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Matt
Matt, Engineer
Category: Mercedes
Satisfied Customers: 20590
Experience:  Mechanical Engineer with 23 years experience in the auto industry, 8 yrs in formula 1 engine testing
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My 1987 mercedes 300d starts when cold, but won't restart

Customer Question

my 1987 mercedes 300d starts when cold, but won't restart after shutting it off
Submitted: 10 months ago.
Category: Mercedes
Expert:  Matt replied 10 months ago.

Hello

Its worth considering fitting a new fuel filter as if only slightly blocked this can lower fuel supply pressure and choke the main pump, the pump will be more sensitive to this when its cold as diesel thickens considerably with low temperatures.

It may have developed an air leak on the fuel supply pipe from the tank to the engine. The engine has no separate lift pump and relies on the built in one in the injection pump. If a leak has developed on the fuel suction pipe then air entering the system will air lock the pump.
Try pumping up the fuel using the hand pump in the engine bay on the filter housing, then crack off / loosen each injector pipe in turn while cranking the engine and getting an assistant to operate the primer. This will expel air from the injection side and the engine should fire up. The problem will be cured by checking the supply pipe along its length for signs of leakage and repairing any you find.

If you daub each pipe joint with heavy grease and the issue goes away then wipe the grease away one at a time to highlight the problem joint

Also if its not injecting at all then check the connections to the crankshaft sensor, unplug it and check for corroded contacts and broken wiring, and consider fitting a new sensor - do the same for the cam position sensor

Customer: replied 10 months ago.
Hi Matt, I haven't had time to try out your suggestions, but hang in there with me and I'll get on it this week end. Have you thought that there might be something wrong with the glow plug relay or a heat sensor or something like that?
All the best, Jerrold
Expert:  Matt replied 10 months ago.

Hello Jerrold

as you said its starts OK when cold make me think the glow plugs are OK ?

I'm not ruling it out but I just think the fuel supply is more likely to be the issue

You can test the glow-plugs with a multimeter by measuring the resistance across the plug ( between the power connection and the body) it should be around 4-8ohms if its tending to infinity / open circuit the plug is faulty or by using a clip on current clamp, either on the supply wire to glow plugs or onto engine earth, with glow plug light on you should see 15-20amps for each glow plug fitted, 4 cylinders would be approximately 60-80amps.

Expert:  Matt replied 9 months ago.

Hi

do you still need help?

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