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Discussion Starter #1
Hi folks,

well, from my other thread, i installed the new Diesel fuel sock that i purchased from my local GM Dealer, but guess what, this truck is still doing the same thing :nope:
sooooooo, i am back and rephrasing the issue according to these specific symptoms :bat:

So, with new sock, it started after i held it to the floor for 25 seconds or more, then idled fine, and i quickly pulled it out of my 60 degree garage to keep from stinking it up.... ran good, except for a few bumps randomly at first, i guess as the air was getting out of the system, then after a min or so, it was smooth and sounded good, i could even quickly punch it and it sounded great, spun right up. I never left the driveway, afraid of stalling, which it did when it reached 0psi.

->> NOW, here are some important other details
1. once it got to 0 PSI and stalled, i shut off the key, then turned it back on, but i got nothing from the lift pump sound like normal & of course the PSI stayed right on 0. so i took the key out and put it back in and turned on the ignition, but nothing, sort of like a dead lift pump.

2. BUTTTTT, get this, after it sat for several minutes (without key in, not sure if matters to the computer knowing my key is in the ignition?), get this, the lift pump starting pumping again and the fuel pressure pumped up to 5/5.5 psi, so this is confusing??? :confuzeld


Components that could produce these symptoms??
Sooooo, what components in the system would act like this. Meaning, is what components would cause something to work great, then go down hill somewhere between a min or so to 3 to 4 mins?
(i didn't time it, but i think i idled in the driveway for atleast 3 mins...)

Would a failing lift pump work fine when cold, then stop running after a maybe 2 mins?

Now i already replaced the OPS & fuel pump relay 2 weeks ago when it was acting up. Buuuuuttttttttttttttt, maybe i have an OPS getting hot and opening up or something, you all ever heard of this?

In a few mins, i am going to swap to another used lift pump, but the other one did the same thing last week, when it still had the gas sock on it.

** UNLESS i have to failing lift pumps that are screwing with me, is this the symptom of a failing lift pump?

*** Unless maybe a vacum suction issue? my rubber fuel lines seemed solid and sturdy, unless rubber fuel lines can trick us?

** or what other component would work great at first, but then stop working after a min or two or three?


Thank you for any tips and help :thumb:
 

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Do your glow plugs work when it stalls? I just had this happen... Try unplugging the fuel heater. It is shorting out overloading the ecm b circuit causing the lp and glow plugs to quit. The circuit breaker internal to the ecm cools down after awhile and it starts working again when it resets. Also check the lift pump ground if you have not its in a bad place behind the fuel tank in drivers rear wheel well... I just cut the wire and redid it on the frame closer to the lp.

Fuel heater connector is the only two wire connector on the big harness running along the back of the firewall next to the FFM. Bring a medium sized flat screwdriver and some patience...
 
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Try to jumper the LP with the paperclip, bypass the OPS and see what happens.

AC Delco OPS?
 

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1996 midyear and up the OPS doesn't effect lift pump operation. At least... Its internal circuitry is not needed to run it like earlier models. I spent an 11 hour day in a snowstorm on my knees under the hood finding this out.
 

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Discussion Starter #5
1996 midyear and up the OPS doesn't effect lift pump operation. At least... Its internal circuitry is not needed to run it like earlier models. I spent an 11 hour day in a snowstorm on my knees under the hood finding this out.
oh no kidding, yuk...
(so now you have me curious, what was your issue, the fuel heater?)
 

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Discussion Starter #6
Do your glow plugs work when it stalls? I just had this happen... Try unplugging the fuel heater. It is shorting out overloading the ecm b circuit causing the lp and glow plugs to quit. The circuit breaker internal to the ecm cools down after awhile and it starts working again when it resets. Also check the lift pump ground if you have not its in a bad place behind the fuel tank in drivers rear wheel well... I just cut the wire and redid it on the frame closer to the lp.

Fuel heater connector is the only two wire connector on the big harness running along the back of the firewall next to the FFM. Bring a medium sized flat screwdriver and some patience...
Hmmm, the lift pump ground, well, if i did the correct one, I did clean it and reattach it when i removed and reinstalled the fuel tank, but tell me, is it the ground i found under my rear bumper on the driver side?

(see, i removed my hitch and bumper because the frame was so rusting, that i wanted to do from the rear past the fuel tank while it was under there and I found that wire and took it off and then used a mini wire wheel on an air grinder and cleaned the 2 grounds and the frame spot and the bolt and then bolted back onto the frame)
 

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Discussion Starter #7
Try to jumper the LP with the paperclip, bypass the OPS and see what happens.

AC Delco OPS?
so your saying to jumper it to make it run and not shut off by jumping it out?
if so, hmmm, how do i jump it out?
 

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First... The lift pump ground last year but truck would still die at idle when fuel was cold. As the fuel heater got worse truck would shut down when you let off the accelerator. Best test for this is when the lp shuts down, wait to start light also will not light.
 
