Fuel System Drain Back Diagnosis
This isnt really about a specific vehicle. more so a condition ive struggled with on several different chassis and engines. im sure we've all heard this customer concern of “have to use starting fluid in the morning but once its running its fine all day”.
the truck im currently working on has intermittent low fuel rail pressure. the injection pump and lift pump pass the return flow test (about to do the injectors now). but i feel the fuel from the return is aerated and foamy. obviously i need to install clear lines to check for aerated fuel but without the correct adapters i always feel like im creating a vacuum leak.
so what im curious about is, does anyone have any good tips or tricks for diagnosing fuel system drain back or a fuel system losing its prime? or for vacuum leaks on the supply side without swapping parts and hoping you find it?
as a side note i do have some of the fittings to isolate return flow that fit cummins and some other engines but i had a mercedes MBE900 that had the same condition that beat us up and took our lunch money a while ago.
Hello Austin, I would suspect an injector leak down issue before a drain back. Any history of slowly rising oil level? It takes a while to catch it on the high-volume systems. There are fuel line anti drain back valves made for many models. Check your fuel system lines to see if one is already there that may have failed.
it passed the injector return test and the pressure relief valve test. my issue with the check valves is theyre on the back of the ECM with the electric lift pump. im not sure how i can bench test them other than just blowing air thru them
Did you jack the rail pressure up with scan too thru the test…..weve had 2 leak bad out pressure relief valve cause the rail pressure to drop fast after turing engine off…are you sure electric pump is working as big pump pulls it around the electric pump after engine is runing ,weird check valve system there as lift pump is just for starting
The 14mm supply bolt does have a check valve in it,,,at supply pump ecu ..other 3 are 12mm
Yea I used insite to do the test and it ramped up the pressure
Pressurize the fuel tank and look for leaks. If it sucks air it might leak diesel..
Tried that a couple times. No external leaks ive noticed.
Every fuel system works different is why I need to know
I have ran across this a couple Times. Same symptoms. I found the mechanical pump the problem. The weep hole sucks air in sometimes when they fail. No sign of a leak. Replace pump and will start no problem every morning.
That makes a lot of sense. The mechanical lift pump on the back of the high pressure pump or the high pressure pump itself? Does cummins sell the mechanical pump separate from the injection pump?
The mechanical pump mounted to high psi pump. This is on HD applications on ISL engines.
You can get the fuel pump mounted on big pump if you need it…does not break it down in pieces just the whole gear pump
Fuel pump pressure from electic pump dhould be 5 psi,,,,at starter speed gear pump presure min 15 psi…both measured at filter…this is from cummins diagnostic..