Spark Knock

[Paul61]

I have a 1986 560sl and It works fine till it gets at full operating temperature. After shutting it down to refuel or some similar short period of time and then restarting the engine exhibits spark knock. I had taken it to an independant garage and the same problem is still evidenced. It seems that the power is some what reduced when the spark knock is evidenced. I have noticed what are apparently thermal sensors in the coolant water jacket with electrical connectors attached to some and vacuum lines to others. Suggestions as to what needs to be done or replaced would be greatly appreciated.

[ctaylor738]

I assume, since you request an answer, that you did not mean to lock the thread.

The coolant sensor you describe is part of the injection system - it helps the control module deterimine how much enrichment the engine needs.

The only thing that I know of that will cause spark knock in a 560 is a bad ignition module. Not what you want to hear. Maybe others have some ideas.

[dugald]

Yes the ignition control module is the only device that alters timing. But it does so based on vacuum, among other things. You should check the vacuum line from the intake manifold under the air cleaner housing to the ICM on the driver's side fender wall next to the coil.

[Paul61]

You are correct regard the locking of the thread. Thanks for unlocking it. I'll look into the ignition module. Thanks again.

[Paul61]

Thanks dugald for the additional information on the line from the intake manifold. I have a small hand held vacuum pump/guage combo, which would no doubt be a start, tool wise. Do you have any recommended procedure to check out this portion of the system?

[dugald]

Paul, the line in question is a single line running from the intake manifold directly to the ICM. Remove the air cleaner housing and you will see the line in its entirety. It's actually a vacuum retard set up, i.e, the spark timing is retarded at idle when the vacuum is high, and advances as the vacuum decreases with throttle opening. A broken vacuum line would leave your timing permanently advanced = premature ignition, aka knock.
If your ICM is dud, these people http://4mercedes.com/index.htm rebuild them at a substantially lower cost than new. Dealers want about $3000 for a new one, other sources $2000. I got a working used one for $50 from these folks
austrianautoparts@yahoo.com. They might have others.
Good luck.

[tomasito]

Originally Posted by Paul61

I have a 1986 560sl and It works fine till it gets at full operating temperature. After shutting it down to refuel or some similar short period of time and then restarting the engine exhibits spark knock. I had taken it to an independant garage and the same problem is still evidenced. It seems that the power is some what reduced when the spark knock is evidenced. I have noticed what are apparently thermal sensors in the coolant water jacket with electrical connectors attached to some and vacuum lines to others. Suggestions as to what needs to be done or replaced would be greatly appreciated.

Have you tried to run your car with a copper based sparg plugs like NGK?