knocking from my 3.0 SC
 

Hey everyone, just became a proud owner of a 1979 911 SC. getting ready to do a bunch of maintenance to the car like cam chain tensioners etc. But before I start I have a question on a pretty annoying knocking sound, it’s really loud when the car is first fired up at around 2000 rpm. And when the car is warmed up its not as bad but still noticeable, and not at all noticeable revving over 3000 rpm. Any thoughts? Again I am a new Porsche owner and any info or advise will be greatly appreciated.

TIA

Is it a tap-tap-tap type sound or a lower frequency, dullen knock? If its the latter, it very well could be a rod knocking. From your description it souds like rod knock. Rod knock is most noticable at idle then slowly goes away as RPM (oil pressure) increases.

At idle, try pulling a sparkplug wire one at a time and listen to see if the knock goes away. Typically, if there is no pressure (combustion), the knock will drastically quiet down.

Other than that, it could be piston slap as this also goes away with RPM and increase in temperature, but has a higher frequency than a knock.

Before spending $$ on tune-up items, I'd hunt this issue down.

did what you said about the spark plugs, didnt change. I should of said its more of a slaping noise, its got a higher pitch than a knock. Its sounds like its coming from the left side cam chain cover, could this be a tentioner gradualy failing?

Use a mechanics stethescope (less then $10 from a local auto parts store), or a wooden dowel, piece of hose, etc. held to ear to see if you can isolate the noise. Chain noise will go away with RPM's, but will usually stay consistant with temperature. Do you have the oil fed tensioners?

Again, it could be piston slap if the noise eventually goes away.

Get a stethescope and see if you can isolate the noise.

No i do not have oil fed tentioners, thats what my main purchase was gunna be. I'll pick up a stethescope and see what i find. Im sure its not rod knock, but if it is piston slap, how should i go about repairing that?

I sure hope i dont have to go through a costly rebuild, i just want to drive and enjoy the car! lol

One thing at a time. Get the stethescope and try to isolate the noise.

Hi, Just wondering what the outcome was on the knocking noise?.

Although it is impossible to accurately diagnose a noise with such a limited forum, I would guess that what you are hearing is a noise very common to early 3.0SC engines.
It is not uncommon for the intermediate shaft drive gears to make a low pitched knocking noise.
The only way to fix it is to split the cases and change the gears. That said, if this is your noise it is nothing to worry about.
I have seen engines that have had that noise for 120K mikes without failure.
If you have want to confirm that it is not a bearing failure you can have the oil analyzed.
Good luck and welcome to the board.

Thanks Harry for your reponse , I was interested in this topic as my Carrera 3 has knock at idle . I was informed by a Porsche service agent here in Australia that it was due to forged pistons (piston slap), however I have since learnt that my car doesn`t have forged pistons ,(according to the porsche factory spec pocket book) it hasn`t caused me any grief as yet and its been over six years since I brought it like that.
Cheers Steve.