View Full Version : Intermittent lift pump issue
Occasionally, my LP will not work. Generally, when I turn the key, I can hear it running, but recently it has died shortly after starting and pulling down the drive. When I open the bleed valve on the fuel filter manager, air comes out. When I jump the LP with the pig tail, it always runs and primes the FFM. I have tried a new pump relay, but the problem continues. I have searched and found that it may be the oil pressure sender, but I have not found if the 96 definitively has the OPS in the LP loop or not.
john8662
07-13-2013, 16:17
Yes, there is an OPS on your 1996. Likely the issue.
The OPS is a backup LP power source on 96+,the ECM controls the LP relay from inputs from OS and CPS when the engine runs.
The OPS normally only activates the LP after engine shutdown when oil pressure bleeds off.
If air bleeds from the filter you might want to source the place where the air is getting in the system,if all is sealed the IP will pull fuel from the tank so air should never be in the filter.
The OPS is a backup LP power source on 96+,the ECM controls the LP relay from inputs from OS and CPS when the engine runs.
The OPS normally only activates the LP after engine shutdown when oil pressure bleeds off.
If air bleeds from the filter you might want to source the place where the air is getting in the system,if all is sealed the IP will pull fuel from the tank so air should never be in the filter.
I will check for air, but that would be two different issues. 1. Air in system and 2. LP not functioning. Oddly, this issue is very, very sporadic once driving. Regardless of the air, my LP is not functioning as it should, and it appears to be a switch or sensor issue up stream, because as I mentioned, the LP always works when I jump it.
Might want to try running with the fuel cap loose?
If the wrong fuel cap or one with a plugged vent is being used-you could end up with excess vacuum in the tank and get air in the filter as well?
Except the pump is sometimes not running. When I jump it it runs. I don't think the fuel cap would effect that. I may be wrong it is not air, but a vacuum in the FFM. Either way, the LP sometimes does not run when it should.
Warren96
07-17-2013, 05:45
The best way to check the OPS is to bypass it, ( temporarily,of course) and see if the air in the fuel manager goes away. If the lift pump stops the injection pump can still pull a vacumm on the FFM. The truck can still temporarily run without the lift pump.Although i wouldn't test that theory in the winter when the fuel thickens.
A more comprehensive narrative.
Generally, I can turn the key, and hear the LP. There are times when it does not appear to run. Those times the truck will die within a couple hundred feet of taking off. When this happens, I will open the bleeder valve on the FFM and hear the rushing of air. To be honest, I don't know if it is blowing or sucking. Sometimes I can cycle the key and the LP will then come on and fuel will come out the bleeder. If I jump the LP from the pig tail just behind the fuse box, it runs always. Once the FFM has been bleed, it runs relatively fine, although there is an occasional bite, but never a stall.
I don't believe there is air entering the system. I have had that issue in the past, and this is not like it. The issue basically comes down to the normal LP signal is not happening. The LP is new, and runs every time it is jumped.
Follow up.
I checked after it stalled, and the FFM had air in it that escaped under pressure. I will check for air leaks in the lines. The odd thing is the stalling only occurs very shortly after starting - within 100 feet of its parking spot. After that initial stall / jump LP / bleed system, no more stalling. Still, the LP is apparently not working as it should. Is there a wiring diagram to try to track down the culprit? Does the LP circuit activate with the key and stay on? Is it governed my system pressure (once FFM is under suitable pressure, the LP shuts off?).
The LP does run when the key is on for a prime cycle and when the engine is running and/or oil pressure is present.
It runs constantly or is supposed to-there is no stop start for pressure.
The LP does run when the key is on for a prime cycle and when the engine is running and/or oil pressure is present.
It runs constantly or is supposed to-there is no stop start for pressure.
Thank you. How long does it run under prime? I will look to the relay. When you say engine is running, is that based on oil pressure only, or is there another source for information? ie: signal from PMD.
Thanks for the info
On 96+ the prime cycle is activated during the glow cycle and is usually about 3 seconds.
As far as the LP activation during running it is controlled by the ECM controlling the LP relay from inputs from the CPS and OS indication that the engine is running,it however can also be powered by the OPS as a backup power source-that is why the LP continues to run after engine shutdown while oil pressure bleeds off.
For 95 and down the OPS is the only power source when the engine is running and an OPS relay mod is encouraged.
Relay tested OK, but I noticed that the LP is not appear to run that brief time after shutoff. I will go after the OPS.
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