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#4115670 - 07/19/21 09:48 PM 04 LX - Fuel Pressure Test
DKH Offline
Stranger

Registered: 06/26/21
Posts: 4
Loc: WA
I just tested the fuel pressure on my 04 (150K) and I have a question.

Key "on" the pressure is at 40psi, bottom end of the normal range. Engine running the pressure drops to and stays at 30psi, 10psi below bottom end of normal range. The hold pressure is within spec.


The car is running slightly rough but not throwing any codes.

Is this definitely a replace the pump situation, or is there something else that I should be checking that could affect fuel pressure other than the pump?

Thanks!

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#4115678 - 07/19/21 10:47 PM Re: 04 LX - Fuel Pressure Test [Re: DKH]
dixiebandit69 Offline
5.4 Club
Member

Registered: 01/17/12
Posts: 317
Loc: Texas
With the engine running, pull the vacuum line off of the fuel pressure regulator.

It should pick up to 40 psi or so.

If not, probably new fuel pump time, or maybe time for a new fuel filter. Or a new fuel pump relay.

Maybe all three.
_________________________
2007 Grand Marquis LS: 5.4 swap, 9.9:1 compression ratio, Comp XE268s, NPI pistons, PI heads ported by me, Accufab upper plenum, 75mm DBW throttle body, Factory PI intake with Chinese spacers, CVPI dual exhaust, CVPI torque converter, CVPI air-box and zip tube, J-Mod, 3.55 gears, FMS Trac-Loc differential
17" Dorman CVPI steelies, BFG Comp T/A 255/45 R17s

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#4115688 - 07/20/21 12:19 AM Re: 04 LX - Fuel Pressure Test [Re: DKH]
RF_Overlord Offline

Poobah

Registered: 12/14/02
Posts: 7614
Loc: MA
KOER FP should be 40 lbs, not 30. I've not measured it KOEO. Only time I've ever seen mine read differently was when I drove to the supermarket and the car sat in the hot sun for a while. When I came out and started it, FP was 51 lbs and as I drove home (about 4 miles) the pressure slowly dropped back to 40. I assumed this was the fuel strategy to prevent vapour lock.
_________________________
'03 Mercury Marauder (The Blackbird)
Trilogy Motorsports supercharged #61
Too many mods to list / Driveway Queen

'04 Grand Marquis LS Limited Edition (Spruce Green)
Daily Driver (Thanks, Adam!)

'00 Grand Marquis GS (Silver)
Rotted frame and broken transmission crossmember
Being resurrected...slowly.

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#4115802 - 07/20/21 06:27 PM Re: 04 LX - Fuel Pressure Test [Re: dixiebandit69]
DKH Offline
Stranger

Registered: 06/26/21
Posts: 4
Loc: WA
Thanks for the suggestions.

With the engine running, I did pull the vacuum line off the fuel injector pressure sensor (mine does not have an actual fuel pressure regulator) and there was no change in fuel pressure. This has me wondering if the sensor is bad, but I'm not going to run out and replace it unless I'm sure, as it costs over $100.

The fuel filter was new when I did the check.

I'm hoping its the relay.

Thanks again.

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#4115804 - 07/20/21 06:28 PM Re: 04 LX - Fuel Pressure Test [Re: RF_Overlord]
DKH Offline
Stranger

Registered: 06/26/21
Posts: 4
Loc: WA
Originally Posted By RF_Overlord
KOER FP should be 40 lbs, not 30. I've not measured it KOEO. Only time I've ever seen mine read differently was when I drove to the supermarket and the car sat in the hot sun for a while. When I came out and started it, FP was 51 lbs and as I drove home (about 4 miles) the pressure slowly dropped back to 40. I assumed this was the fuel strategy to prevent vapour lock.


Good to know. Thanks.

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