airlock problem?

[fresburb]

had some rough starting and hesitation problems with 99 sub so changed fuel filter which was dirty. everything seemed fine for a couple days so i figure problem solved. subsequently i noticed if truck had been run for a while (up to normal operating temperature) then sat for a few hours it may or may not start. or would stall shortly after start up. after completely cooling or the next day it starts no problem and runs ok. with advice from TDP members i removed the new fuel filter and it looked clean so i reinstalled. when putting in the filter i primed the housing with fresh diesel to the point when i pushed filter into place and spun on locking nut the diesel spilled over edge. i left the air bleed valve at top of filter open to see if fuel came out. fuel never came out so i don't think i left key on run mode long enough before starting. so i started it and it started fine and ran fine. but under the same circumstances as mentioned above it might either stall or not start. it seems to only happen after vehicle has been run for a while then cooled for a short time. i'm starting to think it could be an airlock problem because possibly didn't bleed air properly. looking for some direction with this issue

[Kennedy]

Sounds like no lift pump. Try opening the water drain valve with the engine idling. If it stalls, the lift pump is inop. If fuel comes out it is working.

[Hubert]

Do you know your lift pump is working?
Test lift pump: While idling open water drain petcock valve and see if you have a little flow of diesel and truck stays running. Collect diesel in a clear clean container and let it sit for a while too this will sample fuel to see if its grossly contaminated also.

[fresburb]

i can hear what i think is the lift pump when truck goes through wts cycle. not even sure where lift pump is. is it the gold cylindrical thing under drivers side door? the petcock is on it? i'll check it out.

[a5150nut]

Yes lift pump is under drivers side and no the petcock is not on the pump. It is on the fuel filter, air blead knob.

[Doug Thom]

I had a similar problem to this a couple of years ago which I thought was the lift pump but turned out not to be. I installed a new filter, (fuel separator) and upon completion of the filter installation I finally got the truck started (with difficulty) and found that it would stall of a sudden and was difficult to start after it had been run and was hot. The problem was the way I had installed the filter. It wasn't seated into the housing completely and the rubber gasket on the filter was not making proper contact. I noticed the problem after finding some fuel seepage at the top of the housing and the cap was somewhat difficult to put back on. Perhaps check the gasket to see if it'a making perfect contact etc. Hope this helps!
Regards
Doug

[fresburb]

Thom-i don't think that is the problem as there is no seepage from top of filter housing. but thank you for your response. the more info the better

a5150nut-thank you for your response

Kennedy and Hubert- i opened the valve on top of the filter and nothing comes out while truck is running so i guess my problem is lift pump. Now my question is can i get a more robust lift pump because i am running 75%veg oil and 25% gas? The lift pump problem sounds logical as it happened within a couple weeks of the truck running on the higher viscosity fuel.

thank you, rich

[Hubert]

Now my question is can i get a more robust lift pump

thank you, rich

Not really any big gains in the same package. Depends on where you buy from some offer good quality name brand lift pumps with warranty and good custormer service etc but they are nearly the same basic LP as OE.

Some people have reported running (2) OE LP together. There are a few alternatives on the market but costly and much bigger in size and reportely noiser. Search the term FASS for a thread on extra fuel delivery. The DS style pumps like some excess fuel flow to recirculate back to the tank.

What ever you find try and stay with the design of flow through without power. Thats how the OE pump works. The IP can pull fuel through the LP if it goes bad or losses power.

[fresburb]

so is the purpose of the lift pump to aid in the delivery of fuel to the fuel pump when demand is high? if so then wouldn't my truck have problems with acceleration and not stalling at idle or startup?

[Hubert]

The LP pushes fuel to IP. It pushes more fuel than required for engine to run. Excess fuel cools the IP and recirculates back to tank. Thats the idea behind the pump mounted driver (PMD). The IP and fuel act as a heat sink.

Under normal driving with a regular diesel fuel, clean filter, clean tank pickup, leak free lines etc the IP can pull enough fuel for decent performance. Dirty filter, heavy load is different. One idea is: its one more contributing factor for the many reported IP problems is when LP breaks or is real weak the IP runs ok just normal driving around town. But without excess fuel flow I think the IP runs warmer than normal functioning fuel system and adds more stress to the IP and PMD and this shortens thier lifespan.

I don't know about your fuel it might act different. Maybe the added heat or different specific heat has allowed PMD to get a little too warm and you are also getting a sporadic PMD failure. Who knows how long your LP could have been broken. I would fix the LP for now and see if problem continues. Chances are the LP is the only problem as the later years the PMD is a little more robust. But keep it in mind when ever you are not getting fuel to injectors and all else seems to check out fine. Always start diagnostics by ensuring good fuel is delivered to the IP.

Last year I did not know my LP had failed until I tried to bleed air at regular filter change. I had noticed a subtle change in cranking duration before startup sometimes but not excessive. I missed it thinking weather, fuel quality, and age of glowplugs were suspect but not bad enough yet to warrant wrench turning until better weather.

Note some people add a fuel pressure guage to monitor fuel flow. As the oil pressure switch can also act up intermittently to cutout power to the LP.

[fresburb]

thanks hubert , that sounds like the next step for me. new lift pump and we'll see what happens. is replacing easy? by looking at it , its seems like 2 nuts and a plug or am i missing something. thanks again

[Hubert]

Yeah its easy to replace just a little messy. Have a couple of rags ready for the fuel that comes out of the lines and have new LP ready to put on immediately. It can be done pretty quick. Might spray some penetrant on the fuel line connections and clean them up first.

[fresburb]

ok great, i'll try to get to it this weekend. hopefully this will solve my problem but if not at least i've elimnated one possibility.thanks for all the info

rich