harsh noise on deacceleration

hcoles · 05-16-2006, 05:03 AM

About 6k miles ago I rebuilt the top end of my 3.2 G50 89.
So far 7 track DE events and street driving to work.
There is a "harsh" noise and now seems like vibration when the engine/trans is hot and deaccelerating or compression coasting downhill. The only thing I can think of is something is getting more and more worn in the tranny or the exhaust system has some part changing or getting loose. I have the cat out. This has definately gotten more noticable overtime starting after the rebuild. At first I thought is could be that after the rebuild the higher compression was making this sound, now I don't think so.
Any ideas? As far as I know the tranny hasn't been opened up. Now at 78k miles.
Thanks,
-Henry

BReyes · BReyes's Garage

Have you inspected the half shafts?

Good luck!

hcoles · 05-18-2006, 05:42 AM

good thought, I'll check, thanks

hcoles · 05-22-2006, 07:58 PM

ok, I checked the half shafts and all the cv joints felt solid with no play.
I borrowed a muffler today and swapped it in...no help.
This is starting to get worrysome. Vibration very pronounced at 2650 rpm, smoothes out above and below. Not very noticable when things are cold. When put the clutch in and keep the engine rev'ed to 2650 things get better but don't go away...almost like the vibration is coming from the engine and gets transmitted to the car through the transmission. We are sort of thinking loose flywheel at this point. I'm starting not to like this at all. I used new flywheel bolts and red loctite but who knows I may have forgot to torque...I really doubt it. It seems like it is something that is suseptable to a pronounced harmonic vibration.. layshaft? cam chains? We will need to put 2 Carreras side by side and listen with the stethescope again.. maybe we can find something different sounding...may have to get under the car and listen around the bell housing area. Any ideas appriciated.

lindemans · 05-23-2006, 03:31 PM

Could it be one of the 4 engine mounts ?
These are made of metal and rubber and could cause (as we call in Dutch) "distribution of sound/noise" from the engine / gearbox to the metal chassis.
Forces on deceleration may cause metal to metal contact and associated transmission of vibrations to the chassis.

hcoles · 05-23-2006, 03:57 PM

good idea...this was mentioned today over at Jerry Woods place.
They are thinking clutch is falling apart or wheel coming loose, but I like your idea...may be why worse when going downhill and seems to be not as bad when things are cold (rubber stiffer) or maybe the engine is moving and hitting e.g. the sway bar. Maybe I can rig something to pull the engine/mounting system to one side and then see if I hear a difference... thanks very much

hcoles · 05-23-2006, 03:57 PM

make that flywheel coming loose and not wheel

hcoles · 05-24-2006, 07:27 PM

got under car last night and ran it on the lift....very harsh noise coming from the right side header where the three pipes collect...
today pulled plugs and did compression check...fairly consistant compression numbers about 130 plus or minus with engine warm, plugs look very much the same. I got some pictures and can post tomorrow when I get them out of the camera.

At this point I don't think any mounts are grounding out or something hitting the chassis because I can get the noise without the car moving and not changing the revs much...just sweeping up and down across 2650rpm is where I get the harshest noise. But I do need to do more to completely rule out mounts and phyiscal grounding.

Stay tuned...

MBEngineering · 05-25-2006, 09:23 AM

HI has your car got the exhaust support onto the gearbox part No 950 301 401 00, if not try that it was added to stop noises like the one you have. it is approx £18 / $33.93

regards mike

hcoles · 05-25-2006, 09:50 AM

Mike,
never heard of this...do you have a picture?
last night checked plugs and did warm compression check.
compression results..
I was not able to view gauge when cranking and did get a few odd readings

1 - 135, 126 - average = 130.5
2 - 126, 129 - average = 127.5
3 - 140, 129, 125, 133 - average = 131.7
4 - 127, 129 - average = 128
5 - 129, 127 - average = 128
6 - 141, 131 - average = 136

I have a composite picture of the spark plugs and will see if can add the image in this email...

hcoles · 05-25-2006, 09:55 AM

here is a picture of the plugs from last night after driving home from work 7 miles and running the engine real easy...what are the white dots?

