• Welcome to The Truck Stop! We see you haven't REGISTERED yet.

    Your truck knowledge is missing!
    • Registration is FREE , all we need is your birthday and email. (We don't share ANY data with ANYONE)
    • We have tons of knowledge here for your diesel truck!
    • Post your own topics and reply to existing threads to help others out!
    • NO ADS! The site is fully functional and ad free!
    CLICK HERE TO REGISTER!

    Problems registering? Click here to contact us!

    Already registered, but need a PASSWORD RESET? CLICK HERE TO RESET YOUR PASSWORD!

Lift pump question

hookedup50

hookedup50

Well-Known Member
Messages
852
Reaction score
293
Location
MA
My lift pump works fine when there is oil pressure as in always on and good fuel pressure from guage on hood while driving test. It has been years since I DD this truck. I kind of remember that when you turn the key to start position the LP should come on to pressurize the fuel. Is this correct on a 97 truck. This is just one of a bunch of electronic problems with the truck. I'm gonna change out the OPS today as I have a new one and found my basin wrench to get at it without yanking manifold.
 
On all obdII trucks, '96 on up the fuel system is primed while key is turned to on position during wait to start. Be sure you instal a genuine ac delco ops. all others are junk
 
Thank you. It presently only comes on with oil pressure. I'm seeing me changing out any old sensors. I'm trying to stop spending money on this truck, but it has other ideas.
 
Am I misunderstanding? If the lp comes on when you get oil pressure and does not come on with out oil pressure the the ops is working correctly and does not need replacing. The problem is it should come on with the key and wts on and is not.
 
Exactly, trying to find out why lp not on with key. This problem is leftover from before engine change. Is there a common problem causing this issue?
 
I think on the 97 and up the LP is tied in with the glow controller cause prestart the LP turns normally on and off in tune with WTS light.
 
The glow plug controller works for the glow plugs just fine. It starts fine on coldest day. Does anyone know if fuseable links in the ciurcit for the lp at the controller.
 
I ran a jumper wire from the lp plug near the engine bay fuse box to 12 volt source and it runs the pump. No bad fuses so should I suspect the relay. Going out to see if I have another to check it. Also going to write down all the transmission codes and start a new thread as my dtc thread seems to have died.
 
Although I have the older trucks I have never seen the lift pump run during WTS. It may be on some newer trucks, but, lots of confusion on it resulted in me chasing a ghost. IMO it doesn't do much anyway as I have the 1993 wired to key on and it is still hard to start.
 
I agree with You on chasing ghosts. The trucks starts like a new vehicle, I solved my trouble code issue as it was the 98 cartridges on my mt 2500. So I have been chasing ghosts already. I needed to clean the grounds on the truck anyway and I had to move the wires under the intake that were binding for piece of mind. When I have time I will move the ip a hair to try to find the sweet spot, but I am so close I doubt I would see a difference. Thanks everybody who has helped me here. Now bodywork!
 
Check with a gauge under load to 100% verify lift pump output because it can purr but still have weak output, on OBD-II lift pump run is momentary during the glow cycle, does not become "active" until sustained cranking as in a start mode or OPS is seen be OPS, that said PCM will keep lift pump energized if engine is running and OPS is not sending trons to the lift pump, via alternate path part of GMs "fix" to save PMDs that were getting inadequate cooling from IPs struggling to pull own fuel from tank without aid of a lift pump
 
I am gonna tape that same guage that I saw in one of your stickies to the windshield. Your pictures/stickies on grounds and how to set time withn a brick ect... has my truck running awesome. Finally got a relearn. Thank you TD.
 
On 96+ trucks the lift pump is controlled via the ECM by the fuel pump relay. The OPS is still there and will power the lift pump if teh fuel pump relay quits. Sounds like your fuel pump relay has gone bad. The OPS is not a problem on 96+ truck since when they are working teh fuel pump relay carries the load. And teh ECM also turns the lift pump on for 96+ trucks for about 2-3 seconds as a prime feature independant of the glow plug relay.
 
THEFERMANATOR said:
On 96+ trucks the lift pump is controlled via the ECM by the fuel pump relay. The OPS is still there and will power the lift pump if teh fuel pump relay quits. Sounds like your fuel pump relay has gone bad. The OPS is not a problem on 96+ truck since when they are working teh fuel pump relay carries the load. And teh ECM also turns the lift pump on for 96+ trucks for about 2-3 seconds as a prime feature independant of the glow plug relay.
Click to expand...
Explain why on both my 98's the LP quits running as soon the WTS light goes off but the LP comes back on when i turn the key off and on again but only for as long as WTS light remains on
 
Thats how my 97 used to operate before no lp till oil pressure is up.
 
bison said:
[/B]Explain why on both my 98's the LP quits running as soon the WTS light goes off but the LP comes back on when i turn the key off and on again but only for as long as WTS light remains on
Click to expand...

Couldn't tell yeah. The 00 that a friend of mine had for awhile only ran it for a few seconds at key on. And on a hot start when the glow plugs wouldn't come on it would still turn the lift pump on for a few seconds.
 
I've never seen it in writing, but from my dissection of the pump in the TRL thread of lift pumps, I suspect that "chip" & board are used to monitor line/supply pressure to the IP via current monitoring of how hard the pump has to pump since there is no pressure switch in the system to tell the pump when to start/stop.

IE if low pressure in the line would be low resistance head on the pump & low current load in the pumps coil that operates the pumping plunger and pump "purrs/pumps" until pressure head is built sufficiently; then pump is working harder against the head so current would rise in the coil as pump pushes against the head and shuts off.

But there is also logic in the PCM that wants to see oil pressure to keep the power active while engine is running and shuts it off when engine is not running, this is evidenced in the short purr of the lift pump some hear after key off until oil pressure switch bleeds down.

I also have a notion that extended run of lift on run is an early indicator of lift pump becoming weak or open of clearances in the IP as theoretically the fuel system is more/less "closed loop" and there should be minimal bleed down for a lift pump to run at an extended interval on next start up.
 
I swapped out the fuel pump relay and the lp pressurizes with the key again. I still need to stick the guage to the windshield and check pressure under load. I'm gonna install a permanemt fp guage in the cab as well. I just don't know where to install it, as the pillar is full.
 
You must log in or register to reply here.
Back
Top