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My 1985 MB 300 SD will crank but wont start

Started by TimTosser, 27 August 2024, 06:09 PM

TimTosser

So a little info to get you up to speed. I bought this Mercedes from a fellow co worker about a month ago, I needed a car and I've always liked older models compared to the newer ones, the interior looked great and it ran , I was excited to own something so cool and have it for around 5000$ as my first car.  I know nothing of cars at all. I work in a complicated electrical field so I figured wrapping my head around these old diesel systems would be a tight learning curve but doable if I put my mind too it. The car was running, and thats all I needed for now. It was a bit slow to acceleration but would get me up to 80 mph on roads I was allowed to go that fast on. Glow plug sound would go every time and it would start without a hitch every time. Up until last week, It would crank for a minute and not start. I would recycle the glow plugs and try again, nothing. One more time and it would go,  puff of white smoke would come out the muffler and Id let her run for a few minutes and than head out to work. For about a week this was a my morning routine. Once I was at work, and would get let out for lunch, it would start just fine, and on the way home it would start fine. Than the next morning it would repeat. Until this weekend where I went to start it on Saturday and there it goes, cranking but not starting. Tried a few more times, and this time nothing. It'll crank but just wont start. I'm still in the process of moving so funds are a bit tight and Im without tools for now. I looked up a few things, I haven't tried the glow plugs for volts and ohms yet, but I did unscrew the manual primer to give it a few primes just to see if the fuel filter wasn't getting any fuel, to my dismay when I unscrewed the primer and as I pushed in on it a decent amount of diesel came out. I left it at that. I had the guy I bought it from swing by and look at it, we connected batteries cause the battery is dying now due to trying to get it started, with a bit of charge, it still wouldn't start, just cranking. I bought a repair kit for the lift pump and the replacement for the primer, that after a few days of letting sit is leaking now. Just hoping for a few bits of advice, cause Id like to learn this and do it myself before having to save a bit of cash and turning it over to a mechanic.
-TimTosser

ramiro

How long did you crank the engine , if it's not cold it should start without glow plugs if you crank for like 1 minute , also did you see smoke from the exhaust while cranking?

There is also alway the forced way to start an diesel with starting fluid or flamable brake cleaner , if it starts with that and doesn't keep running you have a problem with you fuel supply.

With a diesel there aren't many things needed to get it started just fuel and compression.
They are very sensitiving with getting air into the fuel system , so if you have a small leak somewhere the diesel could flow back to the tank when the engine is off and while doing that pulling air in.

rumb

https://handbook.w116.org/Engine/

Will have lots of info on your engine and jobs to test glow plugs.

'68 250S
'77 6.9 Euro
'91 300SE,
'98 SL500
'14 CLS550,
'16 AMG GTS
'21 E450 Cabrio

benzjag

Hey Tim,

Understand your frustration. Been there about 30 years ago when I bought my 1st diesel.

Since the car started perfectly, at your lunch break and driving home, it appears to be the glow plugs are not working. After getting it started in the mornings, the engine was warm at lunch/going home;therefore, no problems getting it started. One easy test is to turn on the ignition without starting, listen carefully for about 40 seconds and see if you hear a "click" from the glow plug relay (on the front of driver's side inner fender in the engine compartment). If it does NOT click, the module is bad. If it does click, it should be good. It's purpose is to provide power to the glow plugs.

Here's a quick video on how to test the glow plugs at the relay:

https://youtu.be/xqk6tK3WQBg?si=kOil23hN9RLBrr_N

The only other thing I can think about is the fuel system is allowing air to get in while sitting overnight. Priming the pump (as you did) should have gotten the air out. The fact diesel spilled out from the plunger is normal for the "old" pump.

Richard