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Put a 12v. test light on the lift pump relay as in the diagram. Easiest way is to wrap a piece of bare wire around the relay terminal, bend it up and wrap it with tape, then plug it back in.
See if the light goes out when the pressure drops.
This will tell you if the PCM is shutting down.
FYI there is a short red wire with a spade connector on the main harness between the under hood fuse relay center and the firewall. That is a test lead for the lift pump. Apply 12v. to that lead to run the LP when the key is off.

Diagram here: View attachment LPinputTEST.pdf
 

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Discussion Starter #10
Do your glow plugs work when it stalls? I just had this happen... Try unplugging the fuel heater. It is shorting out overloading the ecm b circuit causing the lp and glow plugs to quit. The circuit breaker internal to the ecm cools down after awhile and it starts working again when it resets. Also check the lift pump ground if you have not its in a bad place behind the fuel tank in drivers rear wheel well... I just cut the wire and redid it on the frame closer to the lp.

Fuel heater connector is the only two wire connector on the big harness running along the back of the firewall next to the FFM. Bring a medium sized flat screwdriver and some patience...
Thanks Tr00b,
ok just back in from garage and more testing...
1. So, i unhooked the fuel heater connector (the 2 wire connector red & black).
2. then allowed it to build up to 5.5psi
3. held it to the floor until it fired, then let off for idle.
4. it ran exactly 2 mins before it got down to 0psi and i turned off the key.

5. Then i turned the key to ON position, and the lift pump did NOT run and it stayed at 0psi, and the Glow plugs i assume were working, because the "WAIT to start" light was flashing slowly as normal.

6. then i waited a min, and "wait" to start light was flashing, BUT the Lift pump still would NOT pump up the system.

7. Then after another minute+, the LP finally started working, but it only pumped up about a pound, so i turned the key on and off about 4 times until it got almost to 5 psi, then i started it up and drove it back into the garage.

-->> So unplugging the fuel heater did not stop this condition from occuring, so could there be another component causing the ECM fuse to trip out?

*** hmmm, side note & question. The fuel tank has approx 1/8 tank, and i think it is ok, because the lift pump seems to pump up the system to 6psi easily, so for sanity sake i figure i would mention this. but for another sanity check, i guess i could run and get my 5 gallon jug filled and dump another 5 gals in (but i doubt that will help), but what are your thoughts?
 

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1/8th tank....I think you might be having fuel feed issues from the tank. Get the tank 1/2 full and see if it runs better. Conventional wisdom is dont go below a 1/4 tank of fuel.

Try powering the LP as Glagulator says using the red wire with the spade connector. See if you maintain fuel pressure and the engine runs better.
 

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The LP test lead is only to power the LP when the key is off. With the key on the PCM activates the relay and the test lead is disconnected.
Need to know if the ECM-B fuse has 12v. when the engine dies.
Need to know if the LP relay is activated when the engine dies. see post 9.
These engines do not die because the fuel pressure drops, even to slightly negative, IF the fuel system is sealed..
 

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The fuel system on my Burban was full of all sorts of trash and rust when I bought it. Had similar issues with pressure dropping, eventually replaced the WHOLE fuel system from tank to IP. Might note that I was going crazy trying to fix it and found that the brand new AC delco LP from napa would run for about 30 sec then just dribble fuel. Took it back and got another one and have had no issues since. Tested it by disconnecting the fuel lines and dropping the intake side in at tub of fuel and watching it run and that was when I found the issue.
 

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Discussion Starter #14
Put a 12v. test light on the lift pump relay as in the diagram. Easiest way is to wrap a piece of bare wire around the relay terminal, bend it up and wrap it with tape, then plug it back in.
See if the light goes out when the pressure drops.
This will tell you if the PCM is shutting down.
FYI there is a short red wire with a spade connector on the main harness between the under hood fuse relay center and the firewall. That is a test lead for the lift pump. Apply 12v. to that lead to run the LP when the key is off.

Diagram here: View attachment 286721
Excellent Diag info, and i have 2 questions
1. I looked for the red wire and spade connector last eve, and am going to look right now again. Do you happen to have a picture of this or where it is hiding? Once i find it, sure i can turn on the lift pump, but how long should i run it without the truck running, or is it ok to run endlessly to test it, maybe 5 to 10 mins to see if it fails? Now I have my fuel gauge hooked to the "T" fitting, so should i pump fuel into a oil pan, or just let it run and hold pressure to see if it fails after 2 mins like i keep seeing?

2. on the relay, which pin would i hook a wire took?

(be back in maybe 15 mins, as i try to find the red wire and get a few jumper wires connected to and run it for a few mins under pressure...)
 

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Discussion Starter #15
ok, i found the spade connector, the little sucker was hidden below the bundle of wire :)
will now hook up to 12v, brb.....
 

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Lift Pump Prime Lead

For future reference.

1996 and up lift pump prime connector.

LPprimeLEAD.JPG
 

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For future reference.

1996 and up lift pump prime connector.

