Engine/Transmission (1994 - 1998) Fuel Pressure Issue - Possibly Lift Pump?

[Cooker]

I have a problem/concern I would like some suggestions with.



The truck had been getting pretty crappy mileage lately but I thought it was mainly due to pulling my trailer (usually around 4,000 lbs) 90% of the time. After finally getting fed up with the poor mileage, and the rising cost of fuel, I decided it can't be simply because I am towing such a light trailer.



I started with replacing the fuel filter because it had been almost 10k miles since the last change. No noticable change in MPG's.



I check the fuel pressure this evening and I thought something wasn't right as I was only getting 1-2 psi at idle I pulled of the 40k miles old overflow valve and the spring was at 7/16". I stretched to 9/16" and reinstalled. Fuel pressure is now 12-13psi at idle and 18-20psi at a no load 2000 RPM's



I will be replacing the OF valve as I personally feel stretching the spring is a bandaid repair but I am puzzled that the fuel pressure is not greater with the spring stretched. I am wondering now if I need a new lift pump, the current LP is 138k miles old.



Does this sound like a new lift pump is necessary?

 

[Joe G.]

It may be the lift pump, but probably isn't. You should see at least 18 PSI at idle. I usually have 25. The OF valve spring should only be a tiny bit over 1/2". Check all your hoses. One of them could be sucking air just a little. Also check the fuel heater. Check the large O-rings in the lift pump. One of them may be sucking air. Mine were hard and deformed when I had a problem like that. I suspected the lift pump, but it turned out to be the fuel heater. The input fitting to the fuel heater is a poor excuse for a fitting. It may be leaking a little air.

 

[SEmerson]

While it is running and you are watching the fuel pressure. Pinch of the return. If you get a rise in pressure then it is the overflow valve. If not look elsewhere. I would think it probly is the pump.

 

[Cooker]

Thanks for the replies.



I am going to try a new OF valve first, who knows maybe I will get lucky. If the spring was that crappy after 40k miles maybe it is too weak to keep up even after stretching it? I will try pinching the line at that time too, I'll have the gauge hooked up anyway. I tought of it when I had the gauge hooked up the other day but it was a little rainy so I was want to get back inside ASAP.



I suppose it could be the fuel heater. I replaced the entire thing about 25k miles ago. I wouldn't suspect something to be faulty aready but possibly, I will look there too.



I am planning on a LP. I will try a fuel volume test first but I am not very optimistic that it is going to turn to well.



Thanks again

 

[Joe G.]

The spring may have been stretched too much. If that happens it weakens it. I barely stretched mine. Before I did I had about 21 PSI at the most. Idle was something like 16. I stetched it a tiny bit to get it back to a little over 1/2". I think that may have been too much. It idles at 25 and it pegs my 40 PSI guage many times. I have 230K miles on it and it's the original OF valve.

 

[Cooker]

I think the spring has just had it I can't believe and OF valve, at least this one, has only made it about 40k miles.



I tried it at 1/2" thinking just as you said, I didn't want to weaken it and it was 10-11 psi that it when I took it back out and stretched it to ~9/16" and only got 12-13 psi



Why do I think solving this problem is going to be a PIA Working in the area of the LP looks to be a real pain. I remember when I was replacing my shocks and had the shock tower removed that the LP looked real accessible then. I think I might try to get at it, if it turns out to need replacement, thought the inner fender with the shock tower removed.

 

[Joe G.]

I took my starter and fuel filter off when I took my lift pump off to test it. I also removed the two nuts that fasten the master cylinder to the booster and leaned it toward the wheel well. That reduced, but did not eliminate, the swearing. Long arms and some fingers on your elbows would help a lot.

 

[Cooker]

Well... . I tried a new OF vlave today. Back to 10psi at idle. I see no other possible areas that look like there is any cause for concern in the fuel system. I guess I will be repalcing my LP



On a more positive note it appears that my MPG's have gone back up a little bit. Maybe at 22-25 psi at idle they will get even a little better.



Joe,

Thanks for the suggestions. Hopefully I can eliminate most of the cussing from this project

 

[Joseph Donnelly]

The lift pump runs on unfiltered fuel. After a while it is common to get the bore scratched up and lose some pressure/volume. If your pushrod that rides on the camshaft is OK, I'd re-use it with a new pump.

 

[Cooker]

If your pushrod that rides on the camshaft is OK, I'd re-use it with a new pump.

Joe,

Thanks for your comments. I'm curious now, does a new lift pump typically not come with a new pushrod?

 

[MFranz]

Lift Pump Questions?

Cooker,

I had my lift pump replaced at the dealer at 40000 miles. It took them all off an hour to do it. I was going to do it myself but being under warranty I let

them spend there dollar. Previous to that I had installed a fuel pressure gauge

at the VP44 pump,after the fuel filter and was getting all of 0 psi . At times the pressure would jump to 10 but that was intermittent. At first I thought that the gauge was faulty but it turned out to be the lift pump. New lift pump

gave us a 10 psi at idle and around 7psi under load. In asking around about

changing the lift pump I was told by more then one mechanic that it takes about an hour. Previous to the dealer doing the work I had bought a lift pump for $125 from a local garage in Silver City ,N. M. It was an O. E. M. carterpump,

that now is my backup pump. In reading past post on this subject there seems to be a lot of different readings for fuel pressure. Im curious to where you have taken your readings. I have a 2001 and the pump that it has is electric. I was wondering what the reference to cam shaft was. Isnt your pump electric.

Take Care and Good Luck

Mike

 

[Joe G.]

Mike,



He has a '96 with a P7100 injection pump. The lift pump is not electrical like used on the engines with a VP44 injection pump. It is mechanical and is driven by a push rod on a cam lobe.

 

[MFranz]

Lift Pump

Thanks for the info Joe. With all the problems that Ive read about with the electric lift pump Im wondering what they done with the newer model trucks

Maybe mechanical is the way to go.

Take Care

Mike