have a 80 240d had the headlight smashed out 6 months ago and it sat for a while now just runs like crap.
starts fine. has new filters, def not a priming issue. but after about 2 minutes it idles bad and has no low end power and the few times i hopped on the highway lately and difference in incline and it just started to fade. i thought it was just low compression in a cylinder or maybe my rings finally gave but a mechanic friend said that maybe there is a part in the fuel pump operation that is stuck. not familiar with this on this car.
like i said it starts fine and idles good/drives good for 2 minutes. any tips?

 

When People are stumped as to what to do the often blame the item the understand the least and cannot test. Sometims that is the Fuel Injection Pump.
It is also more Money for the Mechanic.

If your problem presists after the Valve Adjustment:

Concerning the Fuel Filters if Fuel is contaminated I have seen new Filters Plug up in a matter of minutes. You also have a Fuel Tank Screen.

Just for troubleshooting purposes that a good look at the Plastic Primery Filter to see if there is anything in it.
Change the Spin-on Secondary Filter; you can put the used one in a Zip-Lock Bag and save it if you think it might not be plugged.

To see if it is the Fuel Tank Screen plugging. You need to have at least 1/2 Tank of Fuel. You Swap the positions of the Fuel Return Hose (a.k.a Cigar Hose) with the Fuel Inlet Hose where they are attached on the Fender Well.
That bypases the Fuel Tank Screen and draws Fuel from a higher position in the Fuel Tank (why you need to have at least 1/2 tank).
If your problem goes away your Fuel Tank Screen is plugging up.

On the Fuel Injection Pump is a Fuel Supply/Lift Pump and a Fue Pressure Relief/Over Flow Valves. Both of those items have been in use for over 25 Years.
Unfotunately there is no over the counter setup you can buy to test the Fuel Supply/Lift Pump pressure; likely why your Mechanic did not test that.

If you have the Shiny Aluminum Bodied Hand Primer on the Fuel Supply/
Lift Pump and your Engine is running at all rough during idle; that type of Hand Primer is a freaquent cause of Air leaks into the Fuel System.

Try driving without your Fuel Fill Cap on. If that improves things your Fuel Tank Vent is getting plugged.

 

I did valves a year ago and the car drove great so unless one of them didnt get tightened proper and readj. but i will pop the valve cover and check.

also i highly doubt it is either of my fuel filters because i always check them first (and replaced them even though they were clean as soon as i started having problems) considering i run biodiesel (Never ran WVO) The supplier i get it from, i also run through my 1 ton ford and recently dropped the tank on that for a separate issue and the tank was crystal clean so I know the fuel is good quality.
I will try swapping the 2 fuel lines just in case that fuel tank screen is plugged.

I also replaced the fuel primer when i first started having problems(6 months) because i thought the same thing.

also no woosh when i pop cap off, def heard that one on peoples running bad WVO before

gonna go swap fuel lines first then maybe pop valve cover and double check.
thanks for the tips forced induction and diesel9112

 

such a rookie diagnose i did.
swapped lines and ran like a champ.
looks like i have a fun afternoon draining tank and replacing screen.

 

I picked up all that info on the verious Mercedes Forums.

Don't forget to switch the Lines back or you will run out of Fuel way before the Fuel Gauges shows empty.

If there is blackish gray coloed slime on the Fuel Tank Screen you likely have something growing inside of the Fuel Tank. Something like Sartron or Bilbor will kill the organisms.