'78 SC knocking sound at 850rpm
 

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.

:confused:

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.

Oil, temperature and pressure all OK?

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 :D

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)

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)

Bad tensioner, chain slap?
Bruce

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

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.

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

Make sure all the motor mount bolts are tight.

Hi,

the engine was out a few months ago

http://img.photobucket.com/albums/v4...ps5935af04.jpg

no heat shields, and engine mounts were ok.

Like the previous poster said loose wrist pin.

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

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.

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.

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

How do I know if I have nikasils? :)

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

Thanks, but no AC or smog pump present :(

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

I'm going to subscribe and see what I can learn. I have an 80 SC and have the same problem, but it goes along with a fluttering idle. Occasionally I can get a perfect idle. Most of the time, I get a fluttering idle from 1100-1500 RPM that sounds like the engine wants to die and there is some knocking associated with it. Then suddenly it will settle down and idle perfect.

I think I've solved all the vacuum leaks, but I have one more breather hose to go - the one that connects the air metering plate boot to the oil dry sump. It's ancient and in bad shape. I confirmed good pattern and balanced injector flow. This weekend I'm going to check fuel pressure. I"m also going to check timing and pull the plugs to see what they say. Bad spark is worth checking. I'm also going to look into how to test the O2 sensor to see if it is mis-behaving.

I have a good independent mechanic who specializes in air cooled Porsche, and he's confirmed in the past that cyl #2 is basically going along for the ride. He thinks that a dead cylinder will cause irregular pulse in the intake and confuse the idle. So the last step after troubleshooting all the tune-up items is to drop the motor and have him re-ring Cyl #2 as long as the heads are serviceable. May be time for a complete top end. Then again, at 260,000 miles, it's time for the whole thing to be rebuilt.