Help I have an unusual problem
 

I have a 1985 Mercedes 300D all of a sudden it is starting very hard. I live in Florida the temp is 70 now so don’t think it is a cold start problem. I checked the glow plugs per the manual one was bad changed it out still have the problem checked the relay am getting 12 volts at all plugs. I changed both of the fuel filters no change. The problem is it will start after about 20 or more seconds of cranking with the accelerator the floor then a plume of smoke comes out. After it runs for a bit it will start with one crank if I let it sit for 4 or more hours the problem starts over. It has always started the first turn of the key.I have done a search and have not found any threads that described this problem. Any help will be appreciated.

Were there any odd sounds, smells, or vibrations before this happened? Did the performance change? What color is the plume of smoke?

I would get a compression and leakdown test.
Just a hunch, did this happen after you got a new tank of fuel?

Change your fuel filters. After this, check to see if there is any ALGAE in your fuel system....If so, use a BIOCIDE and get ready to change your filters a couple of more times, along with draining the fuel tank.
Good luck and best wishes always,
spo out:cool:

Check the primer pump on the side of the IP for leaks.... sounds like you may be getting air in the fuel system somewhere. Check all the other fuel lines for leaks too if the primer pump isn't the culprit.

I'm with Wasuchi. As I understand it, if there is fuel (with no air) at the injectors, there should be smoke the whole time you're cranking. If the primer pump is the old type or you've developed a leak in a fuel line you could lose prime hence the difficluty after four or more hours of sitting.

Good luck!

At that ambient temperature you really don't even need to preglow to get it started. SO it can't be GP problems...

He did say he replaced the fuel filters already so it can't be a clogged filter. But if the old filters had black specks in them you have algae. It almost sounds like you're not getting good compression.

I doubt that low compression is the culprit as that would have not have happened all of the sudden unless something gummed up the rings. Did it happen right after a refueling? Maybe someone sabotaged the car, is the car left outside? Otherwise my bet is in the primer pump.

Why do you hold the accelerator to the floor?

Change the other four glow plugs regardless of ohmmeter readings.

Good luck,

Come on Larry, why change all of the glow plugs? That will waste 50 bucks and 2 hours. He did the factory test. If he really wants to verify them, he can connect a battery charger to each one individually and verify current flow.

Absolutely no need to change them first.

240Joe

The easiest way to test them without even pulling them out to test them with a battery is to turn on the ignition to let them glow until he hears the thunk from the GP relay once or twice when the engine is cold. Then all is needed to be done is to see if the plugs are hot to the touch. The cold ones will be the bad ones.

I still vote for air in the fuel system.:D

Hi Dave.

When was the last time the valves where
adjusted?

Louis.

I second the air in fuel lines
 

Sorry to say; your problem is common.
Sounds like an O ring or line has cracked.
The injection pump sucks the fuel from the fuel tank.
When an O ring lets go, the first sign is harder starting and longer crank time.
When the leak gets large enough, the injector pump will be sucking air.
You need to check the line from fuel tank to injection pump.

As "Wasuchi" said:
Check the primer pump on the side of the IP for leaks.... sounds like you may be getting air in the fuel system somewhere. Check all the other fuel lines for leaks too if the primer pump isn't the culprit.

Note:
The L shaped clear filter connects to your primer pump.
Have a great day.

Dave1,

Turn on the cabin light before starting. Observe if it dims noticeably when you turn the key to the pre-glow position.

This will tell you if the GP are working. Is should stay dimmed for about for about 40 seconds at which time you should hear the GP relay click open. If it doesn't dim, the GP are not working.

If his test shows the GP are working, go back to checking the fuel system.