Incorrect fuel pressure Key on and Engine Running

Matthew Jeschke · Jun 22, 2024

I'm moving this from another thread. My fuel mileage has been suffering for quite some time and I think this is contributing factor. I honestly, only really know basics of troubleshooting fuel pressure. Maybe this is operating correctly, or maybe not?

Vacuum Check: I did a sanity check, revved the engine and fuel remained low by 20%. So I suspect it's fuel.

Rail Pressure:

Key on Ignition off: 50 PSI (I think should be ~58PSI ?)
Engine Running: 50 PSI when revved the pressure drops quickly down to nearly ~45 PSI
Pressure bleads off slowly when ignition is shut off. I'm not sure if it should hold 50PSI in there? It slowly bleeds down but engine starts like a champ... check ball bad in pump?

FEW NOTES/ BACKSTORY: Truck is a custom built LQ9 w/ Flex Fuel injectors. Note I went from around 24lbs / hr to ~32 lbs per hour... although, I don't think I had the fuel demand so flow should be similar to stock injectors maybe if anything a touch higher? I swapped the Flex Fuel injectors in because my 24# stock LQ9 injectors were leaking. Long story short, I think fuel demand / load on pump is VERY similar to the stock injectors (if anything I shortened the injector duty cycle). I tuned the VE table and am running in speed density mode. I went to log some more data a couple months later and noticed instead of my LTFT being well within 5% as tuned they are now around 20%.

New OEM pump - as spec'ed for LQ9 (not flex fuel, swapping whole system to flex fuel was a LOT of work / didn't have all the parts).
New pressure regulator on order - note when I unplugged the regulator w/ engine running pressure shot to 58 PSI.
Got a new filter to install

VIDEO OF SYMPTOMS LINKED HERE

UPDATE: I attached a copy of the spec for the pump I installed.

Scottydoggs · Jun 22, 2024

seems pretty normal to me. maybe the o2's are high miles and wearing out but not yet setting a code. as the o2's do have a say in fueling.

strutaeng · Jun 22, 2024

Are there any variations between the left and the right banks? Or pretty much identical?

I don't know the fuel pressure specs off the top of my head...

Matthew Jeschke · Jun 22, 2024

@Scottydoggs thanks for tip... The O2's are new also confirmed readings with Wideband. PCM is correctly reading AFR and trims are adjusting fuel as expected. If I reset the trims the engine will stumble and misfire. Just I don't know where the 20% trim / fuel error is coming from. If it was indeed tuning issue I'd be at 60 VE at idle which seems somewhat high (100 is perfect efficiency / pumping).

Running through FSM the Key on Ignition off pressure is wrong. It supposed to be between 55 and 62 PSI. I noticed it will go to that if I (1) start the engine and (2) disconnect the vacuum line from the regulator. But engine has to be running. I don't know if this points to the regulator... The FSM only specs pressure with engine off for that test... and unfortunately, I don't have a fitting to do it as FSM says.

@strutaeng I need to double check that. There's a PID in HP tuners that lets me see how far apart they are. They do vary some but I cannot say to what degree. You mentioned in other post but if they vary would you say exhaust leak?

...

I've been fiddling with the fuel pressure gauge and walking through factory service manual. It leads me to...

1. Bad fuel pressure regulator
OR 2. do a visual inspection of components between the regulator and the pump.

The method to check regulator is to test pressure between regulator and return to tank, should be greater than 62 PSI but lower than 75 PSI. Unfortunately, I don't have fittings to test the return line.

I did a visual inspection of regulator to front of tank and all appears good. I dropped the tank a few months ago to rebuild the evap and lines were good then. I felt up by the pump with my hand and it wasn't wet / leaking.

UPDATE: Engine oil looks good, it doesn't appear to be washed out from any gasoline and I'm near the oil change interval.

strutaeng · Jun 22, 2024

Matthew Jeschke said:
@Scottydoggs thanks for tip... The O2's are new also confirmed readings with Wideband. PCM is correctly reading AFR and trims are adjusting fuel as expected. If I reset the trims the engine will stumble and misfire. Just I don't know where the 20% trim / fuel error is coming from. If it was indeed tuning issue I'd be at 60 VE at idle which seems somewhat high (100 is perfect efficiency / pumping).

