Our 240D has been running well after a recent fuel tank drain and a dose of biostat.
We added a bottle of redline at last fill up. After fifty miles or so (the past couple of days) the motor got noisy at idle. Today I took it out and drove it very hard for twenty miles and the roughness went away.
Now however there is air in the fuel line, that is, a very large bubble in the clear plastic fuel filter, and when I operate the manual fuel pump, small bubbles of air well up from the fuel pump side of the filter.
It's like an air lock. But the air seems to be coming from the fuel pump.
There aren't any loose fuel connections. The in tank fuel filter is clean, and both motor side fuel filters are less than a month old, as is the manual pump.
Does this sound like anything anybody has run into before? Is there some kind of check valve in the fuel pump that might have failed? Thanks for any suggestions.

Harvey

Many of the fuel pre-filters that I see have a bubble in them...I think that it is normal. One way to tell if you are infact getting in air is on start up. If you have VERY hard starts, you have air in fuel system.

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Benzmac:
Donnie Drummonds
1992 500E (very soon I hope :)
1981 280GE SWB
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
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PARTNER IN MERCEDESSHOP.COM

That's my main problem at present. After the car sits for an hour or so, hot, it won't start. When I operate the hand pump I can look at the clear filter and see little bubbles of air coming back up from the pump body. There's a large brass bolt head in the pump body just below the fuel intake line. It must be a 26-28 mm bolt. I was thinking about taking that loose and seeing what was in there. Is this a good idea?

It sounds to me that you have a leak in the fuel line. The leak can be so small that a noticeable amount of fuel will not escape because the fuel line is not under pressure but will still allow the fuel pump to suck in air. There is almost always an air bubble in the first fuel filter. This is OK.

Pressurize your fuel line by pumping fuel into it from the tank end and look for a leak. It may take some time to find a small leak, but eventually fuel will appear. I used an electric fuel pump to find a tiny leak in my truck diesel this way.

P E Haiges

At the very least, replace the fuel hoses on both sides of the clear plastic fuel filter. If they're the old braided-cloth type hoses, they're probably the culprit. Replace them with rubber hose. The dealer game me about two feet of rubber fuel hose for free when I bought both fuel filters. I guess I paid for it, though, through the usual dealership markup.

Anyway, I had the exact same problem a few months ago, and replacing these hoses cured it instantly.

- Nathan
'83 240D, 250k miles

P.S. Maybe the algae you killed with the biostat was what was stopping the leak. When the algae went away, the leak opened up. :D

- Nathan

I will change those hoses. Does anybody know what's going on inside the body of the piece that attaches to the main body of the fuel pump and it receives the manual pump and a fitting for fuel in and fuel out? It's Bosch part number FPI/K22MW21. There's a big brass bolt head sticking out of it below where the manual pump screws in.

Harvey

While I'm thrashing around with those hoses - there's a piece of return hose that's somewhat bulged out across it's length. Maybe four inches. Is it necessary to replace this hose with a specially formed piece of hose from a dealer, or can you use a stock piece of pressure hose?

Harvey