I Wanna Know......

[Marauderjack]

.....why the fuel pressure across my fuel rails differs by as much as 20 PSI!!!!:mad2:

Background: We users of AutoMeter fuel pressure gauges with senders mounted on the end of the driver's side fuel rail have complained about low pressure readings when compared to OBD II readings from the OEM sender on the passenger's side (inlet side) of the fuel rail setup!!:rolleyes:

When I turn my ignition on and let the fuel pressure build it shows 36 PSI on the AM gauge and 38 PSI on my Diablo Tuner plugged into the OBD II port...acceptable.:beer: When I start the cold engine the AM gauge settles in at 19-21 PSI at idle and the OBD II reading is bouncing between 38-44 PSI (the Walbro pump is switched on and off constantly to maintain average pressure in the system).

When accelerating the AM reading goes up momentarily to around 30 PSI while the OBD reading remains fairly stable at 38-44 PSI??:confused: When cruising the AM gauge reads 24-28 PSI while the OBD II is 38-44 PSI!!:rolleyes:

One more thing.....I/we (AM tech) tested the sender voltages and the numbers are right on the money.....the gauge is very accurate according to AM tech guy.:bows:

My question is......How can there be that much pressure drop across the fuel rails and not cause a lean condition on the driver's side bank?? BTW....the Ford OEM pressure spec for our return-less system is 25-45 PSI at the rail (kind of wide range)??

Does the EEC control each injector for Pulse width (rich/lean) based on available pressure at each injector??:confused: If not how come I haven't blown something up over the last 70K miles with the blower since this differential has existed from day one!!?? :argue: Incidentally the plugs are all uniform in color!!

Anyone have a clue?? The AM tech says he has no answer at this point but contends the AM gauge is doing it's job......I agree!!:beer:

Marauderjack:shake:

[RR|Suki]

.....why the fuel pressure across my fuel rails differs by as much as 20 PSI!!!!:mad2:

Background: We users of AutoMeter fuel pressure gauges with senders mounted on the end of the driver's side fuel rail have complained about low pressure readings when compared to OBD II readings from the OEM sender on the passenger's side (inlet side) of the fuel rail setup!!:rolleyes:

When I turn my ignition on and let the fuel pressure build it shows 36 PSI on the AM gauge and 38 PSI on my Diablo Tuner plugged into the OBD II port...acceptable.:beer: When I start the cold engine the AM gauge settles in at 19-21 PSI at idle and the OBD II reading is bouncing between 38-44 PSI (the Walbro pump is switched on and off constantly to maintain average pressure in the system).

When accelerating the AM reading goes up momentarily to around 30 PSI while the OBD reading remains fairly stable at 38-44 PSI??:confused: When cruising the AM gauge reads 24-28 PSI while the OBD II is 38-44 PSI!!:rolleyes:

One more thing.....I/we (AM tech) tested the sender voltages and the numbers are right on the money.....the gauge is very accurate according to AM tech guy.:bows:

My question is......How can there be that much pressure drop across the fuel rails and not cause a lean condition on the driver's side bank?? BTW....the Ford OEM pressure spec for our return-less system is 25-45 PSI at the rail (kind of wide range)??

Does the EEC control each injector for Pulse width (rich/lean) based on available pressure at each injector??:confused: If not how come I haven't blown something up over the last 70K miles with the blower since this differential has existed from day one!!?? :argue: Incidentally the plugs are all uniform in color!!

Anyone have a clue?? The AM tech says he has no answer at this point but contends the AM gauge is doing it's job......I agree!!:beer:

Marauderjack

wouldn't a bad sender give bad voltage to test equipment? Either way back to the question at hand, I just use my scangauge and wideband A/F meter... it is very strange that the pressure would drop 20psi across the rail :eek:

[snowbird]

Jack,

With an Autometer plugged at the very same (end) space on the driver side, I'm getting the very same 20-21 PSI reading at idle. Hope that helps.

I must admit I always had been annoyed but figured if all 8 injectors are sipping before, it must be it !

[HwyCruiser]

The stock fuel rail pressure sensor is referenced to vacuum whereas your Autometer is referenced to atmosphere. This is why with the key on / engine off they jive up, but while in vacuum or boost the gauge isn't getting the other half the story the sensor is getting.

[Marauderjack]

The stock fuel rail pressure sensor is referenced to vacuum whereas your Autometer is referenced to atmosphere. This is why with the key on / engine off they jive up, but while in vacuum or boost the gauge isn't getting the other half the story the sensor is getting.

Huh???

