Fuel Pressure

[FAUST]

2007 Tahoe 5.3L Flex Fuel - some hesitation when accelerating. Periodically throws p0300 - indicates lifter replacement as fix.

There isn’t a loud tick typically associated with collapsed lifters. Talking with a GM master tech, suggested testing fuel pressure before performing lifter DOD AFM delete.

Testing fuel pressure - 59-60 PSI while running - when relief valve is open drops below 30 and doesn’t recover.

Testing with key on engine off - 59-60 PSI. After 25 minutes pressure drops to 35 PSI and continues to drop.

My question is - what is an acceptable loss of pressure over time? Would this be an indication of a bad fuel pump?

Any ideas or advice is great!
Pictures showing PSI before and after relief valve is open - engine is running.

 

[Geotrash]

2007 Tahoe 5.3L Flex Fuel - some hesitation when accelerating. Periodically throws p0300 - indicates lifter replacement as fix.

There isn’t a loud tick typically associated with collapsed lifters. Talking with a GM master tech, suggested testing fuel pressure before performing lifter DOD AFM delete.

Testing fuel pressure - 59-60 PSI while running - when relief valve is open drops below 30 and doesn’t recover.

Testing with key on engine off - 59-60 PSI. After 25 minutes pressure drops to 35 PSI and continues to drop.

My question is - what is an acceptable loss of pressure over time? Would this be an indication of a bad fuel pump?

Any ideas or advice is great!
Pictures showing PSI before and after relief valve is open - engine is running.

How many miles on it? In my opinion, that's an indicator of a failing fuel pressure regulator which is part of the pump assembly in the tank. Your pressure should be a steady 43 psi or so. I had the same thing happen in my 2007 at around 200K miles. Replacing the pump fixed the problem.

 

[Doubeleive]

ya that's high pressure when running, it should drop to 43-45 or so while running. (hybrids may be different)
may or may not be the cause of your random misfire though, not sure why someone would immediately jump to "lifters" because of a random misfire.
plugs, wires, coils, injectors, leaks, wiring, etc should be checked before jumping to anything else

 

[steiny93]

this thread had a reply with some time to pressure lose intel: https://www.tahoeyukonforum.com/threads/2008-5-3-ff-hard-starting-issue.130199/
on the 2008 I have tracks on; it had a fuel pump issue last winter; it would get to 60psi at startup then run in the low 40's; it would lose pressure very slowly

I'd recommend going through the troubleshooting procedure to see if its the pump/regulator or injector(s)

 

[FAUST]

How many miles on it? In my opinion, that's an indicator of a failing fuel pressure regulator which is part of the pump assembly in the tank. Your pressure should be a steady 43 psi or so. I had the same thing happen in my 2007 at around 200K miles. Replacing the pump fixed the problem.

How many miles on it? In my opinion, that's an indicator of a failing fuel pressure regulator which is part of the pump assembly in the tank. Your pressure should be a steady 43 psi or so. I had the same thing happen in my 2007 at around 200K miles. Replacing the pump fixed the problem.

How many miles on it? In my opinion, that's an indicator of a failing fuel pressure regulator which is part of the pump assembly in the tank. Your pressure should be a steady 43 psi or so. I had the same thing happen in my 2007 at around 200K miles. Replacing the pump fixed the problem.

There is about 150k miles.

 

[FAUST]

ya that's high pressure when running, it should drop to 43-45 or so while running. (hybrids may be different)
may or may not be the cause of your random misfire though, not sure why someone would immediately jump to "lifters" because of a random misfire.
plugs, wires, coils, injectors, leaks, wiring, etc should be checked before jumping to anything else

Plugs, wires are new. Basic test on coils (swapped non misfiring to potential misfiring cylinder) - no change. Oh, I’m using a BlueDriver basic scan tool that shows misfiring and which cylinder.

I should say - the original scan from autozone said lifter - that’s why I’m still testing and researching, seemed a bit extreme.

Next is on to injectors - I have done 2 seafoam treatments over the last year and that has not helped.

Any mor ideas to diagnose is welcomed!