Running through FSM the Key on Ignition off pressure is wrong. It supposed to be between 55 and 62 PSI. I noticed it will go to that if I (1) start the engine and (2) disconnect the vacuum line from the regulator. But engine has to be running. I don't know if this points to the regulator... The FSM only specs pressure with engine off for that test... and unfortunately, I don't have a fitting to do it as FSM says.

@strutaeng I need to double check that. There's a PID in HP tuners that lets me see how far apart they are. They do vary some but I cannot say to what degree. You mentioned in other post but if they vary would you say exhaust leak?

...

I've been fiddling with the fuel pressure gauge and walking through factory service manual. It leads me to...

1. Bad fuel pressure regulator
OR 2. do a visual inspection of components between the regulator and the pump.

The method to check regulator is to test pressure between regulator and return to tank, should be greater than 62 PSI but lower than 75 PSI. Unfortunately, I don't have fittings to test the return line.

I did a visual inspection of regulator to front of tank and all appears good. I dropped the tank a few months ago to rebuild the evap and lines were good then. I felt up by the pump with my hand and it wasn't wet / leaking.

UPDATE: Engine oil looks good, it doesn't appear to be washed out from any gasoline and I'm near the oil change interval.

Exhaust leak? I think technically yes, but that would have to be HUGE exhaust leak and between the manifolds and upstream O2. Probably not likely to be culprit.

You can try plugging the return line at the fuel rail (disconnect the hose). Vacuum caps with fuel hose clamps will work. Then prime the system to see how much pressure (or head pressure) the pump is capable of providing. Should be like 80 psi IIRC. This test rules out the pump as being the problem.

Matthew Jeschke · Jun 22, 2024

I will see if there's another fitting for return line. I was trying to use the T adapter. I probably pictured doing it wrong.

I noticed I occasionally get a P0141 O2 Heater Circuit (Bank 1, Sensor 2)... I only replaced the front O2 sensors (ones closest engine).

I revved engine from 600 to 3000 RPM again and watched... The problem does slightly improve

At 2000+ RPM the O2 sensors start to separate... Bank 1 is 2 to 3% richer on up to 3200 RPM

Thanks for tips! I need to get this fixed ASAP as I need to drive it for work.

strutaeng · Jun 22, 2024

Matthew Jeschke said:
I will see if there's another fitting for return line. I was trying to use the T adapter. I probably pictured doing it wrong.

I noticed I occasionally get a P0141 O2 Heater Circuit (Bank 1, Sensor 2)... I only replaced the front O2 sensors (ones closest engine).

I revved engine from 600 to 3000 RPM again and watched... The problem does slightly improve

At 2000+ RPM the O2 sensors start to separate... Bank 1 is 2 to 3% richer on up to 3200 RPM

Thanks for tips! I need to get this fixed ASAP as I need to drive it for work.

View attachment 430923

View attachment 430922

The O2 codes are usually the "effect" of engine running lean. If you fix the lean condition, those usually go away.

Why do you have a misfire code? That would most definitely cause lean running conditions.

Matthew Jeschke · Jun 22, 2024

I thought I maybe aggravated the code by troubleshooting this. It seemed to come up when I started it and it was running really lean.

I will pull some spark plugs and see how they look. hrm...

Fless · Jun 22, 2024

If I read that right your graphic is LT trims -- you should watch short terms trims because they're essentially real time.

Matthew Jeschke · Jun 22, 2024

Yes, I'm watching both short term and long term. There's a split of 3% over 1500 RPM. Bank 2 is running 3% leaner than Bank 1.

Oh God, I went to put in the new fuel filter. What a nightmare. I'm in quite a bit of pain. I depressurized the system. Then disconnected the line set going through the filter along with mounting bracket... It started siphoning fuel out of the tank. Fuel was going everywhere. I tried like hell to swap the new on in quick but I kept getting soaked in gas. I'd have to run spray myself with a garden hose and work some more. It was almost impossible to line up the lines with the new filter while being showered in gas. I finally got it but not before my left arm just absolutely burns soooooo bad.

I know I've lost a little fuel before but holy cow changing a filter but I swear it stopped flowing. I don't think that's normal. I had to revisit the manual to see if I was doing it right, and I was doing it exactly by the book. I'm watching tutorials on YouTube as a sanity check and on all of them the fuel stops coming out the tank side. It kept coming out the tank like a little hose siphoning gas so long as I had the filter disconnected.

Incorrect fuel pressure Key on and Engine Running

Matthew Jeschke

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