, 3rd, 4th, stops when clutch pushed in and engine allowed to go to idle, somewhat noticable when clutch pushed in but engine reved up to the 2600-2700rpm..vibration is connected to engine speed and not transmission gearing change

-can sort of be duplicated when slowly free reving through the 2600-2700 range, also duplicated just static reving in the 26-2700 range

- noise/vibration can be duplicated with car stopped and in neutral but is too a much lesser extent

- using stethescope under the car.. could seem to notice the vibration on the right heat exchanger where the 3 come together before going into the cross pipe

things thought of and done:
- muffler - borrowed a muffler and swapped - no change
- heat exchangers - swapped right for left - no change
- one cylinder not working - compression check and look at exhaust valves - no obivious cylinder not doing work - person mentions exhaust valves look to be running hot -
- found loose bracket holding cross oil line - tightened back up - no change
- half shafts/cv joints - felt and the feel fine with almost no play and feel stiffish

other ideas: flywheel getting loose, piece of clutch came off or steel spring broken or came off (I am running a steel spring clutch compared to stock rubber), partly colapsed cam chain tensioner (I have Jerry Woods tensioner stops installed, rod knock, sump not getting schavanged and crank hitting oil, cam chain ramp broke

sort of sounds like maybe issue is connected to something happening around the input shaft to the transmission, since the noise is really transmitted when the engine is pushing against the transmission...leads me to think the noise is being generated on the input shaft or transmitted primarily through the input shaft




Again, I appriciate the comments and ideas...

[MBEngineering]

HI try the dual-mass flywheel as thay do not like doing what you are doing to it, raging it round a track, i had one, on a 3.3 turbo give up on a track day, at a the track, striped it and had to cut the flywheel to get to the bolts, as it had moved so far round the bolt holes did not line up.
just a thought???

regards mike

[BReyes]

Would Don or Jerry go into the boxes or into the 915. Is dropping it the first step to diagnosisi? Then 4-7k, right? If you drive her on the street you avoid this.

How about this, when did it really start? What is it going to cost me if I don't get back to when I was certain vibration was absent?

You have done so much work to date on this. Thank you for sharing with us. As I write this, I try to put myself in your shoes. I realize the amount of work required, to go in, drop it, go into those components, refurbish necessary ones, clean her up well, re-install drivetrain. The first thing PCars really require is time. The term service comes to mind. If you are going to do all this your self, have you thought about about having a spare (motor/trans) for use.

Regards,

[hcoles]

hope this isn't getting too boring for y'all
I'm getting a bit tired of trying things and thinking about it..

update:

Mike...I found the "bracket" part that goes from the exhaust flange and to the trans case - installed it
also at the same time took both belts off, alt. and ac compressor. I always wondered what that was..came in a box with the car.

went for a very short run...not much change still have the vibration.

after putting the belts back on I left the "bracket" on and went for a longer ride...it was definitely not worse maybe a bit better..I think I still have an issue.

I'm getting ready to pull the motor and trans and check out the flywheel and clutch area and get that assembly balance checked...need to find one around here (San Jose CA)

That's about it for now...thanks for the thoughts

-h

[hcoles]

if anyone is still looking at this..I put in a new post....dropped the engine this morning and looked at the clutch, flywheel and pressure plate. Can't see any issues except the clutch disk heavy springs are loose....is that OK? Seems like there should be no play...the thin springs ( 2 of 6) are in compression. Thanks,
-Henry

[hcoles]

got the engine back in on Sat.
one of the rear engine mounts seems to be cocked.

I'm thinking that I need to check that the rear of the engine is centered in the car and I hope the rear mounts can be loosened and will float to center and then I'll torque everything back down...

is this how it works?

how do I find the center at the rear of the car? is the center of the hold for the trunk latch the center?

[hcoles]

FYI - I asked my mechanic this morning....the engine is not supposed to sit in the center of the car...at least the rear of the engine isn't.. not sure about the tranny end... the offset seems to be about 3/8". We looked at some other Carreras and same thing...engine is offset from center...

[hcoles]

ok, engine back in the car and running.

I would say the issue is fixed....no noticeable vibration at 2650rpm.

The vibration moved a bit to 3000rpm but is different and much less

I would now say the root cause is motor mounts. When I put the engine back in the car I probably shifted the mounts around a bit and the vibration is changed.

The other things I did that might have an effect:
- injectors sent to WitchHunter and cleaned and balanced
- flywheel and pressure plate balanced, clutch disk was balance checked and was fine
- found that the WOT switch was not working, cut it apart and cleaned and JB Weld back together
- cleaned the tracks inside the air flow meter
- changed the air filter to now have the pleats horizontal, I didn't realize that is needed but makes sense..I should know better
- did valve adjustment...they were a bit on the loose side


I guess that's about it for this thread...I'll probably try moving the mounts around a bit and see if that changes things..I now notice that the left rear mount is a bit cocked so I'll see if I can get that to a more relaxed position.

[lindemans]

Originally posted by hcoles
ok, engine back in the car and running.

I would say the issue is fixed....no noticeable vibration at 2650rpm.

The vibration moved a bit to 3000rpm but is different and much less

I would now say the root cause is motor mounts. When I put the engine back in the car I probably shifted the mounts around a bit and the vibration is changed.

The other things I did that might have an effect:
- injectors sent to WitchHunter and cleaned and balanced
- flywheel and pressure plate balanced, clutch disk was balance checked and was fine
- found that the WOT switch was not working, cut it apart and cleaned and JB Weld back together
- cleaned the tracks inside the air flow meter
- changed the air filter to now have the pleats horizontal, I didn't realize that is needed but makes sense..I should know better
- did valve adjustment...they were a bit on the loose side


I guess that's about it for this thread...I'll probably try moving the mounts around a bit and see if that changes things..I now notice that the left rear mount is a bit cocked so I'll see if I can get that to a more relaxed position.

Wow, good to hear you found it; I'm positively surprised my guess was right.....and it's in the end a cheap fix.

What you may do is lubricate the rubber pieces with WD40 or similar to make them slide into place; or slightly jack-up the engine a bit to take the load off the mounts to have them seat....