I have a 1991 350SD Turbo that I recently put back on the road. The vehicle has 190k miles and sat for a period of about 2 years. I know the horror storied about this powerplant, however, the prior owner replaced all the rods - which I have for proof! He got fed up with the little things that were going wrong with the vehicle. With a little TLC, some major annoyances turned out to be minor repairs. The vehicle burns little to no oil and is a good runner.

I took the vehicle on a 600mile highway trip from Denver to KC. The vehicle averaged 17 to 20 mpg. I need to get the average back to where it should be for an easy highway trip.

I had the vehicle aligned before leaving, however, did not do it at a MB shop. In KC, I had an MB shop align the vehicle after replacing tie-rods that were worn. Alignment is much better, mileage a little improved.

When I got the vehicle the remaining fuel in the tank was pitch black and the vehicle smoked excessively. I drove the tank off, replaced all fuel filters and air filter and filled up with fresh diesel. That worked miracles and the smoke levels went down and idle smoothed right out. The engine timing was verified my a good friend at the MB shop.. He is the service manager there and doesn't charge me for those types of things.

Where i think I have a problem is actually figuring the mileage. The fuel tank does not consistently fill. For example, I can fill her up and drive 60miles. Upon returning to the station, I can only get 1/2 a gallon of diesel in her. I can then drive for 80 miles and bring her back and fill it with 6 or 7 gallons. Also, when I open the fuel cap, I hear a vacuum sound that I do not hear on my other diesel vehicles (300SD and E300D).

I am a long way from home and can't get under the vehicle. Is there anything that I should suggest that a mechanic do.

I figure I need to take care of this problem before I try to fix a mileage problem that may be nothing more than a weird tank. what else can I do to help mileage? I have new tires and brakes as well.

I am used to my '78 300SD which can easily achieve 27mpg at 75 - 85 mph. I'm not asking for the world but would like to get to the mid 20's consistently. Does it just need more driving because of all the time it sat?

Please help.

Richard

Wow, that tank thing is kinda strange. Try pumping at half-speed for the last 1/2 of the tank fill that should keep the foaming to a minimum. Regarding the tank developing a vacuum I have seen that problem in some trucks so bad it actually deformed and "imploded" the tank. Try leaving the fuel cap loose so it can breathe for a few tanks and see if the mileage improves. (Don't leave it loose if parked/driving in heavy rain, it may find a way in) Maybe the cap vent is not functioning correctly? Did you change the air filter? If so check the air intake system. Since the car sat maybe varmints built a nest in the air intake system and it is restricting air and killing your mileage? Thats all I can think of... RT

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When all else fails, vote from the rooftops!
84' Mercedes Benz 300D Anthracite/black, 171K
03' Volkswagen Jetta TDI blue/black, 93K
93' Chevrolet C2500HD ExCab 6.5TD, Two-tone blue, 252K

I just filled up on my way home tonight. The foam "burped" out the filler hole. It's like the air vent on the tank is clogged. The caps do not have vents like gas cars. I have never cleaned an air vent out before. When filling it tonight, it kept gurgling air out the filler like it had no other way out of the tank.

Anyone ever clean out a tank vent before?

My other diesels are vented.... RT

__________________
When all else fails, vote from the rooftops!
84' Mercedes Benz 300D Anthracite/black, 171K
03' Volkswagen Jetta TDI blue/black, 93K
93' Chevrolet C2500HD ExCab 6.5TD, Two-tone blue, 252K

bisric,

I know the older cars (W123 and earlier) have a venting system that is prone to being clogged if you overfill the tank a lot. However, the foaming of Diesel fuel at fill up is not handled by this system. Depending on how you fill the tank, the rate of foam making can very quickly fill a quarter of the tank with foam. I typically turn the nozzle on the lowest rate of filling, which lets the foam return to liquid as the tank is being filled.

The main function of the venting system is to allow air back into the tank as you use the Diesel fuel up. When it clogs you cannot, or at least have to work really hard to, get the fuel cap off. Eventually the car will stop running and your tank indicator will begin to read full when the reserve light is on.

The 350SD may not have a venting system as EPA regulations on evaporative emissions may have applied to the 1991 models. Someone with a 350SD or SDL will likely pipe up on this subject. I think the system on my E300D TurboDiesel is sealed and either lets air in or lets vapors out when the cap is removed.

The determination of mileage requires a consistent filling technique, or, the data must be gathered over numerous fill-ups. I fill up at a few stations, and always at the same pumps at those stations.

As for what is in your tank, if it turned black chances are it was alive, and running the tank near empty then filling it up again, will not stop the stuff (algae) from growing again. You need to put some algae killing additive in there and replace the prefilter (the in-line one) pretty frequently until the problem is gone. There is also a screen like filter in the tank that may need to be cleaned.

So, I would not be surprised if your car does not have a simple vent, and the burping of foam out the filler openning is not unusual. Slow down the flow rate out the pump nozzle significantly and you should not have the foam burping out on your hands or the car paint. Good luck and I hope this helps. Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)

Well, I have a turbo diesel myself, never had that kind of problem.

I bought it one year ago, pending is alignment and injector callibration.

My car is yealding about 25 mpg in combined use, but I never attempt to make a calculation with a few liters. You must to that for a whole tank of fuel.
The car has 250k of use.

Besides that, yr tank must breath, your air filter must breath, yr fuel filter must be kept in condition, and you should make an injector callibration and cleaning. As to have a secure base to begin getting normal fuel economy.

Oldbeaver

How did you manage to find this 11 year old thread?

__________________
Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.