tall02

I have a 1992 964 3.6L engine.
It had Good oil pressure and it ran like a champ but wanted to rebuild to my top end due to low compression on 2 cylinders. Finished the rebuild and found the heads needed new valves, springs, guides and seats recut. Reinstalled the engine, primed the engine with oil pressure of 3.5 bars. It started up on the first crank. Sounded good for 10 min at 1500 to 2000 rpm. Decided to take it for an easy drive to cool down. Pulled out of the drive and drove 100 yards to the stop sign. When the clutch was pushed in and coasted to the stop sign a rapping could be heard. I backed up to the drive and garage. It was not as bad as a knocking or light as a tapping but an inbetween rapping sound. Turned it off and disconnected the alternator and A/C belts. Still rapping. Checked the dual distributor and the belt was good. Pulled the valve covers and checked the rocker gaps. Everything looked good. Installed a new clutch and pressure plate also during the rebuild and tried pushing in the clutch at idle but no difference. Drained oil and cut open oil filter. It looked great...very clean.

Anyone have any idea why the engine is rapping now when it sounds great for the first 10 min.?

Puzzled

Thanks

dutchcrunch

did you do the top end yourself?

Larry Herman

If it is a rapping sound my first guess would be a timing chain. Maybe you have a collapsed chain tensioner (outright failure or a piece of debris lodged in it).

tall02

Thanks guys for the feedback.

I performed a few more tests and observations.

- Reset rocker gap - but they looked good
- Checked new distributor belt - both rotating
- Rotated engine by hand - no metal hitting sound
- Turned engine over with starter - no metal hitting sound
- Compression test on cold engine - showed 180# to 195# all around
- Tried to inspect new pressure plate and clutch while rotating engine - no scrapes
- Inspected timing sensor while rotating - no scrapes with a .040" gap
- Disconnected alternator / A/C/ and power steering pump
- Checked exhaust system - nothing loose or contacting metal

Restarted and interesting on a light rev the rapping dissapeared but on releasing the gas the revs fell and the rapping imediately returned lowder until idle which still had the rapping.

The last item I was going to check was my timing chain tensioners. I will double check they were installed correctly. A freind from the Porsche club that races 964 told me he has seen people receive bad tensioners. Luckily I still have my old ones which I will try reinstalling. I just need to figure out which one is the top left tensioner and which one is the bottom right tensioner. My old parts have numbers on them: F-468042 and the other one has F-468031. I would appreciate any help on which one is the right and which is the left.

Or if anyone has any other ideas.

Thanks

Keith

darth

Have you been able to determine if it's a top or bottom end rap?

In order to determine if its a top end valve train component vs. a bottom end component. Is the rap in time with the engine rpm (i.e. indicating a bottom end component - piston, crank etc.) or is it double the engine rpm (i.e indicating a top end component - valve, cam etc.)?

Bill

chris walrod

Curious if the new tensioners are pumped-up completely? Perhaps some more 2K RPM running is needed to get those pumped up?

Oil pressure is lowest at idle. If the 'rapping' sound goes away with increased off idle RPM, might be the tensioners.

dutchcrunch

i do recall a note in the porsche maintenance manual explaining how to properly install and the orientation of the tensioners. so i think you can install them wrong. not at home to check manual

tall02

Hey guys

Reinstalled the old timing chain tensioners. Same sound but it was late and could not run it long or at raised rpm. Will try running at a higher rpm tomorrow. If this does not work I plan to drop the back of the engine and open the timing chain covers to inspect and check timing.

I was wondering after timing the left cam, if you don't have enough tension on the right bank cam would the timing be a smidgen off from the left and cause the left side to fire earlier or later than the left cam? Would this cause a rapping?

Yours truly......puzzled

Thanks for the feedback

Keith

Fred's 964

The tensioner has an oil hole on one end to pump them up. The hole goes away from the guide piece, into the small cover held on by 2 bolts. It is possible to get them in upside down. When they are installed properly, one looks upside down, so it would be easy to get them in wrong. Take the small cover off at the end of the tensioner and make sure you see if you see the oil hole. I hope it is that simple of a fix.

Fred's 964

If the cam timing was a bit off on one side, the plugs would still fire at the same time if the distributer belt is on properly. The power generated by the off cylinders may be different due to slightly different cam timing. Did you time both cams with a dial indicator and temporary tensioner like the manual says?

tall02

Ref: Timing chain tensioners

Yes I checked the new tensioners and reinstalled the old tensioners. Both had the oil holes facing the endcaps.

The odd thing was that the engine started on the first turn over. Then ran smooth and quiet for 7 to 10 minutes. Not until I drove it 100 yards down the road and let off the gas at the stop sign did the rapping start. It runs and sounds good other than the rapping. Now I have not reved the engine above 2000. It has not run long enough for a good heat soak. I am cautious about this rapping.

Did I say I changed the oil and checked the filter......clean as a whistle.

I will not be able to work on the 964 for a few weeks but I plan on opening the timing chain covers and recheck my timing.

Thanks for your interest and any suggestions

Later
Keith

dutchcrunch

i believe it was Kirk F. that said after timing the engine you have to be real careful as the washer covering the dowel is real small and you have to push it toward the dowel so as to make sure it is covered while tightening the cam nut.

i never done this before but i would think the washer should have plenty of surface area to cover the dowels and prevent it from coming out.

but it seems that this can happen and someone else had the dowel come out and destoyed the engine.

i am not sure if this could be your problem but i remember reading it on kirks rebuild info .

Les Dawes

Hi Keith,
I remember doing an engine rebuild on a conventional 4 cylinder car quite a few years ago, in which I removed the old rings and cleaned the piston grooves and then fitted new rings. When running the engine I experienced a tapping noise which turned out to be the rings moving slightly vertically in the grooves as the piston moved through TDC. Previously the rings had been snug in the grooves due to burnt oil and carbon build-up, which obviously I had removed when cleaning.
I know our engines are slightly different in layout, but the piston/rings/bore arrangement is the same.
Did you do a similar exercise when doing your top end rebuild, ie. new rings in existing pistons.
Since then I have always checked the ring to groove gap tolerance, and if too large then I fit new pistons.
Hope this helps.
Regards, Les

ThomasC2

About the tensioners. I changed the seals on their covers and mounted the lower one the wrong way when I put them back. I drove for maybe 10 minutes and then the sound came. The lower one should sit in the opposite way compared to what you think. IIRC there was an arrow or a mark on the tennsioner, but as said, the arrow points the "wrong way". I turned my 180 degrees and then everything was fine.

tall02

I have attached a sound bite of the rapping. I hope it is not too large.

Has anyone know what may be causing this rapping. The sound is during idle, it dissappears on acceleration but reappears louder on deceleration.