Fuel pressure

Im having the same exact problem, except mine is running around 37psi at idle. Its makin my car run like crap...can't be the fuel pump from what i can tell.......its a 99....so im askin the same question.....

FWIW, my 2002 Ford manual calls for a fuel pressure with the engine running of 35-50psi (241-345kPa). With the key on, engine off it indicates 25-40psi (172-275kPa).

When was the fuel filter last replaced?

i've changed my filter 2x in the past 2 months......to no avail.......could the regulator be causing a problem? and what is that lil box on the fuel rail? its got a vacuum line and some wires going into it?

Frankenstang838 said:

i've changed my filter 2x in the past 2 months......to no avail.......could the regulator be causing a problem? and what is that lil box on the fuel rail? its got a vacuum line and some wires going into it?

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Your car doesn't have a fuel pressure regulator. The sensor on the fuel rail is a manifold-referenced pressure transducer that tells the PCM what the pressure in the rail is with respect to the manifold pressure. The PCM uses that sensor to adjust the output duty cycle of the FPDM (fuel pump driver module) which drives the fuel pump in the tank. A higher duty cycle means higher fuel pump speed and output meaning more pressure. Lower duty cycle slows the pump down and output and pressure drop. So the "regulator" in these "returnless" fuel system cars is really a closed-loop control system consisting of the PCM and FPDM with feedback from the pressure sensor.

How did you take your fuel pressure readings? Using the Schrader valve on the fuel rail? Can you hook a scanner up to it and read what the PCM thinks the fuel pressure is?

i took the pressure reading off the valve on the fuel rail. I've had a scanner hooked up to it a couple times.....i might go talk to my buddy with the snap on scanner and have him hook it up again.....the car passed all the tests and has never kicked the check engine light on. Thanks for the explination.......im not real knowledgeable on the returnless fuel system.

trinity_gt said:

Your car doesn't have a fuel pressure regulator. The sensor on the fuel rail is a manifold-referenced pressure transducer that tells the PCM what the pressure in the rail is with respect to the manifold pressure. The PCM uses that sensor to adjust the output duty cycle of the FPDM (fuel pump driver module) which drives the fuel pump in the tank. A higher duty cycle means higher fuel pump speed and output meaning more pressure. Lower duty cycle slows the pump down and output and pressure drop. So the "regulator" in these "returnless" fuel system cars is really a closed-loop control system consisting of the PCM and FPDM with feedback from the pressure sensor.

How did you take your fuel pressure readings? Using the Schrader valve on the fuel rail? Can you hook a scanner up to it and read what the PCM thinks the fuel pressure is?

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my fuel pressure stays at 39-40 at WOT with a peak boost of 10 psi from the supercharger. some people say that is right, some say it should go up 1 psi over 39 as boost increases, but my gauge stays at 39-40. your thoughts?

Lookin at a diff post, im thinkin my problem lies elsewhere.....im gonna check the fuel stuff tho....just to be safe

bergs said:

my fuel pressure stays at 39-40 at WOT with a peak boost of 10 psi from the supercharger. some people say that is right, some say it should go up 1 psi over 39 as boost increases, but my gauge stays at 39-40. your thoughts?

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I believe the following to be correct but do some corroborating research to be sure:

The goal of the OE design was to ensure a relatively constant pressure drop across the fuel injector, a drop of approximately 39psi. The rail pressure is adjusted with respect to the manifold pressure to attain a constant drop of something around 39psi. So if you measure the absolute manifold pressure and see 10psia, then the fuel rail pressure must be 49psia to maintain 39psi across the injectors. In that regard, I'd expect fuel rail pressure to increase one psi per psi of manifold boost.

It's important to know the "base" point when measuring pressure and that the gauges being used are using this same base point. There are "absolute" (psia) and "gage" (psig). A "boost gauge" that reads '0' when exposed to one atmosphere (no boost, no vacuum) is showing a gage reading. If it read 14.7, it would be showing absolute. So you need to make sure when taking readings that all gauges and meters are starting with the same base.

As well, the PCM isn't aware of manifold pressure, per se. It attempts to obtain a 39psi reading from the fuel rail sender but the sender itself is modifying the signal seen because it's referenced off the manifold pressure. If you used a scan gauge, for instance, that reads the fuel pressure from the PCM using the OBD-II port, you'd probably only ever see around 39psi shown because what the PCM is showing is really the pressure drop from the rail to the manifold pressure (it's reference point...) You'd need a rail-mounted transducer/sender and separate gauge to show the absolute fuel rail pressure. Which do you have?

If you're using a separate FP gauge and sender, you might want to have your system checked. As I said earlier, I'd expect the fuel pressure, referenced to the atmosphere (not manifold pressure) to change with engine running conditions, whether at idle or under boost. In those conditions, I'd expect to see 49psi at 10psi of manifold boost. But again, it all depends on what's being used to make the measurement.

trinity_gt said:

I believe the following to be correct but do some corroborating research to be sure:


You'd need a rail-mounted transducer/sender and separate gauge to show the absolute fuel rail pressure. Which do you have?

