964 engine problem

[john f]

Hi All
1991 964 C4

I had a faint knocking on 4-5-6 bank
set the tappets (4 thou) after leaving it to go stone cold and the knocking is the same albeit without tappet tick
When cold its not evident but when hot it knocks
I lifted the car on my ramp and I put a screwdriver to my ear and listened to No 5 and No 6 cylinder pot they both make a knock sound
which isn't there on the other four (hot idle only)
Piston Slap ? wrist pin ?
As regards piston slap I,m sure that would be worse when cold ?
had a top end build 2000 mls ago !
oil and filter changed 10 w 40 semi
Any recommendation of a person who knows their way around air cooled engines based in the Midlands area

John F

[sam]

Are all your plugs firing? and correctly timed, sounds like it could be dizzy belt?

[john f]

Thanks for the reply

All plugs firing
If I disconnect both coils in turn the car will still run on either and will rev well
runs slightly lumpy on idle with reduced revs but no miss fire


John F

[sam]

Knock sensor, Hall sensor? I am not going down the mechanical causes yet because it would be more likely to be associated equipment, they are fairly bomb proof engines after all. How many miles has the engine done, and did the noise start suddenly? There is a very good air cooled firm in Westcott, Bucks called Neil Bainbridge Engineering, excellent people! 01296 658422

[john f]

The car has very little use and it was a few months ago I noticed this faint knocking
I noticed that the final silencer had become detached so I refitted it , then I was sure that it was the alternator bearing so I fitted a new one and on one start up the engine rattled badly and the went back to the faint knock
as mentioned number 5 and 6 are the only piston barrels that I can hear a knock when I put a screwdriver to my ear from underneath the car especially when warmed up
oil pressure is good and the car runs ok without any missfire
After i left the car standing for a month battery flat and all that the car ran as lumpy as hell until I took it for a decent run

John F

[sam]

still reckon it could be knock sensor issue, cam chain adjuster malfunction? sounds like you need to take it to someone with expertise on the engines, the 'scatter gun' approach to car maintenance can work out very expensive, particularly on P.cars. Good luck.

[john f]

I,m pretty sure its mechanical as it is only evident on 2 of the 6 cylinders
and the noise is in the cylinder area
started the car tonight and the noise just i'snt right
off the road again !
spent £5000 addressing top end issues and I still have top end issues

John F