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1985 Mercedes 190D, 1980 Mercedes 300SD, 1981 Mercedes 380SL, 1977 Mercedes 450SL 60k miles
28 Posts
Discussion Starter · #1 ·
I purchased a Mercedes 300SD a few months ago, and every time I cold start it, I have to pump the gas rhythmically and then diesel will start to combust slowly. The best way I can describe this is a "gradual start" where the engine starts to go faster and faster as the fuel burns and it shifts from electric starter power to its own.

After it starts, the car runs great, but the glow plug light blinks for about twenty seconds. At this point, it cannot idle on its own and requires a little gas.

Does this issue sound confined to glow plugs, and if so where would you start? I apologize for my vague description.
Merry Christmas!

78 300SD
46 Posts
Congratulation on your new car.

The blinking glowplug light is an indicator from the glowplug relay that one or two glowplugs are not working properly.
This will cause rough starts beacause the fuel isn't hot enough to ingnite equally well in all 5 cylinders.
You have one or two dead cylinders on startup.
They come to life once the engine temperature raises sufficiently.
Poor starting is also a sign of possible low compression and/or air incursion into the injection pump.

If you haven't done so already, next steps would be:

1) adjust the valves
2) remove the glow plugs, ream the carbon out of the prechambers, test and replace the bad glowplug(s), or all 5 glowplugs with Bosch glowplugs
3) test for air incursion into the IP, a tight fuel system will have no air incursion even after sitting overnight.

Test by temporarily adding a length of clear vinyl hose between the the cigar hose and the fuel return nipple on the top of the fuel filter housing.
This creates a new inspection window that is higher than all the other fuel system components.
Entrained air will rise to the high point.

Use the primer pump to remove any trapped air in the IP supply lines.
Any entrained air that appears afterwards means you have an air leak that needs addressing.
Typical leak points are the old-style primer pump, especially if it it leaks fuel whle pumping the handle, the rubber fuel hoses, the crush washer and o-ring on the fuel filter hollow bolt, and the crush washers on the banjo bolts.

Prompter responses can be had at: Diesel Discussion - PeachParts Mercedes ShopForum for diesel questions
and at: for general W116 questions. (FSM is available online at the Mechanicals tab).
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