So I'm a total newbie when it comes to Mercedes Diesel engines, and I recently acquired a 1987 300D Turbo. The car has 237k and runs strong on the highway. It has several small problems, like a slight knocking noise when idling, low power on take off, not sure if the turbo is working, minor electrical glitches. But the main problem is: when traveling uphill at highway speed, there is severe power loss. It will fall from cruising at 75 all the way down to 45 if the hill (or mountain) is substantial enough.

I've changed both fuel filters and there was no change. It doesn't have the Trap Oxidizer on it anymore, and I don't think my turbo is working. I'm guessing that I should change the Fuel Tank Screen, but truthfully, I'm not sure how to get to it.

If anyone can shed some light on a beginner, I'd be most appreciative.

Check your boost pressure!

You need to monitor/check the pressure on the low pressure side of fuel delivery system. Since many of the lines in the system are nylon/plastic this is not as easy as it should be. I've made a "test" connection part out of an old banjo bolt that I use when I need to check the system pressure.

You should be seeing 10-12 PSI of pressure at idle, my old 2.5 turbo would run fine until the supply pressure dropped to about 1-2 PSI. There is also a specification in the shop manual that details volume flow rates as a function of time at cranking speed.

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98 Dodge-Cummins pickup (137K)
13 GLK250 (157k)
06 E320CDI (341K)
16 C300 (89K)
82 300GD Gelaendewagen (54K)

Start with cleaning the banjo bolt in the overboost protection system. Instructions and pictures here - http://www.dieselgiant.com/mercedesaldaboostsystemservi.htm

If that does not give proper results, try bypassing the solenoid on the firewall.


If you have a boost gauge, definitely check your boost pressure. If you don't, now is a good time to start shopping for one. They are relatively inexpensive. $30~$40 for a VDO

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On some nights I still believe that a car with the fuel gauge on empty can run about fifty more miles if you have the right music very loud on the radio. - HST

1983 300SD - 305000
1984 Toyota Landcruiser - 190000
1994 GMC Jimmy - 203000

https://media.giphy.com/media/X3nnss8PAj5aU/giphy.gif

........no banjo bolt on the 603.......

I had a similar problem and discovered the fuel line was sucking air....not so much as to affect normal driving but after some miles, an air bibble would build up and it actually created an air lock in the fuel line when I was going uphill - the bubble moved inside the fuel line (ONLY uphill)....it drove me nuts 'till I found it and felt pretty stupid...I overthought the issue...hope your'e is similarly an easy and cheap fix.

adding a electric fuel pump to mine was the best thing.Better mpgs,and charging up mountains at 85 mph +.

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1999 w140, quit voting to old, and to old to fight, a god damned veteran, deutschland deutschland uber alles uber alles in der welt

Locate the switchover valve. It is the valve on the driver's side fender that has two vacuum lines to it: one from the bottom to the intake manifold, the other from the middle to the ALDA on the IP. Blow through the line to the intake manifold, if it is blocked, clear it. Blow through the switchover valve, if it is blocked, clear it (WD-40 has worked well for me in cleaning/clearing this valve). Once both of these are clear, drive the car to see if the problem still exists.

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Gone to the dark side

- Jeff

Please disregard previous post......

somehow read it as an '85.....

__________________
On some nights I still believe that a car with the fuel gauge on empty can run about fifty more miles if you have the right music very loud on the radio. - HST

1983 300SD - 305000
1984 Toyota Landcruiser - 190000
1994 GMC Jimmy - 203000

https://media.giphy.com/media/X3nnss8PAj5aU/giphy.gif

clean tank strainer

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2009 E320 Bluetec 117,000
1995 E300D 306,000 Sold
1996 Ford Taurus LX 130,000 Sold
1985 300TD Still 225,000 Sold
2016 Ford Fusion 24,900

Thanks guys! This is great stuff. I'll try these suggestions and get back to you.

Hey man you said there was knocking (like ball bearings moving around), I know the sound, I would suggest using a fuel system cleaner call Diesel Purge it is very inexpensive and it can work wonders on dirty injectors. The procedure is at dieselgiant.com. I hope that this will help you, because it did for me. If you find the the injectors are extremely bad you need to replace them.