TD inconsistent idle hypothesis - PeachParts Mercedes-Benz Forum

kerry · #1 03-23-2007, 02:29 PM

I've posted in the past about a 'hiccup' in the idle on my TD. Others have posted about similar issues with the TD. It is not there all the time. Yesterday on the way home from work, it was 'hiccuping' at idle. This morning, I noticed it was gone. The difference: I filled up with fuel at the end of the day yesterday. Could this be the source of the problem?
The TD has a much lower fuel tank than the D's. Is the TD more susceptible to a weak fuel pump delivering inadequate supply at idle when the tank is low? We hardly ever hear accounts on this board of bad fuel pumps. Is this because the high tank in the D's don't even really require a fuel pump? Others have posted about poor idle on a TD when pointed dowhill. Could this also be explained by the location of the fuel tank and fuel pickup?
I connect these thoughts to my observation that there was no hiccup when I purged. I assumed this was because of the nature of the fuel. Perhaps not. Perhaps the lack of a hiccup while purging is simply explained by the height of the fuel.
I will test this hypothesis over the next few tanks of fuel. Does my hiccup appear once the fuel level in the tank drops below 'X'?

Your thoughts?

Biodiesel300TD · #2 03-23-2007, 03:20 PM

I've not heard of this before, but that doesn't mean it doesn't happen, cause I certainly don't know everything about these cars.

How low was your fuel when you felt the hiccups?

I would think it would have more to do with the fact that the wagon's tank is laying flat instead of upright like the sedan's. So you have larger area for fuel to spread out on the bottom of the tank.

dieseldiehard · #3 03-23-2007, 09:38 PM

Hi, kerry edwards, yes you are right on as to a possibility!
It is true that the lower fuel tank places a lesser pressure on the inlet to the uptake pump than in the sedans which have a tank way higher in the car.
I have a 300TD with this issue and I've been living with it for years, why just today I noticed it running crappy and saw it was near the reserve so I filled it up (ULSD at $2.99 a gallon) and its now running like a top. Same symptoms as yours, right?
Someday I am going to swap the pump off an IP have stored away.

There is another possibility and that is a tiny opening in the fuel line that allows air to be sucked in when the fuel is low as that is when it has less head on it and lower pressure against the hole and then when the pump is pulling fuel some bubbles get sucked in because it offset the slight downward pressure from the tank. At least that's the theory. You should be able to see the bubbles in the prefilter, and the tiny hole would weep fuel when the tank is full perhaps even leaving a few drops on the ground. I was told finding the hole is difficult unless you have the car on a lift and a method of getting fuel to come out good is to pressurize the nearly empty fuel tank by placing a rubber glove over the filler neck and pressurizing it LIGHTLY with an air hose, I guess a couple psi is all. This I learned from an old pro, Larry Thomas who is no longer with us. The holes develop from wear at a clamp or from rust (or both I suppose!)
If you do fix yours let us know which one of these factors it was, I can't imagine anything else that would cause this problem.
DDH

Bruce Kennedy · #4 03-23-2007, 11:28 PM

My td had a similar problem that turned out to be a clogged tank screen. Search my post for a cheap screen removal tool.

dieseldiehard · #5 03-23-2007, 11:54 PM

I looked at my screen and it wasn't plugged. In the wagon that's easy to do compared to the sedan, just lift the floor under the cover deck (unless you have the rear facing seat) and unscrew the level sensor, using a flashlight to see the bottom.