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Truck starts and runs W/O fuel relay....

D

D2 Cat

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Location
south of Kansas City
Friends 95, 6.5 TD starts and runs with out the fuel relay plugged in.

When the engine is turned off the LP is running because it continues for a few seconds.

We also opened the needle valve at the WIF hose with the engine running. Fuel ran out the line and engine continued to run----so doesn't this indicate the LP is pumping?

We installed a new relay, turn key to start position---still no LP.

What would cause this?
 
That would indicate the ops is good and the lp is good. relay only activates lp during cranking ops takes over after it starts. I would look for as mentioned a bad ground or a bad connection possibly at the relay itself. look for corrosion.
 
I could hear the relays (2) we tried click when the key was turned to the start position, but the pump did not purr. If the relay clicked wouldn't that eliminate a bad ground at that relay terminal?
 
i doubt if you can hear the LP run while cranking.
Seems to me judged by your posts all is well.
If I where you, i would install a fuel press gauge,you'll be surprized the peace of mind it'll give you to see that all is well or not at a glance
 
The LP relay only powers the LP during crank in an OBD1 truck. LP does not need to be operational to start the truck. Would be good read the whole LP and OPS sticky. Factory wired, your truck wont run the LP with only Ignition in the On position.

The relay is done as soon as you stop cranking. Then OPS has independent battery source so that will keep running and after key is off until oil pressure bleeds off below 6psi. sounds like everything is working. Good to do the relay modification for the OPS, or will have a failed OPS in a matter of months.
 
buddy said:
The LP relay only powers the LP during crank in an OBD1 truck. LP does not need to be operational to start the truck. Would be good read the whole LP and OPS sticky. Factory wired, your truck wont run the LP with only Ignition in the On position.

The relay is done as soon as you stop cranking. Then OPS has independent battery source so that will keep running and after key is off until oil pressure bleeds off below 6psi. sounds like everything is working. Good to do the relay modification for the OPS, or will have a failed OPS in a matter of months.
Click to expand...

buddy, I'm missing something here. Help me understand it, 'cause electrical systems are not my strength!!

"LP relay only powers LP during crank in OBD1 truck."

"LP does not need to be operational to start truck."

"...sounds like everything is working."

If the truck starts w/o the relay in the circuit, doesn't the IP then have to provide the fuel (which is not ideal)? So, how can everything be working? Why is there even a LP relay?
 
D2 Cat said:
Why is there even a LP relay?
Click to expand...

Because GM was a bunch of IDIOTS, and didn't understand how the circuit should work. The lift pump on 94-95 only runs at 2 times. The first is by the relay which is only energized to power the lift pump while the starter is being engaged. Second is once the engine is running the OPS will complete the circuit and power the lift pump off of it's own power source once the engine reaches roughly 2.5-5 pounds of oil pressure. The OPS works WITHOUT the relay in any way shape or form, and the only thing the relay does is power the lift pump during cranking. The IP can pull fuel on it's own, just not very well. This is why it will run and start, but does better when you make the lift pump work with the glow plugs. 96+ uses a seperate circuit in the PCM to trigger the fuel pump relay and makes the circuit work like it should've from the get go.
 
Well, you should have the relay in, it wasnt working while cranking without the relay installed obviously. But its not supposed to run with just the ignition on and it didnt, so what are you looking for it to do? It was working while the engine was idling, so OPS was functioning. If its not working while cranking then check out the CRK fuse.

You can test if the relay circuit is functioning with the engine off, put it in gear, turn the key to start position (like as if you were cranking). It wont crank, but LP relay will be powered and should be pumping fuel.
 
THEFERMANATOR said:
This is why it will run and start, but does better when you make the lift pump work with the glow plugs. 96+ uses a seperate circuit in the PCM to trigger the fuel pump relay and makes the circuit work like it should've from the get go.
Click to expand...

This can be done with a couple of diodes and a lil wire and solder.....
 
Another reason why a fuel pressure gauge is recommended to all, is because these trucks run so damn well with a non-working lift pump that most times you'd have no idea your LP isn't working unless you have a guage or are pulling hard. This greatly shortens the IP's lifespan, as well as drops in MPG's. (Possibly because the extra IP work throws timing off a little???)
 
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