If you're using a separate FP gauge and sender, you might want to have your system checked. As I said earlier, I'd expect the fuel pressure, referenced to the atmosphere (not manifold pressure) to change with engine running conditions, whether at idle or under boost. In those conditions, I'd expect to see 49psi at 10psi of manifold boost. But again, it all depends on what's being used to make the measurement.

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i have an autometer gauge with the sender on the fuel fail where the shrader valve was.

bergs said:

i have an autometer gauge with the sender on the fuel fail where the shrader valve was.

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Then I'm not sure I'd expect the gauge to stay at 39psi all the time. Although this vid is pretty shakey, you can see the FP gauge increase under boost:

Stewart Warner Boost/Fuel pressure gauge in Kenne - Car Videos on StreetFire

bergs said:

i have an autometer gauge with the sender on the fuel fail where the shrader valve was.

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I have the same gage and it fluctuates between 35psi & 40psi, but I’m N/A.

As stated fuel flow is based on pressure differential. If you have a 42 lbs/hr injector that is rated @ 39 psi and you have 10 psi boost with 39 psi fuel pressure you have 29 psi pressure differential and your injector now flows 36.22 lbs/hr.


On my web site
Calculate Change in Fuel Injector Flow from change in Fuel Pressure:
Automotive Performance Software / Interactive JavaScripts Calculators

I datalogged fuel pressure at the rail when I was making my dyno pulls, and it went from around 32 at idle to 42ish under load.

a good rule of thumb is for every 1 psi boost you should see 1 psi increase over 39psi and for every 2" of vacuum you should see a 1 psi drop from 39 psi.

20" of vacuum-~ 30 psi
wot(0")- 39 psi
10lbs boost- ~ 49psi

Stan Weiss said:

As stated fuel flow is based on pressure differential. If you have a 42 lbs/hr injector that is rated @ 39 psi and you have 10 psi boost with 39 psi fuel pressure you have 29 psi pressure differential and your injector now flows 36.22 lbs/hr.


On my web site
Calculate Change in Fuel Injector Flow from change in Fuel Pressure:
Automotive Performance Software / Interactive JavaScripts Calculators

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i have 42lb. injectors and a gt "supercar" fuel pump. at idle the fuel pressure is 29 and at WOT with 10 pounds of boost i see 39 psi of fuel pressure. sound right?

The presssure increase is correct.

OK 42 lbs/hr are only 42 lbs/hr at their pressure rating below that they flow less and above that they flow more. At what pressure either psi or bars is your injector rated 42 lbs/hr. Then we can calculate what they flow at 29 psi (2 bars).

Stan

Edit: If you are running FORD injectors FMS-M-9593-F302 they are rated 42 lbs/hr @ 39.15 PSI

Stan Weiss said:

The presssure increase is correct.

OK 42 lbs/hr are only 42 lbs/hr at their pressure rating below that they flow less and above that they flow more. At what pressure either psi or bars is your injector rated 42 lbs/hr. Then we can calculate what they flow at 29 psi (2 bars).

Stan

Edit: If you are running FORD injectors FMS-M-9593-F302 they are rated 42 lbs/hr @ 39.15 PSI

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yes, they are the ford racing green top injectors. so i guess that means that the 39psi that i see at wot is correct?

NO!

If you have a 42 lbs/hr injector that is rated @ 39 psi and at WOT you have 10 psi boost with 39 psi fuel pressure you have a 29 psi pressure differential and your injectors are flowing a max of 36.22 lbs/hr. The question becomes how much fuel does your engine need?

I calculate that a 4.6l at 6000 RPM with 10 psi boost needs a mass air flow of around 3460 lbs/hr. With an AF of 12.5:1 each injector need to flow 34.6 lbs/hr

(3460 / 12.5 ) / 8 = 34.6

Stan Weiss said:

NO!

If you have a 42 lbs/hr injector that is rated @ 39 psi and at WOT you have 10 psi boost with 39 psi fuel pressure you have a 29 psi pressure differential and your injectors are flowing a max of 36.22 lbs/hr. The question becomes how much fuel does your engine need?

I calculate that a 4.6l at 6000 RPM with 10 psi boost needs a mass air flow of around 3460 lbs/hr. With an AF of 12.5:1 each injector need to flow 34.6 lbs/hr

(3460 / 12.5 ) / 8 = 34.6

Click to expand...

I am a little concerned /confused on the info posted. for one 12:5 afr at 10 psi is not considered safe, these figures seem to apply at 100% duty cycle, which you don't want, and lastly what about the effects of the pressure/vacuum the injector see's at the tip. he was asking about pressure not flow, he cannot see flow on a guage. 39psi at 10psi boost is too low, no matter what size of inj. imho(I won't run one with that low of pressure at rail)