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Discussion Starter · #1 ·
When I start my truck in the morning the wts light comes on for a few seconds, goes off and I start. I hear the lift pump on the rail right below me as soon as I turn the key and even after the wts light goes off. Lift pump running the whole time. Fast forward to one hour of driving. I get to a job, can't work for various reasons and need to leave. Now, when I turn on the key the wts light comes on and is obviously shorter because the truck is warm. I notice as soon as the light goes off though, the lift pump stops. I can turn it over to no end without firing. It's as if it is starving for fuel. Normally I have to wait a few hours for it to cool before it will start. It will run to fuel runs out with no problems. Starting when hot is the problem. I've gotten to the point of keeping a generator and battery charging with me because my impatience has killed my batteries and left me stranded in remote substations. I also noticed if I try to start immediately after driving that hour, it will start. Waiting 10+ minutes, I just turns over with no fire. Almost like fuel is draining back into the tank, aka losing my prime?

I've researched the forums and most GP relay problems revolve around cold starts. I did read about the timing of the relay and how it is affected by coolant temp. This seems like a plausible place to start. A year ago I realized the top coolant sensor was completely corroded to the point of not being being attached to the sensor. Reminded me of battery corrosion. The harness had the connection attached without being connected to the sensor anymore. Cleaned the harness and replaced the sensor and this seemed to fix the problem. I'm forced to leave it running if stopping somewhere on the way home. The old lock doors, extra door key move which I hate doing. I did replace the glow plugs in 2019 with AC Delco 60G, so I am aware of that situation. Any thoughts?
 

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1995 GMC Suburban
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Did you open the t-valve when you said the LP is not running? just to test the flow.
Do this at home, though, to test because if the LP is weak or bad, you may not be able to start it again after you open the t-valve.

LP may run even though you are not hearing anything.
Unfortunately, LP may run but does not have enough power to get sufficient pressure.

The GP or GP relay have no effect when engine is running.
That does not mean that the GP relay cannot go bad, though.
 
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Discussion Starter · #3 ·
Did not check t-valve yet. Seems to be a heat related issue which made me deduct the LP. It always purrs in the morning (engine cool) no hesitation to fire, start. Hot engine = 1 sec wts light with LP also purring for 1 sec and a no fire situation. Let cool for two hours+..... the wts and LP operate longer and fires right up. GP and IP(new) three years old. The fact the GP relay is dictated by coolant temp. makes me think possible problem.
 

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Discussion Starter · #4 ·
The GP or GP relay have no effect when engine is running.
That does not mean that the GP relay cannot go bad, though.
Does the GP relay have any say into how long the LP runs BEFORE starting? Probably the question I'm trying to get answered at this juncture.
 

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Circuit Description (from FSM)

When the key is in turned ON, the PCM energizes
the fuel pump relay which sends voltage to the lift
pump. The lift pump remains ON during the glow
plug cycle. If the engine is not started immediately
after the glow plug cycle, the PCM shuts the fuel lift
pump OFF and waits until the engine starts. As a
backup system to the fuel pump relay, the fuel lift
pump can (IS) also be turned ON by the fuel pump/oil
pressure switch. When the engine oil pressure
reaches 28 kPa (psi), and the fuel pump relay does
not complete the circuit, the fuel pump/oil pressure
switch will close and complete the circuit to run the
fuel lift pump.

I will add that the OPS energizes the LP circuit at all
times when there is oil pressure no matter what the
LP relay is doing.
 
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Discussion Starter · #6 ·
Switch out GP relay last night. Started rough, I shut off quickly to assess in morning start-up. No start. Check t-valve, had flow. Bled at filter, looked good. Noticed smoke while turning over. Looked like possibly GP's. Put old GP relay back in and started. CEL now on. Going to work to salvage some day. Napa GP relay is what I put in. I don't get it.
 
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