While traveling on a road trip my 85 300TD wagon started slowing down on hills. It seems the problem may be air bubbles in the fuel line because when I rev the engine the primary filter gets extremely foamy. I tried bleeding the air the normal way with the primer and loosening the top bolt on the fuel line but after the air seems cleared out the whole thing begins again. I checked connections at the tank and at the filters but all seem tight and with no leaks. The weird thing is that it is intermittent—like day to weeks before it occurs again. A possible clue: Before this all happened, a mechanic that blew air back to the tank as a kind of maintenance.

Also, the fuel flows regularly out of the tank (I disconnected the lower hose and checked)

any thoughts?

It seems like you would have to have a bad connection somewhere or your fuel line is allowing air to get in. You might not see it, but I would feel the lines to see if they are soft, and replace them if so. if a mechanic was messing with your lines then maybe they didn't make a good seal when connected back together. If you have enough slack in your lines, you could cut just the end of the line of at the primary filter so that you have a fresh end of hose to reconnect. Maybe the primary filter itself is letting in air.

look for rust on the fuel lines...pay special attention under the clips that hold it to the body...also look for leaking rubber hose at tank...sounds like you have the beginings of an air leak between the tank and the plastic filter.

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Proud owner of ....
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1979 300SD W116
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

Can't help you much on your problem, but:

Shubh Devali

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1998 W202 C230 - The money pit of late.
1984 W126 300SD (356,800 miles) Gone to the wrenchapart
1984 W123 300D Gone to the wrenchapart
1972 W108 280SE 3.5 (sold but not forgotten)
1986 Buick Grand National 3.8l Turbo (86k miles)
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Replace all your rubber lines. Not that expensive and not that hard. When the original rubber lines (they have the cloth outer wrap) start to go, they leach air in when running and leach fuel out when stopped. Make sure you include the line with the threaded peice that comes out of the tank. I just ran the tank low on fuel then drained what was left into a bucket.

I've done this to both the benzes that I've had, both were beginning to drip below the tank. I noticed I went from some bubbles in the primary filter to none after the job was completed, plus no more leaks

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'85 300D - 206K (sold)
'81 240D - 149K (sold)
'03 Jetta TDI - (sold)
'79 300D - (sold)

It is normal to see small air bubbles in the clear line from the IP to the secondery filter. When did you change filters last? Your mechanic coulda stured up crud in the tank and its now plugging the filters. (and/or tank screen)

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"

A clogged filter or tank screen can cause something like that.. What can happen is that as the filter or screen clogs, the pump keeps on pulling, and since it can't suck enough fuel through, it begins sucking whatever it can, through whatever orifice, meaning air bubbles. A connection that is normally secure might begin to leak air when subjected to serious suction.

I'd check the filters and tank strainer first, and if they are OK, then go on to the hoses and hose connections like people have mentioned.

peace,
sam

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"That f***in' biodiesel is makin' me hungry."

1982 300TD Astral Silver w/ 250k (BIO BNZ)
2001 Aprilia SR50 Corsa Red w/ 5.5k (>100 MPG)

Thanks everybody, I changed the pimary filter and put new hoses on the fuel line and the foamy bubbles are gone.

Priya