You lost me JD!!??:cool4:

Someone used the terms "gross pressure" which adds atmospheric and "net pressure" which subtracts atmospheric pressure......kind of makes sense but the AM tech say "BS"??:confused:

Marauderjack:shake:

[JMan]

The stock fuel rail pressure sensor is referenced to vacuum whereas your Autometer is referenced to atmosphere. This is why with the key on / engine off they jive up, but while in vacuum or boost the gauge isn't getting the other half the story the sensor is getting.

Lemme help MJ-
Your cockpit gauge shows the pressure difference between the source (fuel rail) and the atmosphere. The fuel rail sensor, the one you're reading in OBDII, shows the difference between the source (again the fuel rail) and the absolute pressure in the intake manifold. This pressure varies with conditions - throttle opening, boost etc.

Lemme see if I can lose everyone here-
Standard atmospheric pressure can be displayed in several ways and is approximate and varying-
A. 14.8psi
B. 29.92"hg
C. 101.6 kPa
D. 1016 hPa or millibars
E. 1 atmosphere or Bar

Using (A) and (B) above but rounding them off to 15psi and 30.00"hg for ease of calculation -
Typical engine vacuum at idle is around 20"hg. Since it started with an absolute pressure of ~30" and we have a vacuum of ~20", that leaves an absolute pressure of ~10"hg or one third the pressure of the atmosphere inside the intake manifold. Using this in pounds per square inch (That's what your gauge reads), we started with 15psi and end with 5psi. The atmosphere is 15psi and the intake manifold is 5psi at idle for a difference of 10psi.

Summary -
You should see around 10psi higher on the OBDII reading than your gauge to atmosphere at idle and they should be very close key on engine off!

HTH

J

[Marauderjack]

Lemme help MJ-
Your cockpit gauge shows the pressure difference between the source (fuel rail) and the atmosphere. The fuel rail sensor, the one you're reading in OBDII, shows the difference between the source (again the fuel rail) and the absolute pressure in the intake manifold. This pressure varies with conditions - throttle opening, boost etc.

Lemme see if I can lose everyone here-
Standard atmospheric pressure can be displayed in several ways and is approximate and varying-
A. 14.8psi
B. 29.92"hg
C. 101.6 kPa
D. 1016 hPa or millibars
E. 1 atmosphere or Bar

Using (A) and (B) above but rounding them off to 15psi and 30.00"hg for ease of calculation -
Typical engine vacuum at idle is around 20"hg. Since it started with an absolute pressure of ~30" and we have a vacuum of ~20", that leaves an absolute pressure of ~10"hg or one third the pressure of the atmosphere inside the intake manifold. Using this in pounds per square inch (That's what your gauge reads), we started with 15psi and end with 5psi. The atmosphere is 15psi and the intake manifold is 5psi at idle for a difference of 10psi.

Summary -
You should see around 10psi higher on the OBDII reading than your gauge to atmosphere at idle and they should be very close key on engine off!

HTH

J

Soooo......

If the AM gauge sender was placed before the fuel rail it would still read about 10 PSI low.....Right???:confused:

[Zack]

Trust the OBDII reading, Autometer FP gauges (including mine) are way off.

[JMan]

Soooo......

If the AM gauge sender was placed before the fuel rail it would still read about 10 PSI low.....Right???:confused:

Yes, it will read different than the fuel rail pressure sensor when at idle. The reason for all that technical crap was to help you understand how much difference is normal and/or how far off your gauge really is.

Best luck,

J

[Marauderjack]

Yes, it will read different than the fuel rail pressure sensor when at idle. The reason for all that technical crap was to help you understand how much difference is normal and/or how far off your gauge really is.

Best luck,

J

Well JMan....

I guess my next question is.....IS THE OBD II PRESSURE WITHIN REASON FOR MY APPLICATION???:confused:

I have 39# injectors and have never had any pinging so I guess it is OK??:cool: As I recall the pressures have remained constant for the past 2 years.......:confused:

Just wonder how long the Walbro pump will last....I do have a spare and may just go ahead and put it in to be safe!!:beer:


Thanks for your input!!:beer:

Marauderjack:burnout:

[Zack]

Jack, your car is doing exactly what its supposed to be doing.
I wouldnt worry about that pump either.
Go have a cold beer, its 5:00 somewhere :beer:

[FordNut]

The pressure across the injector (what you see on the OBD II port)should normally be around 40 psi. Sometimes, if your injectors are not quite big enough, a tuner will program the tune to bump the pressure up to something closer to 50 psi. The injector size is a different subject. The injector size is normally quoted as 39#, but that is abbreviated. They are really rated as 39#/hr and that flow rate is at a particular pressure (across the injector). I believe a 39#/hr injector is the same as a 42#/hr injector, the only difference is that one is measured at 35 psi and the other at 40 psi (approximately).

