Engine/Transmission (1998.5 - 2002) Better Fuel Pressure = Better Fuel Economy

[wcoomes]

I replaced the lift pump with the Glacier 391 system and I am amazed. In checking fuel economy I got 16. 2 mpg last tank, the most MPG's ever with my truck since I bought it in 2002 with 3996 miles. Anyone else notice an increase in MPG since changing their pump?

 

[rweis]

I do know that the difference between the input fuel temperature and the output fuel temperature gets bigger for all other conditions the same when the input fuel pressure goes over about 16#.



In the old post "pump, lines, and what not... " They also saw elevated fuel temperatures as fuel pressures rose above about 20#.



This tells me that the pump is working harder to do the same job.



Calibration fuel pressure is 13. 5#, and that is about where I try to stay with mine. I think it is generally thought acceptable to be +- about 3# from calibration pressure.



Could it be that your VP44 is getting sufficient volume (and pressure) now rather than the way it was running before?



I have seen threads about when someone replaced a VP44 the mileage went up. I have seen threads from the pump rebuilders that falling mpg is an indication the VP44 is on the way out.



Generally if the mpg is getting better then the system is running better. Which is an easy measure to take and track.



Bob Weis

 

[Michael Hughes]

I have had two VP44's go bad and I never saw any change in my mileage. I have also changed lift pump three times and I didn't see any change in mileage.

When my first lift pump went out I was ready surprised, because I had read that your mileage would go down when the lift pump got weak.

Just my 2 cents.

 

[jeepit]

I am now consistently getting 20 mpg unloaded on the highway since LP replaced (see sig). Befoe I could not get above the 20 mark.

 

[DSpencer]

Higher pressure should mean more fuel circulated back to the tank. The injector pump will cause most of the heating of diesel. I doubt that the low pressure pump would have much effect on that.



I have heard just the opposite on fuel efficiency, but not in some time. I used to hear threads from people that had changed their IP's and suddenly had lower mpg's.

 

[Diesel Gunner]

I have just relocated my LP to the frame rail. Will be traveling to Arizona in a few weeks and will keep track of fuel mileage via GPS and station fuel pump readings. Last trip to Oregon I averaged 21. 6 and that was on LP #2. This is LP #3 so we will see.



Oh, by the way. My LP pressure post filter went from 10 psi to 14. 5 psi after I moved the LP. Don't get me wrong, I am not singing praise, yet. Just letting you know that if there is any MPG change I will post.

 

[rweis]

Heating fuel by the injector pump -



I have monitored this for about a year.



RPM's have a direct relationship to the amount of heat imparted to the fuel.



Load (empty / towing) have a direct relationship to the amount of heat imparted to the fuel. ( Thanks to Gary. I was sure there was no relationship to load, he was sure there was. I set out to prove him wrong. I was the one wrong, he was right)



Generally:

Empty

55 mph (ETC 4. 10) ~ 1800 rpm flat output fuel temp = input fuel temp + 12*

60 mph (ETC 4. 10) ~ 2000 rpm flat output fuel temp = input fuel temp + 15*



Towing 13k 5er

55 mph (ETC 4. 10) ~ 1800 rpm flat output fuel temp = input fuel temp + 17*

60 mph (ETC 4. 10) ~ 2000 rpm flat output fuel temp = input fuel temp + 20*



Then the KEY is do/can you get rid of the increase fuel temperature or does it increase and increase as the quantity of fuel decreases.



Empty

I have found that the input fuel is OAT + 10*



Towing

I have found that the input fuel is OAT + 20*



I have 4 fuel coolers (2 frame rail, 1 fined 18" cylinder, 1 H7B w/1000 cfm fan) and return the VP44 return fuel to the tank vent line to mix better with the entire tank.



Generally I can keep the input fuel to the VP44 to no more than OAT + 20*



95* here today, empty, input fuel 105* (+10*), output fuel 60 mph 2000 rpm 120* (+15*). Towing would give input fuel 115* output fuel 135*



I can limit fuel temp increases as the fuel quantity decreases and the fuel would get hotter the fuel coolers become more efficient due to the spread of OAT to fuel temp (I think).



Bosch max input fuel temp is 160*



Bob Weis



Opps forgot one other thing: When the input fuel temp goes over 100* the input pressure to the VP44 falls 1 psi. Thinner hotter fuel?

 

[PRout]

So, in your opinion Bob,

Would it be helpful to add a fuel cooler (like an oil cooler unit) on the input side of the VP44? What I have in mind is using an oil cooler and tapping it in just before the OEM fuel filter so fuel would be filtered by the fass, it would go up through the cooler in front next to the AC unit (i think if there is room) and be filtered by the OEM unit then into the VP.

Do you think that would make a significant difference in cooling the VP?

The only thing I dont like is it adds more restriction to the input line if the FASS were to fail. Right now, with big fuel lines if the FASS were to fail, Im hoping the VP could pull its own fuel to get home.

Edited: To try to veer back on subject, I havent seen any mileage differences by changing fuel pressure. Have been up to 25psi and down to 8. All I know is mileage didnt change noticeably, but the lower the pressure, the easier it started, and "maybe" idled smoother. I like the 13. 5 number also.

 

[rweis]

I do not want to hijack the thread - Shortshift you have a pm



Bob Weis

 

[JRBurton]

Just back from vacation , had a FASS installed 2 weeks ago . Increase in HP & a little on fuel mileage running empty about 21. 5 at present . Towing 8000# TT 75 MPH used to get 10 MPG now get 12 MPG and thats a combination of hills and flat .

 

[wcjp]

The 08' 6. 4l Ford Superduty's have a fuel cooler mounted to the block. Also, they have 4micron filtration.

 

[Melton]

Added a fass pump and a edge/attitude, I tow a 8000 TT just returned from the Colo. Mtns, and got 12 MPG @ 75 MPH and got 13. 4 running 65 MPH. I thought that it was the Edge but it could be the Fass pump. I had never got above 10 MPG before. Next I am going to add 4" exhausts. I blew a power stroke off the road on La Veta Pass and he had a lighter trailer then I do, made me feel that it was all worth the time and effort.