View attachment 286865
Fwiw I had some difficulty getting a good connection thru this connector .I kind of ended up blaming the lift pump ground . And could have been my cobbled up 12 volt lead .Just saying so as to be aware that a good connection needed to get the lift pump to run . Just saying because I was blaming the liftpump & it turned out to be just fine.
 

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Discussion Starter #18
Put a 12v. test light on the lift pump relay as in the diagram. Easiest way is to wrap a piece of bare wire around the relay terminal, bend it up and wrap it with tape, then plug it back in.
See if the light goes out when the pressure drops.
This will tell you if the PCM is shutting down.
FYI there is a short red wire with a spade connector on the main harness between the under hood fuse relay center and the firewall. That is a test lead for the lift pump. Apply 12v. to that lead to run the LP when the key is off.

Diagram here: View attachment 286721
Thanks Glagulator & other Gents!!!

OK, i now have the wires running under hood and then wrapping around thru the windows in a temporary testing setup.

1. NOTE: The fuel heater is still unhooked. (I will hook it up after i finish this post and test the truck again)

2. of course now that i have all this hooked up, the truck will NOT stahl when it reaches 0psi (gee whiz, the sucker idled for 10 mins & idled PERFECTLY, so i pulled it back into the garage to hook up the fuel heater. Soo i can surmise this, now that i have the proper diesel sock installed, i guess the Injection Pump is pulling enough fuel to keep itself idling, would you agree??? (i didn't want to drive anywhere on 0psi)

3. I have a 12+ volt reading on the selenoid leg while it is running and idling, and yes even at 0psi (of coarse I have 12+v as soon as the key is turned on & i never loose the 12+v). So this seems like good news.
(I have voltmeter hooked up to the selenoid and sitting on my dash temporarily)

->>YES the 12+ volts never dropped or went to zero, even though i had 0psi. So it seems the LP is not blowing the breaker in the ECM, which you think it would, so this is strange to me??? I assume the lift pump stops and maybe freezes up and is still drawing some amperage, which should kick out the ECM breaker (hmmm, let me go measure that now)


4. Also I hooked up temporary switch & sitting on my dash for turning on the Lift pump when the key is off, and that worked fine. (hooked up to that LP tap by the firewall)

5. ***Get this*** once the lift pump quit working, it has NOW refused to come back on even after 10+ mins so far.... really??? :confuzeld urggg
i even climbed underneath and tap it with a mini hammer to see if it would jar loose, sort of like a starter in the old days where you would smack it to get it to catch a good armature contact and spin so you could get on your way (i know, showing my age, i did this many times in the early 80s when i was 18 :bigglasse )

6. **Amperage that this lift pump is pulling, now that it seems stuck or frozen, or basically junk, but none the less, it is pulling 1.80amps (thru my digital amp meter via the 10 amp connections (DC 10amp connection) )

7. Get this, now that i pulled it into the garage and shut the truck off, it dawned on me a few mins later to jump back in and turn on the key, and sure enough, i get the 12+volts, and NO lift pump sound, but guess what, i hear a small metal type sound below my drivers seat and the 12v disappears....
hmmmmm So i can only assume, that the lift pump after shutting off the truck is now maybe kicking off the ECM breaker just by simply turning on the key and after maybe 5 seconds or more, something happens and the voltage drops to zero...(and yes the "WAIT" light is flashing as normal for start, even after the 12v drops to 0volts???) So that is weird, because

and YES, i can turn off and immdeiately turn the key back and do it again and the voltage goes right back to 12+v, and then i hear a tiny sound and the voltage drops to 0v. So i did this at least 3 times. So i assume the ECM can not trip and immediately allow me to turn the key off and back on and get the 12v??? or can it? (not sure if the ECM is running a breaker like a protable generator that kicks out, or does the ECM run an amp meter and sees high amps and then kicks off a selenoid and that is why i can immediately turn the key off and on and get 12volts again???) (yes, i am an electronics guy :coffee: )

8. oh get this, while i was typing this, giving it maybe 10 to 15mins to cool down, the stupid lift pump is now working again when i flip my firewall tap LP switch, wow... So do i assume the ECM is NOT doing this, but the LP cooling down and now working again??

## Sooooo, i am putting on my other "SPARE" older/used lift pump. I will test it and be in 15 mins or so....
 

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Discussion Starter #19
Fwiw I had some difficulty getting a good connection thru this connector .I kind of ended up blaming the lift pump ground . And could have been my cobbled up 12 volt lead .Just saying so as to be aware that a good connection needed to get the lift pump to run . Just saying because I was blaming the liftpump & it turned out to be just fine.
Hmm, you have me curious :idea:

Did you clean up your ground connection like i did underneath your rear bumper on the drivers side?
was it as simple as that and your LP and whole situation was fixed?

(so far for me, i did clean that ground and the other grounds up on the intake manifold, but still had my same bad situation the past 2 weeks of bucking and kicking and not drivable from stahling)
 

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By "12+ volt reading on the selenoid leg" you mean the Lift Pump Relay?

Another test, When the 12v. drops to 0 after the metal type sound, need to know what the voltage is at the ECM-B fuse.
 
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