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Lift pump not working and truck keeps stalling

HD Chrome Dome

HD Chrome Dome

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sydney, ns
I have a 1995 GMC 1500 6.5 turbo, I recently swapped out the tranny from my 1995 DONOR truck (same engine and trans) and seemed to be working great for a week and then started stalling while driving and then while at idle. I have experienced this before and just changed the PMD and all was fine. I used the DONOR truck to change the OPS,PMD(external mount),LP,GPS and even tried the ECM. The DONOR truck seemed to run fine so I assumed all these parts would be usable. The truck will only run for about 30 sec on the donor pmd (was working fine on this pmd for a few days) and about 5 min or so on the old pmd(also external mount). My LP is no longer running, have voltage on the meter at the pump but will not even light a test light (runs fine off a direct 12v feed). I have also changed the pump relay(tried three different ones) and tested the fuse but still no change. Any help or advice would be great, walking sucks when you live 20 kms from work, lol. Thanks in advance.
 
Oil pressure switches burn the contacts so you get "no load" voltage, but, any amperage to light a lamp or work the lift pump isn't there. The fuel pump relay is more or less useless. It simply isn't used while the engine is running as GM decided to fry the OPS contacts with this job directly. Although a new OPS will fix it awhile most do a lift pump relay mod.

Back probe the lift pump while it's hooked up engine running and I bet you get 7v or less (not enough to run the pump) - with open circuit, lift pump unplugged, 12v+.
 
WarWagon said:
Oil pressure switches burn the contacts so you get "no load" voltage, but, any amperage to light a lamp or work the lift pump isn't there. The fuel pump relay is more or less useless. It simply isn't used while the engine is running as GM decided to fry the OPS contacts with this job directly. Although a new OPS will fix it awhile most do a lift pump relay mod.

Back probe the lift pump while it's hooked up engine running and I bet you get 7v or less (not enough to run the pump) - with open circuit, lift pump unplugged, 12v+.
Click to expand...
 
you are exactly right WarWagon, I have already done this and the voltage is pretty much what you have stated. I tried running a direct power to the pump and it still stalls after about 30 sec with the pump running so I am thinking that the 2 PMD's I have may be cooked as well. running out of patients with this thing.
this relay mod you speak of, are you able to tell me how to do that?
 
I thought you meant the engine ran with a direct wire lift pump...

At this point check for fuel pressure at the water drain as lift pumps can rattle while doing nothing. Check for air via a clear 1/4" fuel line off the IP return to the return rail. Check for air first!!!

http://www.thetruckstop.us/forum/threads/lift-pumps-ops-operation-and-troubleshooting.6872/

Then grounds need to be cleaned. Check for damage to the grounds (for the ECM) on the trans dipstick bolt back of the head. Wiggle the PMD extension cable to see if that kills the engine (and consider any PMD run without a heatsink deep fried).
 
Thanks WarWagon, I already ran the pump direct and opened the drain cock at the front of the engine and have a good steady flow of fuel there, didn't notice any air in the flow but not sure if it would show up from that drain. Will have to try the clear line you said to try. I have noticed a puddle of fuel on the floor at the tank so I need to find where that's coming from also, guess it could be a line. Both of the PMD's I have are attached to coolers and worked fine until all these troubles started in the past week. Not sure if there is any way to check the PMD to see if they are cooked. when it does start and you try to rev the engine it seems to have a bad MISS as well.
 
Just want to say a HUGE THANKS to all you guys that been helping me out over the last few days. I believe that I can conclude that air in the fuel was my problem. I decided to take a chance on the fuel filter and was surprized to find that the screw top was a little loose, so I did the filter change and made sure that the screw top was good and tight, after a bit of cranking and bleeding air the truck fired up and continued to run for the first time in a week. Havent gone for a test run yet but seems fine thus far. Still have the lift pump problem but that will be an OPS change for another day. Thanks again to all of you who offered up your knowledge.
 
You still need to find the fuel tank leak unless it was a leak from the Fuel Filter dripping from the bottom of the bell housing.
 
And the clear fuel line is a good permanent addition. A few years from now something will hiccup and you can do diagnostics at a glance.

Glad you have it running better.
 
There are certain things they make the best of. The tendency is everyone to assume there is better than stock brands, however AC Delco spends tons of cash into r&d. iirc reading an article a couple years ago, over 40% of their sales are to GM, The rest is to the open market.
There are other things that there are better brands to go with.
 
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