compression test through GPs or injectors?

Factory service manual uses the glow plug holes, some mechanics prefer to remove the injectors rather than the glow plugs. If you choose to remove the injectors you will need to replace the fabric covered bleed off lines between the nozzles and remember, the high pressure steel pipes to the injectors have compression fittings that should never be overtightened - just snug enough not to leak. Over tightening may crush the compression fitting and a new line will then be needed.

Added caution with respect to the injectors. It's vital that NO DIRT gets into their fuel inlets. Use a pristine clean deep socket (1 1/16" works) and keep the nozzles vertical when not in the motor for they're full of fuel that will otherwise spill out.

Thanks T.G. for advice. I'll go through the glow plug ports to check compression and install new glow plugs as well.

Jeff

OK, a couple of tricks to the glow plug route.

Use a small socket to remove and replace the nuts that secure their power wires, that helps avoid dropping the nut and guides it onto the GP stud.

Glow plugs extend into the prechambers. The space between the GP and its prechamber hole is filled with carbon so the old plugs sometimes bind upon their exit and the new plugs won't go in. Once the old GP is out twirl a 1/4" twist drill in the GP hole to cut the carbon deposits. Just use your fingers to turn it - no effort required - works easily and the new GP goes right in.

Buy either Beru or Bosch glow plugs for trouble free driving.

T.G. Thanks again for sharing your valuable experience.

It's those little details that can mean the difference between something going smoothly and pulling your hair out after a long Saturday under the hood.

Jeff

I have been a tech for 28 years at a MB only dealership and we ALWAYS test compression via the injector holes. Never ever do we even touch the glow plugs. Good luck getting anything in the last glow plug hole on 77 and up diesels with the oil filter housing in the way. And you have never tried to R&R the glow plugs on a 606 diesel! The MB factory tester just comes with the adaptor for the injector hole. Here is a compression tester that says you can use either hole:

"-->> T.G. Lambach

Karl, Yea, I'm going to do the test through the injector ports. After thinking about it, it made more sense to do it that way. I am going to go ahead and change out the glow plugs while I'm at it.

One question I have is should I do the compression test with the engine hot or cold. The instructions I have say to do it cold, but I've read on here suggestions to have the engine hot.

So, which is best... hot or cold?

Thanks Jeff

Cold is OK. Sometimes when compression is below 15 bar, the engine will not start when the temp is below 40 deg F.

Remember, a diesel ignites via compression. So when you check compression you want to know what the 1st compression stroke amount is! Unlike a gas engine where you let it go around for 6 or so compression strokes.