MBEngineering · 05-25-2006, 12:57 PM

HI do you have the PET system, if you do look under 911 1970>1989, and then1989,group 2, illustration No 202-05 No 24, it fits from the exhaust flange bolt to the gearbox casing, the plugs look a little weak and the dots could just be the bad grade of fuel, if the light is the same on the No1 plug which looks ok then i would say you have some faulty injectors/spray pattern, injector cleaner in the tank and richer mix'

regards mike

jadeblaquiere · 05-25-2006, 01:49 PM

Is there perhaps a crack in the exhaust header or flange?

My experience is that cracks in the exhaust can make some really horrific sounds (imagine whacking something with a ball pein hammer)... these tend to be more pronounced when warm and a little backfiring caused by richness on overrun would certainly emphasize it...

It's a bit of a stab in the dark, but it should be much easier to look for than a lot of problems...

MBEngineering · 05-25-2006, 02:15 PM

HI it's a 3.2 1989 carrera and i asume is still STD injection, with that the injectors will be SHUT OFF on the over run so no fuel !!

regards mike

hcoles · 05-25-2006, 02:59 PM

Mike, I have PET. I'll look and it could be a good addition...but seemed to run ok for 4 years that I had the car without. I'll check it.

Tonight I'm going to switch the exchangers...one hope is that a tube inside the right exchanger has a crack...I hope it is something like that. The other night as mentioned very harsh noise on right header compared to left header...I hope it is the header/exchanger.

Re. injectors... yes...I've been worried about these for sometime now. I need to send and get matched/cleaned. Not sure of a place there is Maron and EBS and I think now a bunch of places because the tuner market guys are sending in injectors to get checked. I have run Techron a number of different times. I should just bite the bullet and send them to EBS and be done with that.

-Henry

hcoles · 05-25-2006, 03:07 PM

Mike, I now notice your comment re. no fuel and getting the harsh noise..means during deaccel all the fuel related/combustion related contributors should be gone or reduced....does this point to crack or ? inside one of the header/exchangers? I'm just sort of thinking out loud here, is that your point? Good thinking...

and jadeblaq...good comment as well re. cracked exhaust parts...and I have had some back firing when shifting at full throttle at the track... e.g. on the front straight at Laguna Seca from 3rd to 4th..I wonder if this has caused a break in the "header" or vis. versa... I've been short shifting a bit and that issue is reduced now...

I really appriciate the comments and all the good thoughts..

MBEngineering · 05-26-2006, 12:57 AM

HI check the micro switch on the throttle housing, as this tel's the ECU the throttle is closed and to shut the fuel off, it could not be working correctly and over fueling???

regards mike

Jandrews · 05-26-2006, 05:35 AM

Does it run strong under acceleration/load?

JA

hcoles · 05-26-2006, 06:37 AM

John,
It seems to me to run strong under WOT...but...when Rich and head engine mechanic at Jerry Woods took it out they said it seemed to be down on torque....who knows they may be used to driving cars that actually have power to weight ratios that are mentionable

hard for me to tell, I'll run it against some other cars around here that are the same and see if the accelerations are the same.

Some guys are coming over and we are going to work/think about this some more.

Last night I switched right for left the exchangers and filled them with water to see if a leak....no leaks and no increase/decrease in vibration

I'll see re. the switch closing when the throttle is up...

Thanks for thinking on this...sooner or later I'm going to find it I hope...I don't really want to track the car too much more until I find the root cause

-h

hcoles · 05-26-2006, 06:46 AM

My latest thought...one of the cam chain tensioners has collapsed and is now resting on the "Jerry Woods spacer" I placed inside the tensioner. This means the chain could be operating OK but would be loose causing a vibration being worse at a given rpm...