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Topic: Fuel Vacuum at lift pump? (Read 449 times) previous topic - next topic

Fuel Vacuum at lift pump?

For those of you that have or have installed a vacuum gauge on the suction side if the lift pump (between the lift pump and primary fuel filter) I would like to know what readings you are getting with a clean (new) filter? Pulling fuel from 25' away thru a 3/8" fuel line, my gauge normally reads between 2" and 4".  I have been changing my filter when the gauge gets to 8" or so.  It does fluctuate somewhat between a full tank and 3/8 full as that is when I normally fuel up.

Just curios as to what others are seeing and also thinking that a 3/8" fuel line is a little small for a 25' run.

 
Justin & Cathy Byrd
1995 U280 "Old Faithful"
36' Build #4673
C8.3 Cummins
Allison MD3060R 6 speed - retarder
Powertech 10KW  4cyl Kubota

Re: Fuel Vacuum at lift pump?

Reply #1 –
Would a fuel pressure gauge readout after the lift pump tell us more about performance? Of course, also having a vacuum gauge before lift pump would be helpful..

Our dash fuel pressure gauge is helpful.

What vacuum reading are you seeing?

Re: Fuel Vacuum at lift pump?

Reply #2 –
Several of us have fuel pressure gauges mounted between the lift pump and the injection pump.  The most useful thing we learn from a gauge in that position is whether or not the overflow valve is performing properly.  It is very important to maintain adequate fuel pressure at the inlet of the injection pump.  On my mechanical C8.3L "adequate" pressure is around 20 psi at idle.

Fuel System Science Project

I can't recall reading about any members installing vacuum gauges between the primary fuel filter and the lift pump.  I will be interested to hear from anyone who has done so.
1993 U280 SE 40' WTBI, Build: 4359
C8.3 300hp, 6-Speed, Exhaust Brake
960 watts on the roof (6 x 160)
Sorento (or BOLT) on a Kar Kaddy SS
"Nature abhors a vacuum"

Re: Fuel Vacuum at lift pump?

Reply #3 –
Quote from: Barry & Cindy –
Would a fuel pressure gauge readout after the lift pump tell us more about performance? Of course, also having a vacuum gauge before lift pump would be helpful..

Our dash fuel pressure gauge is helpful.

What vacuum reading are you seeing?

Typical vacuum ahead of the lift pump runs 2" to 6" depending on engine RPM and fuel level in the tank.

Pressure between the lift pump and injection pump typically runs around 22 to 24 psi at idle and 28 to 30 at highway speeds.

I don't think I have a pic of gauges while running...........but they are mounted low on the dash




Justin & Cathy Byrd
1995 U280 "Old Faithful"
36' Build #4673
C8.3 Cummins
Allison MD3060R 6 speed - retarder
Powertech 10KW  4cyl Kubota

Re: Fuel Vacuum at lift pump?

Reply #4 –
I have installed a vacuum gauge between the primary and secondary fuel filters on my 3208t.  My objective was to watch for a plugging primary filter.  That vacuum runs about 3 in of vacuum at idle.  I expect that the vacuum will increase if the filter starts plugging.  All is well so far.  Have a great day. ----  Fritz
Fritz & Kathy Johnson
1991 36'