any way to test if lift pump is good? - PeachParts Mercedes-Benz Forum

karmamechanic · #1 01-03-2009, 12:36 PM

I can get my 85 300SD to idle but dies soon after.
I put in new filters, a new button primer pump.
I cleaned/tested the injectors, they crack at 1800 psi
I bled the system, now it wont start and seems like maybe the lift pump.
I already removed it and would like to test it before
I buy another one.
when I press down in on the pumps ball I hear suction
but very little.
How much suction should these have?
any sure way to test?
appreciate any adn all feedback
thanks

Diesel911 · #2 01-03-2009, 12:47 PM

The test for the lift pump is done when it is mounted on the IP.
You need to T in a test gauge between the Supply Pump Outlet and the Secondary/Spin On filter inlet.
This means removing that hard plastic tubing and replacing it with 1/4" rubber Fuel Hose.
This will measure the pressure between the Secondary filter and the Fuel Supply Pump.
You can also test the pressure that is going into then IP in a similar manner but you need to T in the test gauge between the Secondary Filter Outlet and the IP Inlet on the IP housing. Again the involves removing the plastic tubing.

There is some info on testing the Fuel Supply Pump in this thead:
IP expert HELP needed - 85 300DT

When you bled the air out of your system did you leave the Injector Hard Line Nuts loose at the Injectors; crank the engine and observe Fuel comming out up by the Injectors?

funola · #3 01-03-2009, 12:54 PM

Sounds like air leaks. That lift pump is pretty robust and I have not heard of it going bad often. Do the easy stuff first before testing the lift pump (which you cna do with a pressure gauge and a tee fitting). Put a clear 5/16 PVC line in place of the cigar hose temporarily. After you get the fuel system primed and the engine running, there should be a solid stream of fuel in the clear hose. If you see any air in the clear line, you have a leak somewhere and you need to find it and fix it first before suspecting the lift pump.