It takes a lot of Cranking to get the Engine Started; if caused by the Fuel migrating back towards the Fuel tank due to the Pump Valves not sealing. (However, a small Air leak in the Fuel Inlet Hoses or a crack in the Plastic Filter couls also cause this.)
Ounce the Engine is going the Engine does not want to idle well due to Fuel staration; not enough Fuel Volume moving through the Pump to maintain the minimum pressure. I this case the Engine will run fine ounce it is off idle. But, again this symptom can be due to other causes.
There are several Test in the Factory Service Manual.
For some of the Tests you need a Liquid Filled/dampend Gauge or some sort of restricted fitting to dampen out the impulses from the Fuel Supply/Lift Pump because the Gauge Needle will bounce around too much to be sure of your reading.
There is also a test where you see how much Vacuum the Pump pulls on the inlet side.
I do not remember the details of the tests so I cannot describe them.
i have an 82 300d with same symptoms, have to depress accelerator pedal to start car and when the engine is cold soaked say 50 deg or less first start of day car has hard time idling, even stalling a couple of times, have to keep slight pedal pressure to keep idle. once car is driven a mile or two and starts to warm up it is fine. i drive the car alot, i have put 29,000 miles on it since april 2009. total mileage is 169,000. since i bought the car then i have adjusted valves twice, replaced all 5 glow plugs and updated to a monarch glow plug relay, done 2 diesel purges, replaced both fuel filters twice and add diesel fuel lubricant additive at every fill up. i will be throwing a lift pump at it this week and i will definately update this page with my result, should have it installed by the new year at the latest. has anybody changed a lift pump before that could give some useful information on what to expect , that would greatly appreciated. if this is not the problem i just hope it is not a tired engine. i guess i will do a compression test eventually to find out. FYI i found the lift pump on europartsdirect.com for $139 with free shipping, comes with gasket and copper washers.
well i replaced the lift pump, took about an hour and a half. problem still there, so i can only assume that compression is the problem. when i get the urge i will do a compression test. it may be a while before i get around to it but will update when finished
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