OM 603.961/71 Rebuilt Upper End Lumpy Cold Start
 

Hello All,

Looking for some feedback before I start poking around.

Cars runs fantastic except for immediately after cold start for about a minute. Glow plug light functions and start up is attempted after light turns off. Vehicle intermittently misfires. No lingering pressure in the coolant systems or indications of combustion leaks.

Rebuilt the upper end of my 87 300d with the following:

017 head completely rebuilt. Guides, seals, valve job, decked.

Inclined injectors with Monark 314's rebuilt by Greazer (thank G).

New Bosch 017 specific glow plugs. 001-159-17-01-M14. Installed with torque wrench...

Couple of thoughts:

Does the .971 use a different GP system to light the GPs or keep them lit after start up? If yes should I adapt?

I fully understand that I need to check for individual glow plug function, but would like some other ideas to follow up on as I have limited time per week to chase non critical problems.

Thanks in advance fellow diesel nerds.

JM

The .97's used an afterglow relay and use a different glow plug from the .96x engines. If you're using your original W124 relay, it doesn't have afterglow and you can expect an occasional chug or clatter in the first 30-60 seconds of operation when cold.

Excessive missing out or chugging can indicate air in the fuel system. Check your rubber hoses and banjo bolts for wetness and/or sweating. If it's still in the circuit, be aware that the fuel heater thermostat is an extremely common source of air ingress since it's on the suction side of the lift pump and has an O-ring seal that perishes with age.

That is what I suspected.

Is there a link in how to update this system?

My 971 is have these issues too. Starts great then lumpy with smoke.

Not sure if there's a DIY on this forum or not. The .97 glow plug relay uses an extra pin in the control wiring plug that runs out to an extra temp sensor in the head. Essentially all you need to do is add the extra wire and temp sensor and swap the relay out.

The poor man's version of afterglow is to lift the purple wire on the .96 glow plug relay control wiring harness. The purple wire is the starter interrupt signal, so lifting it forces to the glow plug relay to run to it's full 30 second cycle, even if the engine has been cranked.

Be aware that afterglow can help mask injection or compression problems but will not eliminate it.

http://www.peachparts.com/shopforum/diesel-discussion/236848-w124-om603-afterglow-diy.html

You can try lifting the purple wire to do a test but not as a fix. It will leave the gp on for 30 sec even with hot engine. It will burn out gp quickly.