just installed a fuel pressure guage to try and diagnose problem

i ordered a new lift pump from marty of precision diesel, got it the other day but am trying to make sure that mine is bad.


at idle my truck is at 18-24psi then under light load it drops to 10psi under full throttle drops below 0psi.


is this lift pump or overflow valve?

this is on my 1998 12v has 112k miles both lift pump and overflow are original as far as i know.

 

Squeeze the rubber portion of the return line at the rear of the engine with pliers. If your gauge shoots up above 50 psi the problem is the overflow valve.
If it doesn't shoot up problem is the lift pump or one of the three filters being plugged.

 

shot up to the 50 psi like you said. guess i'll call and get an overflow valve from marty. thanks ifidel

should have ordered the overflow valve before.

 

spend an extra 10 bucks and get an OEM bosch one from tstproducts.com

 

so i did the bic pen spring mod on the overflow valve.... and now i get 40-45psi at idle but it drops to 0 under full throttle still.... takes a lil longer to drop to 0 but it still does.

so is this lift pump or overflow?

 

Replace the OF valve.
The one Marty sells is the same used in all Mercedes diesels.
I don't see how this could of lesser quality than the Bosch valve.
In fact both of my OEM valves failed under 50k. Almost 200k on two of Marty's valves with no problem.

 

Don't return the lift pump. It is probably as weak as the valve.
Are you doing the checks while driving or standing & running the engine?

This is where a chassis dyno comes in really handy. Lift pumps (& overflow valves) will pass a check out in static testing, but drop once you get a load.

 

did you do the pen spring mod on the old one or did you get a new OFV?

 

Sounds like a weak LP to me

best to change it out as well...

Rick

 

Air leaks on the suction side will mimic those symptoms too. Especially that bit about the pressure dropping under load. I also increased the spring preload in the overflow valve with the same results. Check your hoses and the electrical connector on the fuel heater.
Might be a good idea to pressure test the metal fuel lines from the tank too..
I just replaced my lines and pickups outright so I wouldnt have to second guess myself. If youre in the rust belt thats going to happen eventually.

 

did the bic mod on the old overflow... and now its at 45psi at idle...

drops as low as 18psi at 3100 rpm's at wide open throttle. but hasnt dropped to 0 in 2 days and prolly 140 miles of driving now, so i'm not sure what it is...


when the gauge drops to 0 the truck didnt run any different, felt the same, same power it seemed, didnt sputter or anything, thats what seems wierd to me.

 

Notes for u to consider

mine idles at 40 psi and it only drops 15psi at WOT, and it seems to stay there, I replaced the LP i believe in august.. Just for ur consideration...

Rick

 

replacing pump tomorrow and also doing kdp... and going to order a overflow valve.

 

Just a thought here

would WVO affect fuel pressure as well? Im just curious if anyone has made any note of this...

Rick

 

Stupis question here but where's the 3rd filter ? How much is a new lift pump ?