I've been having a weird power loss issue with my '83 300D. The car runs fine for the first 5 miles, then slows down to about 55 mph. Pushing the accelerator in doesn't make any difference. A few months ago I decided the problem was a leaky vacuum line messing with the wastegate; so I replaced some rubber lines that looked really bad. This fixed the problem for about a month but now it's even worse. I know the fuel tank is venting properly; and the fuel filter is new.

When you say the fuel filter is new, which one are you talking about....The spin on one, or the in-line plastic one. I had a similar issue a while back, and cleaning the tank screen and changing BOTH fuel filters fixed it

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2012 Mercedes ML350 Bluetec 102K (hers)
2005 Corvette 55K (fun car)
2002 VW Jetta TDI 238K (mine)
1998 Volvo S70 T5 Turbo 211K (kids)
1994 Ford F150 4WD 246K (firewood hauler)
1983 Mercedes 300D 384K (diesel commuter)

Recommend a new tank screen. Found a large hole in the screen on my 1987, had the part on hand. Don't risk having to do the job twice.

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Present
1987 300SDL
1991 300SL
1990 560SEL
2001 ML320

Past
1969 200D 1979-82
1983 300SD 1984-85
1972 250C 1982-02
1995 S350 2011
1997 S320 2012

Probably a fuel delivery problem of some kind. You could try switching the feed and return lines in the engine compartment. If the problem goes away, it's a plugged tank screen. If the problem continues, the issue lies elsewhere. I wouldn't pull the tank screen before running this test.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

That's interesting. Could you explain the concept in greater detail?

Just after I posted my question I read elsewhere on this forum about the inline filter. I didn't know the car had one. It was very dirty. I replaced it, and it seemed to help a little. I didn't know there is a screen in the tank. I'll check that. I really thought it seemed like a fuel pressure issue, but I was out of ideas. Thank you for the advice.

tangofox007- I read about that on this forum somewhere. I'm not sure exactly how it works but as far as I can tell there is a vacuum valve connected to the throttle linkage. A line from this valve connects to the wastegate controller. This adjusts boost relative to throttle position.

Before you pull the tank screen filter (with can be a pain as the tank has to be either drained or empty) do what Kerry posted earlier (see below) Sounds like you are getting closer to fixing it

"Probably a fuel delivery problem of some kind. You could try switching the feed and return lines in the engine compartment. If the problem goes away, it's a plugged tank screen. If the problem continues, the issue lies elsewhere. I wouldn't pull the tank screen before running this test".

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2012 Mercedes ML350 Bluetec 102K (hers)
2005 Corvette 55K (fun car)
2002 VW Jetta TDI 238K (mine)
1998 Volvo S70 T5 Turbo 211K (kids)
1994 Ford F150 4WD 246K (firewood hauler)
1983 Mercedes 300D 384K (diesel commuter)

Suggest draining the gas tank. This is a long shot, but I was having similar problems and discovered someone had put some q-tip swabs into my gas tank. Car ran okay for a few minutes until one of the q-tip swabs got sucked in and blocked the fuel line. Just my $0.02.

I switched the fuel lines tonight and that didn't fix the problem. I took the cap off the tank even though I'm sure it's venting properly and that didn't make a difference either. Is there a way I can put a temporary boost guage or fuel pressure guage on it that I can read while driving?

On the top of the valve cover is a hose that runs to the air cleaner. Remove this hose and check for any restrictions in the hose. As a test, disconnect the hose from the air cleaner and take a test drive. Report back with results.

If you switched the fuel lines and it did not help, most likely the tank screen is OK. How much fuel was in the tank when you switched the lines?

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2012 Mercedes ML350 Bluetec 102K (hers)
2005 Corvette 55K (fun car)
2002 VW Jetta TDI 238K (mine)
1998 Volvo S70 T5 Turbo 211K (kids)
1994 Ford F150 4WD 246K (firewood hauler)
1983 Mercedes 300D 384K (diesel commuter)

@Bassmaster: LOL . . . reminds me of the days when the prank of putting a condom in the tank would drive the owner crazy and cause the same exact problem!!!! LOL! Wondering if it was done by idiotic mistake . . . or deliberate to the PO.

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1983 300D, the "Avocado"
1976 240D, 4-spd the "Pumpkin", SOLD to Pierre
1984 190D, 2.2L, 5-spd, my intro to MBZ diesels, crashed into in 2002

Since switching the lines did not fix the problem, it is likely that the problem is not in the screen. However, since back flushing the lines is a very easy procedure you might give it a try anyway.
Try the feed line first. Disconnect the line at in-line filter in the engine bay, remove the tank cap and apply the compressed air to the line. Try short shots at first. You should hear a lot of bubbling at the back of the car. Please note that you must have enough fuel to totally cover the screen for the compressed air to clear the screen. If part of the screen is above the fuel level, the air wil take the easy route and not clear the screen.

Some time ago I removed the screen and let the crap go all the way to the in-line filter. This one is much easier to clean/replace than the tank screen.
This is not to say it is the correct thing to do, though.

good luck
gonzalo

Sounds like maybe the lift pump is going bad then. Another member here (Surveyguy, lives not to far from me) was having issues with random fuel starvation, and I pulled a used lift pump for him and it cured his problems.

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I haven't had much time to work on it recently; that's why I haven't posted anything. I did blow out the lines, that didn't help. Today I finally put a gallon can of deisel in the engine compartment and connected hoses from it to the pump and return. (Tip for anyone who wants to try this: remove the windshield washer reservoir to make room for that can.) I took it for a drive and experienced the same power loss; though it didn't slow down quite as much ( about 5 mph faster). I think this indicates a weak lift pump. I reason that it ran a little faster because it wasn't experiencing the resistance in the tank screen and the fuel line under the car. Do you agree? I'll remove the lift pump and examine it.