1986 300SDL & Biodiesel Issue - Need Help - Mercedes Forum - Mercedes Benz Enthusiast Forums


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Old 04-24-2011, 09:29 PM
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Default 1986 300SDL & Biodiesel Issue - Need Help

I have a 300SDL that kept going and going for 245,000 miles. Then I filled the tank with 9/10th biodiesel. Car started normally about 20 times and drove fine for about 350 miles or 3/4 tank. Then this morning I turned the switch and engine turned but wouldn't start as if no fuel got to the injectors.
I heard that biodiesel loosens the junk left from regular diesel from the tank & hoses and brings them into filters so I changed 2 filters, the small, plastic prefilter (on a hose going directly toward bottom of engine) and the metal screw-on larger filter( right behind top of radiator). Car still can't start, I turned the switch twice, each time 4-5 seconds.
I did not bleed/re-prime the fuel line, again I had read somewhere that 300SDL automatically bleeds itself.
What am I doing wrong or not doing?
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Old 04-25-2011, 03:42 AM
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Thanks, I heard about the strainer before but my car is parked in front of my condo I can't possibly access the tank here, any method of cleaning that strainer without emptying and lowering the tank? is the strainer inside or at the bottom visible from outside?
The car was running fine until I parked it 2 days ago, now it won't start. Also didn't understand what you mean by switching the supply/return line, switch at what spot? and why?
Thanks again
My direct E.mail is: [email protected]
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Old 04-26-2011, 04:05 AM
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Mine looks just like that picture but I still don't understand what you mean by switching those two lines. How do we switch them? Where do we disconnect them to switch? Do we start the car after switching?
One more thing, after I installed the primary-filter and started 2-3 times each 4-5 seconds, the plastic filter slowly filled with fuel little over half. Wouldn't that mean that strainer is passing the fuel and is not clogged?
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Old 04-27-2011, 05:08 AM
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There are no metal lines to switch inside engine compartment. There are a shorter rubber hose with primary-filter that goes to the lower side of engine & the longer rubber hose that goes up under secondary screw-on filter.
I meant since there is fuel inside primary filter shouldn't the car start? or does it need more pressure which clogged strainer blocks?
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Old 04-28-2011, 01:37 AM
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The return line takes fuel that isn't used by the injectors back to the tank, what ForcedInduction is trying to tell you is to take the return line that goes to the tank and switch that with your supply line, making the supply line the return line temporarily. This will by-pass the strainer and allow you to run till you get the tank pulled down low enough to replace the strainer and switch the lines back.
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Old 04-28-2011, 01:39 AM
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The clogged strainer might not be giving you enough flow th let the engine start
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Old 05-02-2011, 05:42 PM
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I just loosened the 6 nuts on top of injector pump, cranked the engine and a few drops of fuel came out of each line. I'm not sure how much fuel is enough, it seems fuel is coming to injector-pump but too little with little pressure. Black smoke came out of tailpipe while starting. Anyone knows if partly clogged strainer and low fuel pressure can stop the car from running or those few drops into injector-pump should get it started?
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Old 05-03-2011, 08:26 AM
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It dosen't take much, but if there isn't enough to fire the injectors it won't run. If you're getting smoke you are getting fuel into the cylinders, the question is if it's warm enough to start the engine. Check the glow plugs, and the glow plug relay.
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Old 05-05-2011, 09:01 PM
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How do I make sure my pre-glow relay is bad before I change it? The cheapest one is around $105. I'm still not sure that's the reason unless 95-100 degree temoperature outside isn't enough for half-biodiesel fuel to start the car assuming relay is bad. By the way, I've heard about a special fuse for relay, does anyone know where that fuse is, if any?
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Old 05-09-2011, 06:46 PM
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Default Update - found the problem

It was the Pre-Glow Relay. The silver metal connection between the 2 screws at the top left was melted and cut off. Anyone knows if I can just buy that metal piece or have to buy a new Relay?
What causes that silver metal to burn?
I'm afraid an outside cause might burn the new Relay too. Is burning caused by bad Relay?
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1986, 300, 300sdl, 86, benz, biodiesel, bypass, diesel, filter, fuel, hose, injector, mercedes, radiator, removal, sdl, strainer, tank

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