 

[FAUST]

this thread had a reply with some time to pressure lose intel: https://www.tahoeyukonforum.com/threads/2008-5-3-ff-hard-starting-issue.130199/
on the 2008 I have tracks on; it had a fuel pump issue last winter; it would get to 60psi at startup then run in the low 40's; it would lose pressure very slowly

I'd recommend going through the troubleshooting procedure to see if its the pump/regulator or injector(s)

Awesome! Thank you for this! I’ll get after it and let y’all know what I find.

 

[Geotrash]

As I said above, I'll wager $100 that the problem is the pressure regulator in the fuel pump. When mine went, I got the same occasional P0300 and similar pressure readings.

Here is the diagnostic procedure to be sure. You will need a Tech2 or equivalent to perform the diagnostics. They are available for $250 or so from chinacardiags.com.

Circuit/System Testing

Note:

• The fuel pump may need to be commanded ON a few times in order to obtain the highest possible fuel pressure.
• DO NOT start the engine.

1. Ignition ON, engine OFF, command the FPCM Fuel Pump ON with a scan tool and observe the fuel pressure gauge while the fuel pump is operating. Verify the fuel pressure is between 345-690 kPa (50-100 psi).

¤	If the fuel pressure is greater than the specified range, replace the fuel tank fuel pump module.
¤	If the fuel pressure is less than the specified range, test, inspect, and repair the items listed below. If all items test normal, replace the fuel tank fuel pump module.

• Restricted fuel feed pipe
• Inspect the harness connectors and the ground circuits of the fuel pump for poor connections.

1. After the fuel pump is turned OFF, verify the fuel pressure stabilizes between 500-599 kPa (72-87 psi).

¤	If greater than the specified range, replace the fuel tank fuel pump module.

1. After the fuel pump is turned OFF, verify the fuel pressure does not decrease more than 34 kPa (5 psi) in 1 minute.

¤	If the pressure decreases more than the specified value, perform the following procedure:

1. Ignition OFF, relieve the fuel pressure. Refer to Fuel Pressure Relief (CH-48027) See: Fuel Pressure Release\Service and Repair\Fuel Pressure Relief (CH-48027)Fuel Pressure Relief (Without CH-48027) See: Fuel Pressure Release\Service and Repair\Fuel Pressure Relief (Without CH-48027).
2. Install the J 37287 between the fuel feed pipe and the fuel rail.
3. Open the valve on the J 37287 .
4. Ignition ON, command the FPCM Fuel Pump ON with a scan tool and bleed the air from the CH-48027 .
5. Command the FPCM Fuel Pump ON and then OFF with a scan tool.
6. Close the valve on the J 37287 .
7. Monitor the fuel pressure for 1 minute.

¤	If the fuel pressure decreases more than 34 kPa (5 psi) within the specified time, locate and replace the leaking fuel injector(s).
¤	If the fuel pressure does not decrease more than 34 kPa (5 psi) within the specified time, replace the fuel tank fuel pump module.

1. Relieve the fuel pressure to 69 kPa (10 psi). Verify that the fuel pressure does not decrease more than 14 kPa (2 psi) in 5 minutes.

¤	If the fuel pressure decreases more than the specified value, replace the fuel tank fuel pump module.

1. Operate the vehicle within the conditions of the customer's concern while monitoring the fuel pressure with the CH-48027 . The fuel pressure should not drop off during acceleration, cruise or hard cornering.

¤	If the fuel pressure drops off, test, inspect, and repair the items listed below. If all items test normal, replace the fuel tank fuel pump module.

• Restricted fuel feed pipe
• Inspect the harness connectors and the ground circuits of the fuel pump for poor connections

1. If the fuel system tests normal, refer to Symptoms - Engine Controls

 

[FAUST]

This is quite the process. I’ll start here and see what happens. The challenge is buying a new scan tool at the moment.

Thank you for the help on this. I’ll reply once I’ve been able to run the diagnostic.

 

[Geotrash]

This is quite the process. I’ll start here and see what happens. The challenge is buying a new scan tool at the moment.

Thank you for the help on this. I’ll reply once I’ve been able to run the diagnostic.

Check the Tech 2 owners map. If one of us lives near you, you'd be welcome to borrow one.