Low Fuel = Rough Idle + Uphill Battles - PeachParts Mercedes-Benz Forum

GreenMedallion · #1 03-11-2008, 02:40 PM

I had a bit of a scare this past weekend. My tank was below the 1/4 full mark. This usually means my idle is a bit rough. Otherwise, with a full tank, it is rather smooth. However, with my low tank, and climbing a long uphill, my 300TD simply lost power. I made it to the top, but even then it seemed to be sputtering. I would accelerate slightly, and then it would slow down. With cars passing at 75, and my car refusing to get over 45, it was getting a little tense! After a minute or two of this back and forth, acceleration and stoppage, it finally started to run normal. I suspect this was simply due to low fuel, and sucking on air during the uphill. But my fuel gauge was not in the red by any means, and I have never had this problem before.

So my question - is there some problem here other than just having low fuel? Is there a vacuum issue hidden in here somewhere?

TMAllison · #2 03-11-2008, 02:51 PM

I would expect clogged filters or tank screen allowing insufficient fuel at higher demands.

barry123400 · #3 03-11-2008, 03:00 PM

Air leaking into fuel system? As fuel demand goes up more air is sucked in as well? Weak lift pump possible as you state normal with tank above 1/4 full? Dirty tank fuel screen forcing fuel aquisition to be too hard? When fuel level is higher gravity contributes to helping fuel flow through the screen.
It is not pre and main filters I suspect but they could contribute to the problem somewhat if partially restricted.
Fuel lines need cleaned out? In otherwords it sounds like a pretty economical problem to fix just a little labour on your part basically. Kind of a tuneup of the fuel system.
Also make sure your tank vent is totally free and you are not working against tank suction. It may only be mostly obstructed and not totally for example. Never hurts to clean the vent system out anyways as the problem is usually on the exposed end of the vent line.
You should not get a whoosh of air entering when removing fuel cap. If vent totally obstructed car would totally quit after about 100 miles on average. Thats if there were no reverse leakage of air from the fuel line back to the tank to equalise the negative pressure a little.

nhdoc · #4 03-11-2008, 03:06 PM

Weak lift pump and/or air leak into the suction side of the pump fuel line also comes to mind as a possible cause of the symptoms you describe.

GreenMedallion · #5 03-11-2008, 03:11 PM

I looked up some past posts on the vent line, but many of the pics or diagrams are for a sedan tank. Anyone know where the vent line is on a wagon tank?

I will check that first, as I have been pretty good about checking and changing all my filters. I did the fuel tank screen about 5000 miles ago. My first thought was air got into the system, so hopefully cleaning out the vent line will solve this.

barry, can you explain more about this lift pump thing? not sure what that is, or how i can investigate.

mytimeyet · #6 03-11-2008, 03:16 PM

I had similar symptoms with a old style prime pump that was leaking. Apparently when the tank was full the pressure was such that the fuel leaked and when the tank was less than 1/4 the pressure was such to allow air into the lines.

MB-Owner-in-ind · #7 03-11-2008, 03:21 PM

I ahve the same problem with my 77, under 1/4 tank and it actually runs out of fuel. Try interchanging the return and supply lines right at the first filter and see if this fixes it.

GreenMedallion · #8 03-11-2008, 03:42 PM

Maybe I should add that I primarily run on B100 (have for a year), and that I have an older style primer pump (unscrew+pump). Also, I replaced all the rubber fuel lines, so there should be no problem there.

GreenMedallion · #9 03-11-2008, 04:24 PM

Just went out and checked a couple things.

Vent line is clear. It is located in front of the rear passenger side tire. The third of three lines leaving the tank, the only line that does not continue toward the engine.

Primer pump is leaking. Maybe no surprise there. Not something I've noticed in the past. Each pump provides so fuel overflow out of the pump. I suspect it also leaks air. Seems like an easy fix.

GreenMedallion · #10 03-13-2008, 06:38 PM

This forum is awesome.
Using this wiki - http://www.peachparts.com/Wikka/PrimerPump - I replaced my pump in about 5 minutes. Good as new. Thanks everyone.