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Old 11-24-2009, 03:05 AM
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Join Date: Jun 2007
Posts: 9
Fuel system trouble shooting

Does anyone have a systematic approach to diagnose a problem with the fuel system? I've got a starting problem that's been getting progressively worse, and now the 85 300CD just won't run.

Previously I would have to crank it for a minute in the morning (in FL and about 75*F in am) to get it started. Car would run fine until turned off. The longer the car was turned off the longer it would take to start during the day. A longer than normal start would occur when the engine was fully warmed up, so I don't think it's a glow plug issue, and all 5 were replaced 6 months ago. I suppose compression could be the problem, but I'm leaning toward the fuel system.

I've tried switching the intake and return fuel lines to elliminate a clogged tank screen issue. Have bleed the system several times at the filter housing, and the injectors getting air bubbles every time. And now I'm just stuck. There must be a factory trouble shouting tree for diesels. If anyone can share such an approach I'd really appreciate it.

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Old 11-24-2009, 03:12 AM
Posts: n/a
Valve adjustment?
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Old 11-24-2009, 05:20 AM
oldsinner111's Avatar
a parent should die first
Join Date: Nov 2008
Location: bristol,tn
Posts: 5,037
One major problem with Mercedes Diesels is that you really need German Hose Clamps on any conection.Don't buy the hose clamps that bite into fuel lines.
With your German clamps now you can tighten the hoses so they don't spin on a conection.I always trim the ends of a hose,when I replace a filter.
Check your tank hoses too.I mean a fuel line may not leak,but suck air.
usaf veteran ecm repair, ex hippy from california,sober,no drugs or alcohol since 1985,bitter old man,waiting for WW3 lost my daughter in 2015, do to her druggie husband,s320 best time with nos 14.6 1/4 mile
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Old 11-24-2009, 08:46 AM
moon161's Avatar
Formerly of Car Hell
Join Date: Mar 2008
Location: Buffalo NY
Posts: 1,689
If you always have bubbles going to the injectors, I suspect fuel.

A few questions:
Ever change primary or secondary fuel filters- when?
Does the primer pump make it better?

A few things to try:

Changing the fuel filters.

The primer pump.

Get it running, look at the plastic secondary fuel filter- You should see it trapping bubbles, you may see air getting sucked down.

Put supply & return in a jar of diesel (mark them first, hard lines as well) prime it and start it. Get it running well, crack lines clear injectors of air. Shut it down, put the jar ABOVE the IP and walk away for a while. If it starts good, I think your system may be losing it's prime. Might be easier to just jack the rear end.

It looks like it might be a bad lift pump or leaky lines causing you to lose prime.

All things are burning, know this and be released.

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Old 11-24-2009, 09:40 AM
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Join Date: Sep 2004
Posts: 8,413
Originally Posted by moon161 View Post
Get it running, look at the plastic secondary fuel filter- You should see it trapping bubbles, you may see air getting sucked down.
That would be the primary filter.
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