For instance, I have 60#/hr injectors in my car and the pressure across them is 40 psi. When the blower was on the car, my tuner had the fuel pressure programmed to go up to 50 psi at high rpm. I guess he figured the 60 # injectors were almost too small.

[JMan]

Well JMan....

I guess my next question is.....IS THE OBD II PRESSURE WITHIN REASON FOR MY APPLICATION???:confused:

I have 39# injectors and have never had any pinging so I guess it is OK??:cool: As I recall the pressures have remained constant for the past 2 years.......:confused:

Just wonder how long the Walbro pump will last....I do have a spare and may just go ahead and put it in to be safe!!:beer:


Thanks for your input!!:beer:

Marauderjack:burnout:


38-44psi is fine. Look at your Fuel Pump Module Duty Cycle in OBDII. If the Duty Cycle is less than 50% you're doing okay and have plenty to spare! Keep in mind I'm not a hot rodder or engine builder, I'm just stating the normal stuff. Your application is pretty extreme!



Jack, your car is doing exactly what its supposed to be doing.
I wouldnt worry about that pump either.
Go have a cold beer, its 5:00 somewhere :beer:

I agree with Zack ! ! !

[Marauderjack]

I have forwarded all of your comments to Cory at AutoMeter and he has forwarded them to his engineers!!!:rolleyes: I'll bet he wishes he never got my inquiry now but we will all (including him) learn something here...I hope!!??:confused:

I made an interesting observation today.....If I take my manifold vacuum reading at any one time and add it to the AutoMeter fuel pressure reading it almost exactly duplicates the OBD II reading!! Example: 20 PSI at idle and 20" of vacuum makes the 40 PSI that I see at the OBD II port?? At 10" of vacuum I see 28-30 PSI giving 38-40, etc.!!:beer:

When the boost kicks in the fuel pressure more or less duplicates the OBD II readings.......it peaks out at 40-45 PSI on the AM gauge :cool:

Stay tuned!!:rolleyes:

Marauderjack:burnout:

[HwyCruiser]

Jack, it takes about 2" hg of vacuum to equal 1 psia. At 20" of vacuum your ODBII reading should be about 10 psi higher than your gauge reading.

OK, so here's the long answer I didn't have time to complete before...

The fuel rail pressure sensor is a differential pressure sensor, where the engine manifold vacuum pulls down the low side (the reference) of the sensor adding about 1 psi to the reading for every 2" of hg of engine vacuum. It's basically reading the pressure drop across the injector from the pressurized fuel rail to the engine manifold vacuum to control the duty cycle of the fuel pump to maintain a programmed value. For the 39# injectors it should be right around 39 psid. A little higher isn't going to kill anyone.

From how I understand it, if the fuel pump can't keep the pressure drop at design the PCM will compensate by increasing the injector duty cycle to maintain the required flow. The lower the fuel pressure the longer the injector has to stay open to deliver the same amount of fuel. Ideally there is enough pump to keep the pressure drop at design, but when increasing the injector duty cycle still doesn't get enough fuel then you're looking at upgrading the pump or adding a boost-a-pump.

Kenne Bell has a pretty good article that explains all this in detail:
www.kennebell.net/techinfo/general-info/BAPtheory_kens.pdf

Boil it all down and your fuel pressure gauge is giving you gauge pressure. Your FRPS via the ODBII port is giving you differential pressure. Two different physical measurements. The only time they are going to read the same is when the engine manifold vacuum is 0 psig or at atmospheric pressure.

Also FWIW, it seems like the FRPS logged from the ODBII port is filtered out quite a bit, probably slow to change to keep the driver signal from bouncing all over the place. From what I understand the fuel pressure is controlled by a Proportional-Integral-Derivative (PID) loop, which are inherently unstable during change. I deal with them all the time in industrial applications are are pretty much "old technology" in the controls world.

[Marauderjack]

QUOTE....Also FWIW, it seems like the FRPS logged from the ODBII port is filtered out quite a bit, probably slow to change to keep the driver signal from bouncing all over the place. From what I understand the fuel pressure is controlled by a Proportional-Integral-Derivative (PID) loop, which are inherently unstable during change. I deal with them all the time in industrial applications are are pretty much "old technology" in the controls world.
Reply With Quote

Actually the FRPS data changes very rapidly on the Diablo tool!!:beer:

I am currently awaiting the commands from Ron at Linear Logic to be able to read FP with my ScanGauge II......it will probably duplicate the Diablo readings but then I can discard the AM FP gauge and put something worthwhile in the hole it occupies now!!!:bows:

I'll post whatever AutoMeter comes back to me with!!:rolleyes:

Thanks for all replies!!:beer:

Marauderjack:burnout: