My car is a '95 E300D OM606 Non-Turbo Engine. Its starts, Idles, and drives flawlessly with one problem, but here is a bit of a background first. I park the car indoors on a perfectly level heated garage two floors below street level. When I start it up in the morning as I said its starts fine, but if I step on the accelerator and let go, it will stumble, and almost stall. It seemed to have been doing it randomly and that was the frustrating part. Occasionally it will never do it and sometimes it will do it consistently. When it does stumble it only does it for the first few minutes, and its fine from then on.

Its not ambient temperature related, as it will do it when its either 80F or -20F outside. Its been doing it since the summer. Its strictly time dependent in the sense that it depends on how long the car has been parked. The glow plugs are relatively new, and their resistance checks out fine so please don't send me in that direction. (It will start at -20F without a block heater)

I finally however did discover a pattern as to when it will stumble. If the fuel Tank is anywhere above the 1/2 mark it will NOT stumble, as soon as the fuel in the tank goes below the 1/2 mark level it will start stumbling in the mornings, and the stumbling will get progressively worse as the tank empties. When I fill it up again there is absolutely no stumble.

Here is a bit more info. Fuel Filter and pre-filter are OK. Tried releasing the Fuel cap prior to starting on a less than 1/2 tank, but it made no difference. I can't tell if there is air in the lines because my lines are brown, and are not clear anymore, but if there was air in them why would it only be there when the fuel tank is less than 1/2 full?

I am stumped. Any ideas.

Thanks

Phil

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'95 E300 Diesel, 264,000 Miles. [Sold it]

I'm not familiar with your car's fuel system. Is there a strainer in the fuel tank that may be partially clogged?

When was the last time you changed the primary and secondary fuel filters? Sounds like a restriction issue since you mention no problems above 1/2 full!

Both the primary and pre-filter were changed within the last year or so. Coincidentally I removed the pre-filter today to look at it. It looks brand-new. Not a speck of dirt on it. No algae, no nothing.

Phil

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'95 E300 Diesel, 264,000 Miles. [Sold it]

When you get below 1/2 tank, un-cap the fuel tank.....if you hear a vacuum swush in, see then if you continue the problem!

Clogged fuel vents happen!

Vacuum leak test the fuel system.

Sounds like a micro air leak.

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bet you have a computer problem

don

Sounds like you are getting air in the system, perhaps there is a pin hole in the pickup and the fuel level has to be that low for it to bleed off. Regardless your symptoms definitely sound like air related. If your injection return hoses are weeping it could also contribute to this symptom. If fuel can get out air can get in.

I second the air infusion theory of the fuel lines. If they are brown, they're old and probably need replacement. Each has an O-ring on each end that can fail. Some failures leak fuel, others admit air which causes stumbling or hard starts. I replaced the fuel lines in mine and still had a massive air leak that could be traced back to the prefilter, which had a bad O-ring. The new clear fuel lines made it easy to view the air leak and follow it to its source.

Your responses convinced me that an air leak is the most likely cause, but it still makes no sense. The reason I am saying that is that if the air leak is past the pre-filter than It should not make a difference how full my tank is. It should be there wether I have a full tank or not. On the other hand, if the air leak was somewhere in the vicinity of the fuel tank, than, once I start the car, it should take awhile for the air to make its way to the Injectors. Yet the condition that I described happens immediately after I start the car.

Having said that, and agreeing that a pin hole is the most likely cause, in which section of the fuel delivery system would such a pin hole likely be. Would it necessarily be a visible leak? What is the best way to find it?

Thanks

Phillip

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'95 E300 Diesel, 264,000 Miles. [Sold it]

Phil, I've been mulling over this for a bit and have the following theory:

When the tank is full, there is a slight head pressure in the fuel lines due to the height of the fuel in the tank. This pressure is sufficient to keep the air from leaking into the line at the point of the air leak. On an low tank, the head pressure drops, and more air will infuse at the location of the leak.

The time that the vehicle remains not running should also be a variable in this situation. The longer it sits with a low tank, the more pronounced the problem will be.

Roy probably has the best suggestion..........a vacuum check of the fuel lines........but, I don't know the proper procedure or tools for this.

I still agree with the theory, BUT keep in mind that the symptom only occur when the fuel tank is less than 1/2 full. So if the leak was somewhere in the rear, or at least before the pre-filter, than it should take the air a few minutes to reach the injection pump once the car is started. This is not the case, as it happens immediately after starting the car.

On the other hand, if the leak was somewhere more forward, ie: past the pre-filter, than the amount of fuel in the tank should not make a differenece.

Keeping the above in mind, than where would such a leak likely be?

Phil

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'95 E300 Diesel, 264,000 Miles. [Sold it]

Since the stumble is immediate, it says that the leak must be very close to the IP.........after the secondary.

The amount of fuel still matters. The pressure in the entire system is greater with more fuel in the tank........thereby mimimizing the capability of the system to take on air through the point of the leak.

Do you have a tank strainer? Is it clear?

A car enthusiast friend of mine just called me to suggest another possibility. Although he is not a Diesel expert he said that most cars have a "Check Valve" that prevents the fuel from going back into the tank when the engine is stopped.

If that's true in my case than maybe that valve is malfunctioning letting the fuel drip back into the tank, but only when the tank is less than 1/2 full, as there is less back pressure in that case. When the tank is full, than the back pressure in the tank prevents the fuel from going back into the tank. It makes sense. Does anyone know if this car has such a valve, If so where is it?

I also used a very strong flashlight, and looked at the formerly clear (now almost opaque) fuel line that enters the pre-fuel filter. I noticed a 1/2" air gap there. When I started the car, the fuel was only dripping in, not ever filling that void. That does not sound normal either.

Phil

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'95 E300 Diesel, 264,000 Miles. [Sold it]