When I switch the key to run to let the fuel pump prime it's not coming on. Once I crank the truck the pump works fine. What could be causing this? I've noticed that if I disconnect the batteries it will prime, but only for about a month or two and it'll go back to doing what it is now.

Any suggestions?

 

[I] may be wrong here but if I am someone will set you straight .The pump doesnt start till the starter engages. You must "bump" the starter then the pump will run for about 25 secs. Bump it again 25 secs. more. I am experiencing similar problems with fuel draining out of the line. But I am dilligently troubleshooting component by component. Frank[/ I]

 

That is not true. When you first turn the ignition to on, the Lift Pump should run for a few seconds then shut off. Same as if you have the truck off, drain the fuel filter housing a bit, shut that valve, then go turn the key to on. The lift pump will come on and fill the fuel filter canister and then turn off. Once started the LP will come back on and run normally.

I am not sure about why it wouln't be coming on. If the truck has been sitting over night will it come on if it's been sitting for awhile?

 

It wont come on and run for 25 seconds if I bump the starter either. However it used to and will if I unhook the batteries. How can I reset the computer on my truck?

 

Have you checked the relay that runs the LP. I am assuming there is a relay for it. So maybe when you disconnect the batteries you are resetting the Relay?

 

if your lp is not running after you bump the key, pull the plug on the lp and get a VOM on it, read the voltage when you bump the key. if you are not seeing voltage you have issues besides the pump, if you are seeing voltage get a new lp

 

What are we talking about here ? It's a relay, it doesn't reset. Relays cannot be reset. A circuit breaker can be reset but not a relay.
Pump will come on at first key start for 30 seconds, shut key off then on, pump will not come on unless you wait at least 1 minute, but if you "bump" the starter it will come on again for 30 seconds.
LP will not come on every time you turn the key.

 

Oops, sorry about that. I was thinking about a circuit breaker yes..

And my LP does the same, I could sit there and flick the key to on and off a few times and the pump wouldn't come on. So I thought there was something wrong. I waited a bit and it came on. However, he is saying his is never coming on before start up. I would THINK that it would come on at least once in a whole before start up...IMO

 

Try the key On - key Off - key On routine - LP should run ~25 seconds without bumping the starter

 

gmctd, I tried that also and it didn't not work.

Also I installed my AirDog 150 and it's still doing the same stuff it was doing when I was running just my Holley Blue pump and Vulcan Big line/Relocation kit.

 

Sounds like an ECM problem. I really have no idea but, normally the ECM will activate the fuel pump until pressure is reached and then turn it off until the engine is started. Looks like an ECM problem.

Charlie

 

You said,"the ECM will activate the fuel pump until pressure is reached"
How does the ECM detect pressure?

 

Probably through a pressure sensor. I have opened the water separator/ unscrewed it so the pump will only suck air and not build pressure. The pump will continue to run...and run...and run. The indicator light stays on. As soon as I tighten up the water separator, you hear it run freely for a moment, then you can hear the change in sound as the pumps gets fuel. Then as pressure begins building, the light goes off and a few moments later, the pump stops running. Must be a pressure sensor.

Charlie

 

It's not that sophisticated - ECM runs the lift pump enuff to prime the system, usually about 2-5seconds, then shuts down if no input from the Crank Position Sensor.

When the crank begins turning, pulsing the CPS, ECM again powers the lift pump, running it till the crank stops turning, then immediately breaks power for safety reasons, like in an accident.

If the starter is bumped and released, ECM will run the lift pump for ~25seconds, intended as a system prime function to purge air and fill the filter after maintenance.

Reason I asked about the key routine is the Jeep Bosch CRD system uses that routine to initiate the system prime mode - just seems odd that DCX doesn't standardize various common EFI functions.

 

So can I have my ECM reflashed or what? It's getting cold down here and I wouldn't mind have the high idle also.