Bad fuel pressure, is reason for bad mpg?

[Pauls14]

Hi guys. Today checked my fuel pressure. When engine works is 65 psi. When I turn off, pressure is go down until 20psi during 3 minutes.
Engine works good, smooth, but I have 12 mpg. Is it bad pressure? Could be problems with mpg? How can I fix it. Thanks.

 

[cbxer55]

Well, the 98 requires 56 to 72 psi when running. I had to put a new pump in mine in late 2016, cause mine was down to 22 running. Wouldn't hardly get out of it's own way. Even now, with the new pump, mine is 00 when key on engine off. Running it's 65, like yours. Mine gets 18 or better highway. Around town, not so much. Maybe 15. Not highly likely low pressure key off will result in bad mileage. Usual culprit is a lead foot. LMAO! Ask me how I know. I drive mine like I stole it.

 

[Pauls14]

Well, the 98 requires 56 to 72 psi when running. I had to put a new pump in mine in late 2016, cause mine was down to 22 running. Wouldn't hardly get out of it's own way. Even now, with the new pump, mine is 00 when key on engine off. Running it's 65, like yours. Mine gets 18 or better highway. Around town, not so much. Maybe 15. Not highly likely low pressure key off will result in bad mileage. Usual culprit is a lead foot. LMAO! Ask me how I know. I drive mine like I stole it.

I try to drive like a grandma)
Is it bad if pressure down when engine turn off?

 

[Dirtman]

Well, the 98 requires 56 to 72 psi when running. I had to put a new pump in mine in late 2016, cause mine was down to 22 running. Wouldn't hardly get out of it's own way. Even now, with the new pump, mine is 00 when key on engine off. Running it's 65, like yours. Mine gets 18 or better highway. Around town, not so much. Maybe 15. Not highly likely low pressure key off will result in bad mileage. Usual culprit is a lead foot. LMAO! Ask me how I know. I drive mine like I stole it.

A. 65 psi. It requires 65 psi. Not a random range of numbers.

B. It should be 65psi with the engine off. Not zero, not 20. 65psi.

C. Low pressure with the engine off could mean a leaking injector which will definitely cause poor fuel mileage.

 

[cbxer55]

View attachment 54912

A. 65 psi. It requires 65 psi. Not a random range of numbers.

B. It should be 65psi with the engine off. Not zero, not 20. 65psi.

C. Low pressure with the engine off could mean a leaking injector which will definitely cause poor fuel mileage.

My shop manual says 56 to 72 psi. Mine runs at 65 psi, so it's all good. Mine is zero key on, engine off, still runs great. It's a new fuel pump, as of November 2017. It was that way from the get-go. I don't lose any sleep over it as long as the pressure is good running. Starts first turn of the key, very time, even when the temperature is in the teens, as it was this morning when I got off work at 2am.

 

[Uncle Gump]

This is the continuation of the plugged cat thread...

What you describe could be related to leaky injectors.

Maybe try a can of injector cleaner in the tank and see what happens.

 

[RonD]

Fuel pressure running or key off will be in a range, not a specific number, for a few reasons, one of course is the pressure gauge, lol, these are not calibrated daily, so grain of salt

Yes, 55 to 75 is expected, and as long as it steady at idle any number in between those is fine
Raise RPMs to 2,500 or so and hold there, should also see a steady pressure, slow drop as you continue to hold higher RPMs means clogged fuel filter or failing fuel pump

You should see a 10 to 20psi drop in fuel pressure with key off, but it should hold above 20psi for MONTHS and MONTHS

If you had a leaking injector you WOULD GET Rich codes and O2 sensor codes
So pressure drop is most likely check valve inside fuel pump
If its hard to start then cycle key on and off 3 times then start engine, the key off and on builds up lost pressure

Low MPG can be a few things, first would be upstream O2 sensors, they need to be changed every 100k miles or 10 years
Old O2s lower MPG, and there would be no codes because O2 is how computer refines air/fuel mix

Second would be larger tires, yes more weight but the bigger deal is that the Odometer is now off, its showing LOWER miles that actually traveled, so if you are using odometer vs gallons used, the math is wrong so result is wrong
You can look on drivers door label to see stock tire size, if its smaller than what you have then odometer is wrong

 

[Uncle Gump]

I had to go back and read the other thread... That's why it's beneficial to just add to the existing thread and not create a new one for the same concern.

So it runs "perfect" no misses no sputters starts great hot or cold and no lack of power.

You say the tires are the same size from the factory and they're 265/75/16 and speedo is accurate. I wasn't aware they used 265/75/16's... I thought they were 265/70/16's. If so... it could account for a bit of mileage.

