Can a bad glow plug cause my 85 300D to knock at 2500rpm when cold? Everymorning while accelerating to speed, I have a loud nailing, knocking sound at 2500rpm. It goes away once warm. Any suggestions?

Randall,

It is unlikely a GP is causing your knock. Once the engine is started, the GP are shut off so they have no effect on the engine. The knock is probably caused by a bad injector.

When the engine is knocking, loosen one injector line at a time to determine which cylinder is knocking. Remove the injector from the knocking cylinder and get a new tip put in that injector at a Diesel injection shop. You might have all the injectors tested there and replace any other margional tips. NOTE, the tips are available from Fastlane probably at a lower cost.

One other thing, when you have the injectors out, check the ball pins to make sure they are not broken.

P E H

Thing is, it only happens at 2500 rpm and under load. So, disconecting the injector is not an option. I think its dirty fuel. Maybe a purge is in order... Thanks for the insight though.

Randell.

You can loosen one injector line and drive car up to 2500 RPM underload and listen for the knock. When the knock isn't appearant, the injector with the line loose is the bad one. You will spray a little fuel around but it will evaporate after a few days.

Or having the injectors tested at a Diesel injection shop may find a bad one. But this necessates removing all the injectors. Usually a knock is caused by a leaking injector that releases the fuel too early and this can be found by testing them.

One other way is to get a good injector and replace one injector at a time to determine which injector is bad. Then get that bad injector rebuilt and keep it for a spare.

To remove an injector you need a 27mm or 1 1/16 deep socket wrench.

P E H

Seems like a straight forward operation, thanks for the insight. So to remove an injector, all I have to do is unscrew it... Thanks!

Randell,

You have to remove the injector line from the IP to the injector at both ends and the vibration damper clamp on the lines too. Make sure only the line nut is turned, not the part that is screwed into the IP. You will probably need a flare wrench. Try not to bend the injection lines so they go back on easily.

The rubber/fabric overflow fuel lines also need to be removed from the overflow nipples. If they are old and hard, you will have to replace them or they will leak. You can get bulk fuel line at MB dealer.

Then the injector screws out with the deep socket wrench and ratchet handle. Assemble in reverse order. Make sure the same or new washers (one each) are put back under the injectors.

P E H