Engine/Transmission (1998.5 - 2002) Lift pump will run then not run.

[rono]

When I bump the starter the lift pump will run for the 20 to 25 seconds with 12 psi. Start the motor and when the timer runs out pressure goes to 0. What could be causing this.



Thanks, Ron

 

[JLandry]

Ron,



The lift pump is controlled by the ECM. Since you were able to verify the lift pump actually works (by bumping the starter), then a reasonable conclusion is the ECM is malfunctioning.



Question: Did this problem suddenly start on its own, or did it start after you changed the fuel filter?



I ask because are you sure the fuel pressure is dropping to zero 25 seconds at the end of the timer, or could it just be the demand from the VP44 pulling the pressure down to zero due to fuel starvation or air trapped in the low pressure side of the fuel system?



If you suspect it could be trapped air, place a suitable container or large wad of rags under the VP44, loosen the fuel inlet fitting, then bump the starter to activate the lift pump. This will push any air out of the fuel inlet fitting.



Good luck,



John L.

 

[rono]

Thanks for your reply John. I have a switch that I can select between two sending units, one before the filter and one after the filter. The problem started all of a sudden. I'm going to look a the wiring diagram to see where all the signals come from to the lift pump. If it is the ECM I may just wire around it and run the pump anytime the engine is running.



Ron

 

[JLandry]

Ron,



This thread might be helpful:



Loss of Power to Lift Pump



John L.

 

[mhenon]

I let the ECM throw an external relay that powers the LP. I can bypass the ECM altogether if need be but so far it's been providing the small amperage needed to trip the relay.

Always was suspicious of letting the ECM run the pump and I've had intermittent problems like yours in the past. Since installing the relay I haven't had LP problems for the past 4 or 5 years.

Mike

 

[rono]

Thanks for the replies, everyone. After looking at the diagram like some of you did, I also thought that the pump could be run from an external relay. Since it draws up to 12 amps. I also thought that was to much for the ecm, but could be handled by some scr's or similar.



Anyway, I am going to get a relay and socket assy. and wire mine to work thru the relay and bypass the ecm.



Thanks again.



Ron

 

[mhenon]

I've had good luck with this relay from Summit Racing:

MagnaFuel MP-1025 - MagnaFuel Fuel Pump Relay Harnesses - Overview - SummitRacing.com



Mike

 

[KCanepari]

hey. having the opposite problem on mine. Won't run when I bump the starter, but will run with engine running. my question on the relay is: if you look in the book it states that the pump has two duty cycles. it runs at 25% when engine is cranking. is this important or should I not worry about to much pressure at injecting port while cranking?

 

[Bob Cochran]

hey. having the opposite problem on mine. Won't run when I bump the starter, but will run with engine running. my question on the relay is: if you look in the book it states that the pump has two duty cycles. it runs at 25% when engine is cranking. is this important or should I not worry about to much pressure at injecting port while cranking?

If it starts ok then don't worry. Some have had starting problems if the pressure is to high though. I don't recall how high is to high maybe some can provide that information for you.

 

[mhenon]

hey. having the opposite problem on mine. Won't run when I bump the starter, but will run with engine running. my question on the relay is: if you look in the book it states that the pump has two duty cycles. it runs at 25% when engine is cranking. is this important or should I not worry about to much pressure at injecting port while cranking?

You're right. . the ECM sends pulses of power to the LP during start-up to keep the pressure down. With a relay running off the ECM you get those same pulses to the relay (the relay is triggered on and off) and the pressure is about the same. At least that seems to be what's happening.

I've been running my LP off the relay for 3 years or so and never had a problem with too much pressure. Much rather have the LP running off the battery than straight through the ECM.

You need to keep a spare relay as they can go bad too. I've had a few relays go south on me over the years but it was a quick side of the road fix with no tools required.

Mike

 

[KCanepari]

You're right. . the ECM sends pulses of power to the LP during start-up to keep the pressure down. With a relay running off the ECM you get those same pulses to the relay (the relay is triggered on and off) and the pressure is about the same. At least that seems to be what's happening.

I've been running my LP off the relay for 3 years or so and never had a problem with too much pressure. Much rather have the LP running off the battery than straight through the ECM.

You need to keep a spare relay as they can go bad too. I've had a few relays go south on me over the years but it was a quick side of the road fix with no tools required.

Mike

ya, but the ecm is malfunctioning, so that makes no sense. I would run the relay off of a key on circuit.

 

[mhenon]

I was just talking about duty cycles on my last post. If your ECM is really shot that's another issue. Running the LP off an accessory circuit "might" get the pressure too high on startup. It would depend on the pump you're using and the pressure it puts out. I think the VP44 wants 7lbs or so at start.

My reason for putting a relay in the ECM-LP circiut is to reduce the load on the ECM. Can't understand the logic behind using a computor to supply 10-12 amps @ 12 volts to run a pump. Hell they didn't even put a fuse in the system. The load on the ECM to trip a relay can't be much (milliamps?) and the battery does the real work. JMO

Mike

 

[JLandry]

Can't understand the logic behind using a computor to supply 10-12 amps @ 12 volts to run a pump. Hell they didn't even put a fuse in the system.

Mike,



Wanna bet increasing profit somehow weighed in on the decision?



John L.

 

[Chief USA]

When I bump the starter the lift pump will run for the 20 to 25 seconds with 12 psi. Start the motor and when the timer runs out pressure goes to 0. What could be causing this.



Thanks, Ron

12 psi fuel pressure static is the bottom of the barrel and I suspect you get less than that with the engine running. The owners manual says that much:



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In my opinion anytime fuel pressure drops below 12 psi during engine operation, the VP-44 can be damaged.



I would strongly recommend you replace that lift pump with at least an Airtex lift OEM replacement unit or preferably an Air Dog or FASS.

 

[mhenon]

Mike,



Wanna bet increasing profit somehow weighed in on the decision?



John L.

Wouldn't doubt that at all! Bean counters ruin everything they get their calculator on. If the engineers made all the decisions 90% of these problems wouldn't have happened in the first place.

Mike

 

[Bob Cochran]

Wouldn't doubt that at all! Bean counters ruin everything they get their calculator on. If the engineers made all the decisions 90% of these problems wouldn't have happened in the first place.

Mike

Having been an engineer that is I agree bean counters cause or trucks to not be $80,000 vehicles new. SOOOOO what is needed is low cost, reliable parts. For that it takes 2 to tango!!!! Not easy to do but when successful all are happy.