You also say the PO recently tuned up before selling... I would still pull plugs and have a look. If they aren't Motorcraft or Autolites... I would throw them away and install the proper Motorcraft/Autolite plugs. I've never had any luck with anything else.

I mentioned in the other other thread that it was a bit strange you weren't getting a DTC. I don't ever doubt what RonD says... He has probably forgot more then I will ever know about these trucks.

 

[cbxer55]

So, my fuel pump was brand new in December 2016. Since installed, it's been no pressure key on engine off. But maintains 65 running. As stated, starts first turn of the key every time, even when the temps are in the teens. I figured it was the check valve, but on a brand new pump? Go figure! I'm not replacing it again any time soon, it runs fine.

 

[Pauls14]

I had to go back and read the other thread... That's why it's beneficial to just add to the existing thread and not create a new one for the same concern.

So it runs "perfect" no misses no sputters starts great hot or cold and no lack of power.

You say the tires are the same size from the factory and they're 265/75/16 and speedo is accurate. I wasn't aware they used 265/75/16's... I thought they were 265/70/16's. If so... it could account for a bit of mileage.

You also say the PO recently tuned up before selling... I would still pull plugs and have a look. If they aren't Motorcraft or Autolites... I would throw them away and install the proper Motorcraft/Autolite plugs. I've never had any luck with anything else.

I mentioned in the other other thread that it was a bit strange you weren't getting a DTC. I don't ever doubt what RonD says... He has probably forgot more then I will ever know about these trucks.

Spark plugs is Autolite.
Will take off fuel injectors in next week.

 

[Broosedamoose]

Does it have a K&N filter or a cold air intake on it? The reason I ask is because the PO could have cleaned the filter and over oiled it and buggered up the MAF sensor. You could get some electronics cleaner and spray the MAF sensor to clean it off. You have to be very gentle when doing it though! You don't want to damage it. Don't use a brush or anything! Just spray it lightly and let it air dry.

 

[Pauls14]

Fuel pressure running or key off will be in a range, not a specific number, for a few reasons, one of course is the pressure gauge, lol, these are not calibrated daily, so grain of salt

Yes, 55 to 75 is expected, and as long as it steady at idle any number in between those is fine
Raise RPMs to 2,500 or so and hold there, should also see a steady pressure, slow drop as you continue to hold higher RPMs means clogged fuel filter or failing fuel pump

You should see a 10 to 20psi drop in fuel pressure with key off, but it should hold above 20psi for MONTHS and MONTHS

If you had a leaking injector you WOULD GET Rich codes and O2 sensor codes
So pressure drop is most likely check valve inside fuel pump
If its hard to start then cycle key on and off 3 times then start engine, the key off and on builds up lost pressure

Low MPG can be a few things, first would be upstream O2 sensors, they need to be changed every 100k miles or 10 years
Old O2s lower MPG, and there would be no codes because O2 is how computer refines air/fuel mix

Second would be larger tires, yes more weight but the bigger deal is that the Odometer is now off, its showing LOWER miles that actually traveled, so if you are using odometer vs gallons used, the math is wrong so result is wrong
You can look on drivers door label to see stock tire size, if its smaller than what you have then odometer is wrong

What is brand of sensors should I buy?
Bosch (2pcs upstream and 1 pcs downstream)? If yes, what is part number. Thanks.

 

[Pauls14]

Does it have a K&N filter or a cold air intake on it? The reason I ask is because the PO could have cleaned the filter and over oiled it and buggered up the MAF sensor. You could get some electronics cleaner and spray the MAF sensor to clean it off. You have to be very gentle when doing it though! You don't want to damage it. Don't use a brush or anything! Just spray it lightly and let it air dry.

Air filter is usual. MAF is clean

 

[RonD]

Change both Upstream O2s, bosch are fine to use, these have the most effect on MPG

Downstream O2s generally last longer since they see cleaner exhaust(after Cats) and don't effect MPG much, but if you want change all 3
Because the computer compares upstream and downstream data to make sure Cats are working, so with new Upstream O2s and old downstream it could set a code saying bank 1 sensor 2 has low voltage, but usually not until 2nd set of upstream O2s

 

[Pauls14]

Change both Upstream O2s, bosch are fine to use, these have the most effect on MPG

Downstream O2s generally last longer since they see cleaner exhaust(after Cats) and don't effect MPG much, but if you want change all 3
Because the computer compares upstream and downstream data to make sure Cats are working, so with new Upstream O2s and old downstream it could set a code saying bank 1 sensor 2 has low voltage, but usually not until 2nd set of upstream O2s

Ok, thank you. Will try to change.