'78 SC knocking sound at 850rpm

[Hellraiser]

Hello,

since I have the car, I always had this knocking sound while idling at 850rpm.
sounds alot like a con rod bearing past its lifetime.
but when I increase rpm's to 950, the sound disappears.

[bh912e]

Usually con rod knocks increase when engine is revved and quickly become worse. Did you check valve clearance, I noticed if one cylinder has a loose valve clearance it sounds similar to a light rod knock.

[tuchan]

Oil, temperature and pressure all OK?

[Hellraiser]

Hi,

no we didn't check valve clearance yet because the covers
weren't leaking and I wanted to keep it that way for a while

but, I guess it's about time now

a friend of mine has it too, but only if the engine is warm
(also a '78 SC)

[Hellraiser]

Hi, oil, temp and pressure all perfect

maybe a little bit low at cold startup (3-4 bar)
but normal pressures when warm (1 bar idling and +1 bar per 1000 rpm)

[Flat6pac]

Bad tensioner, chain slap?
Bruce

[Hellraiser]

Hi Bruce, allready had a bad tensioner on another 911, souded more rattling.

[thumbdoctor]

Try disconnecting spark plug wires one at a time. If the noise decays, it could be piston slap, sometimes a tight wrist pin that gets worse when warm. Generally rods and mains with excessive clearance also cause oil pressure problems.

[clem911]

check your heat shield over the cat for broken/ cracked mounts.

[Norm Faustino]

Make sure all the motor mount bolts are tight.

[Hellraiser]

Hi,

the engine was out a few months ago



no heat shields, and engine mounts were ok.

[kodioneill]

Like the previous poster said loose wrist pin.

[oneblueyedog]

Old intermediate shaft bearing end play? Maybe hunt the answer in the engine rebuilding forums too.

[Reiver]

If you have Mahle nikasil forged pistons they tend to make a knocking sound at cold start until warm, a trait of forged pistons.
Accelerate and the noise goes away.
If you have Alusils it's a dif. issue.

[Lapkritis]

You can get load knock if you drop rpm too low. You'll hear this on even new engines when they're about to stall.

[Hellraiser]

I was also thinking load knock, but 850rpm is normal idling speed.
Unless my ignition or fuel delivery is way off...

[Hellraiser]

Originally Posted by Reiver

If you have Mahle nikasil forged pistons they tend to make a knocking sound at cold start until warm, a trait of forged pistons.
Accelerate and the noise goes away.
If you have Alusils it's a dif. issue.

How do I know if I have nikasils?

[Rich Gas]

This may be a shot in the dark but I had something knocking at low rpm, turned out the POs wrench had screwed up the AC mounting bolts, which loosened, and the AC unit was bouncing while turning...creating a knocking sound. I retapped the bolts... Tightened it down, Knocking gone. I don't run the AC belt now anyway.
Rich

[Hellraiser]

Thanks, but no AC or smog pump present

[Rich Gas]

One more shot...my fan pully was not aligned properly as well...I made sure that the shims were in correctly, tight, and not skewed... This was also a source of chatter, knock.

Rich