300SD Fuel Issues - PeachParts Mercedes-Benz Forum

Rocketrider · #1 10-07-2008, 11:10 AM

I'm having an issue with my 82 300SD and now it's time to consult with the Grand Poobas! I bought the car with 152K miles and a bad tranny for $450. I just happened to have a parts car of same model with a bad engine and good tranny. The car had been sitting for almost two years, with very little history before that. I replaced the tranny and now that I'm driving the car, I have discovered several issues.

The engine ran great when I first drove it with 1/4 tank. Then I filled it up and it ran great until I got to 1/2 tank. Then it started acting like I was running out of fuel. It spits and sputters, loses power, and eventually dies. It gets worse and then won't do anything other than idle at 3/8 tank or less. I fill it up and it runs great until it hits 1/2 tank again.

Now here's the kicker! When I was left stranded, I pumped the primer at the lift pump and had lots of air in the lines. The engine cranked immediately and ran great for a quarter mile or so, then it died again. Pumped the primer again and the engine cranked immediately. I finally got the car home and now the primer doesn't work at all. It takes ZERO effort to actuate the primer and it never gets firm. Moreover, it is leaking fuel around the shaft of the primer. Obviously it needs to be replaced and is leaking air into my fuel system.

Now, is it possible that the bad primer is what was causing the engine to run like crap below 1/2 tank? I'm told that above 1/2 tank the fuel level is higher than the lift pump, which would explain why it ran so well. Is it possible that I have a bad lift pump as well?????

Graplr · #2 10-07-2008, 11:20 AM

I'm guessing that how full the tank is, is totally unrelated to the problem.

Here is my guess-

You first only put 1/4 worth of fuel in the tank. No noticeable problems.

Then you filled it up more. The primer pump leak got worse and it is taking in more air. So it seems how full the tank is, is related but in actuallity it is not.

My vote. Replace the primer pump to stop the air leaks. See how it runs. Always start with the simple stuff first.

blackestate · #3 10-07-2008, 11:24 AM

Replace the primer pump, and the filters. You could have all kinds of stuff in that old fuel.
May need to pull the screen in the tank also, but I would do the other filters and see what happens from there. Also treat the tank with some biocide if in doubt

Rocketrider · #4 10-07-2008, 11:25 AM

That's what I plan to do. I know the primer is busted, so I'll definitely be replacing that first. However, I drove the car for 3 weeks on 3 tanks of fuel and I could not get more than 1/2 tank and 220 miles without the issue rearing its ugly head. I guess we'll see. Thanks for your input.

Biodiesel300TD · #5 10-07-2008, 11:29 AM

I too would suggest new filters, and then if that doesn't solve the problem I would pull the tank screen and have a look at it. Make sure you put on a new o-ring when you put the tank screen back in. You don't want it leaking with a fuel tank.

Rocketrider · #6 10-07-2008, 01:06 PM

The previous owner replaced both fuel filters, oil change, adjusted valves prior to transfer. While I am not ruling out that the filters may be the problem or contribute to the problem, I can't help but question why it only acts up at 1/2 tank and below. If I were starving the engine due to clogged supply then it should act up all the time. I am ordering a primer this afternoon. If that doesn't fix it, then I'll go after the filters and strainers.

There are a couple different types of primer out there. Any recommendation on which to purchase? There's the one that looks like the OEM with the white plastic plunger and another that has a black plunger (looks like a shotgun shell).

rrgrassi · #7 10-07-2008, 01:37 PM

Fuel lines could also be cracked, as well as the injector fuel return lines. Just because the PO replaced the filters, does not mean a thing since the car sat for two years.

Biodiesel300TD · #8 10-07-2008, 02:29 PM

Might also check the tank vent. If the car sat for a while a mud dobber might have decided the end of your tank vent line was a good residence. You can check to see if this is the issue by driving for a good while then pulling the fuel cap off. If you get a big hissing noise then the tank is under too much vac, and your vent line is clogged.

johnathan1 · #9 10-10-2008, 04:45 PM

Where is the fuel tank vent line located, and how can I unclog it?

Rocketrider · #10 10-10-2008, 06:31 PM

Okay, I replaced the primer and fuel filters. I also replaced the fuel injector return line that goes to the banjo bolt on the filter housing as it was leaking. It seems to be getting fuel, but I have to constantly pump the primer to keep the car running. I was thinking it could be the primer pump assembly, so I removed it to have it checked out.

When I removed the primer (lift) pump, about a pint of engine oil came out of the injector pump. Is this normal? Will I need to add oil back in there or does it get oil from the engine? I didn't notice any oil supply lines to the injector pump. I hope I didn't mess anything up.

Rocketrider · #11 10-11-2008, 08:53 AM

Bump...can anyone help?

Rocketrider · #12 10-11-2008, 09:03 AM

Bump...can anyone help?

Maxbumpo · #13 10-11-2008, 09:06 AM

Oil is supplied to the IP by the engine, no worries there.

Could you have mixed up the supply and return fuel lines? That will cause exactly what you describe (but usually at a lower fuel level).

Try blowing air back into the fuel tank; if the 'supply' line blows easily but the 'return' line takes more effort because it's really the supply line and the other end is at the bottom of the fuel tank, then you've found a problem.

Once you replace the main fuel filter, it takes like 50 to 100 strokes of the primer pump to get all the air out of the system. You need to vigorously work the primer until the noise changes to a whistling sound and the force required goes up.

Rocketrider · #14 10-11-2008, 09:13 AM

That's a great idea! I'll give it a shot today. Thanks.

jim16671836 · #15 10-11-2008, 09:36 AM

I always fill the large filter with partial gas & diesel and have never had to pump anything... Never install the filter empty... The small filter will be fine installed empty and you will not need to pump unless the mechanical pump is going bad or you have other issues... You can disconnect the line from the large filter to the injection pump and check for pressure... should have 4-5 pounds when the engine is running... If not then you have either a bad pump or blockage.. Yes the fuel lines can be hooked up backwards.. It would surprise me if the engine would run. I also run pure uvo(used veggie oil) and sometimes it is necessary to use compressed air and blow out the fuel line to the fuel tank.. You might want to cover the fuel inlet with a rag if you don't want a mess... If you have alge then you need to check and see if the fuel is black ot at least darker than normal... that stuff can plug your filters... Half gasoline and diesel in your large filter will clean out your injector lines and your injectors and is much cheaper than using the wall street cleaner..Jim