I promise this isn't like the "the car runs better after it's washed" theories. My car honestly runs better with five gallons of fuel or less. Any thoughts why that might be? I've speculated that it may be because the fuel is being circulate more through the return lines which could heat it up, or possibly filter the fuel more. Or is it symptomatic of a problem that exists? i.e. lift or injection pump? Thanks in advance.

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1998 E300D, 287k, barely broken in.

Less fuel = Less weight?

I should have addressed that previously. I've thought about the less weight. But it's more dynamic than that I think. The car runs a little smoother, less smoke at idle (I normally have a little whisp of grey, where it goes down to almost nothing), and runs a little more quiet.

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1998 E300D, 287k, barely broken in.

Plugged fuel tank breather?

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

back pressure from the fuel return lines to the injectors.

?

I am not sure of the routing, but the return line to the tank is pretty close to the bottom of the tank, it was designed like this for a reason..

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Fill the tank and try driving the car with the fuel return hose going into a separate container and see if it runs bette. If it does you have something restsricting the return flow.
Another thought; after a fill up try driving with your Fuel Tank Cap off. If it runs better you have a Tank Vent problem

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84 300D, 82 Volvo 244Gl Diesel

Mine sure doesn't run any better unless you look at the weight issue.

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- Brian


1989 500SEL Euro
1966 250SE Cabriolet
1958 BMW Isetta 600

Excellent ideas, guys, thanks! Just went poking around under the hood looking at return lines, etc. Quick question, how important is it to have a "cigar hose" as opposed to a regular piece of 3/8 fuel line? Perhaps that's my problem? I'll try rerouting the return line and leaving the fuel cap off after fill up and report back.

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1998 E300D, 287k, barely broken in.

The cigar hose was added by the factory to stop some kind of pulsation noise or vibration – or so I read here a couple of years ago.

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

reason being that it if it returned at the op of tank, and just poured in there, it would cause foaming- like at the pump. and that would not be good.

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1984 123.193 372,xxx miles, room for Seven.

1999 Dodge Durango Cummins 4BTAA 47RE 5k lb 4x4 getting 25+mpgs, room for Seven.

How would one go about inspecting a fuel tank breather?

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1998 E300D, 287k, barely broken in.

Test drive with the fill cap loose (just don't turn left too fast)

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine

First things first. Install the cigar hose. It does have a real purpose or function to modulate the fluxuating output of the lift pump after the regulator. Right now the output regulator is trying to function properly as designed but cannot.

There are even cautions somewhere not to substitute a normal hose. I believe it will interfere with the injection pumps base fuel supply reguators function to some extent. That is not explained. Just the warning not to do it. Next look through Cervans fairly recent thread labeled rack damper pin and decide if you want to examine the injection pumps base fuel pressure.

What I think is really happening here. Subject to correction of course. Plus others input from better minds than my own that may be posted. With the cigar hose eliminated and the return line in the tank submerged free release of the overflow regulators output is impaired even more. The rapid changing output pressure cannot be dealt with or much less so when the line is kept full with even more restriction. This is also impacting the elements in the injection pump with abnormal large fuel pressure pulse swings as they are loading. Or trying to load.

Yet when the fuel is lower and the return line not submerged in the tank some moderation of the massive spread of fuel supply pressure is occuring . Not totally for the best performance and milage mind you just less so. Yet when the fuel gets higher in the tank than the return all hell breaks loose so to speak.

Just be happy you added the missing cigar hose in your question. Otherwise a possible causitive for your problem could not have been though of I think. So you posted a good question with an important detail included in my opinion anyways. We are just on the threshhold of learning. Or at least I am of exactly what does impact the loading function in the pump and the results of diferent variables. Some distance to go yet but we are on our way.

The cigar hose dampens the pulses and prevents a thumping noise in the cabin from the fuel lines.

Its a human comfort item, like the engine shocks.

It may do that as well I have no doubt as I said better minds will post. Still without the cigar hose there and the return under the fuel level causing a problem. Obviously there is a reasonable chance there is a relationship.

The hydralic nature of the diesel fuel being pulse fed into a non flexable reciever is going to have an effect. More so when you increase the restriction to deal with the dynamic pulses by submerging the output of that line.

Simple installation of a cigar hose will do away with the problem I think. I could be wrong of course and respect everyones viewpoint. I also do not have any compulsion about having to be right all the time. I can, do, and will make errors from time to time. That will always be the price of moving forward.
I am slowly arriving at the conclusion that mercedes may have not released all information to their cars over the years. Or a better way perhaps to state it is too much was left out. Or perhaps the pumps actual manufacturer did not release how fussy this injection pumps fuel feed can be to it's operational health.

I will even go a little farther. I do not even know if this can occur. Maybe someone can quote sometime comparing a new one to an old one. If your cigar hose has hardened with age. It should be changed out as it is probably having a negative influence to some unknown at present extent. Soon a gauge check of abnormal pressure swings of the injection pumps fuel supply if they were occuring will point to this area.