2008 5.3 FF hard starting issue

[mxking1712]

hello everyone. im having a issue with my 08 tahoe. 5.3L flex fuel engine
Ill try to list everything ive done or looked into thus far
engine sometimes starts just fine, other times it turns over maybe 5 or 6 times before it stumbles and eventually starts idling, other times its like it starts too soon maybe the first crank or crank and a half which also causes it to stumble and then idle normal. It seems if i hold my foot on the accel pedal just slightly. this issue is less prevalent.

Now onto what ive tried so far...
when i bought the tahoe 2 years ago i changed plugs, wires
at first i believed i had a ad fuel pump, as fuel pressure would slowly bleed off and figured it must be the check valve inside the pump. replaced fuel pump with no change... then also replaced fuel pressure reg on the fuel rail with no change...
i have removed throttle body, cleaned that, and also did the relearn procedure...

now onto what i think is the issue but im not sure how everything works... Fuel pressure with vehicle idling is around 58psi, which i gather is normal, fuel pressure does slowly bleed away when vehicle is off, which also seems normal, however every time i cycle the key, the pump does not run... i have seen fuel pressure down as low as 20psi and its not until the engine is actually cranking that the fuel pressure comes back up... so it seems like sometimes when key is in run position the pump runs, but other times it does not.... should the pump run every time the key is cycled? is ignition switch telling the fuel pump relay to go when the key is turned?
what controls fuel pump once the vehicle is running? same relay? or something else?

i have tried swapping the fuel pump relay also with no improvements. sorry for the long winded post but figured the more info the better. i cant find much online as far as the same issue im having. thank you

 

[Doubeleive]

fpcm fuel pump control module, replacing it requires a relearn with a tech2 or similar scan tool

 

[swathdiver]

hello everyone. im having a issue with my 08 tahoe. 5.3L flex fuel engine
Ill try to list everything ive done or looked into thus far
engine sometimes starts just fine, other times it turns over maybe 5 or 6 times before it stumbles and eventually starts idling, other times its like it starts too soon maybe the first crank or crank and a half which also causes it to stumble and then idle normal. It seems if i hold my foot on the accel pedal just slightly. this issue is less prevalent.

Now onto what ive tried so far...
when i bought the tahoe 2 years ago i changed plugs, wires
at first i believed i had a ad fuel pump, as fuel pressure would slowly bleed off and figured it must be the check valve inside the pump. replaced fuel pump with no change... then also replaced fuel pressure reg on the fuel rail with no change...
i have removed throttle body, cleaned that, and also did the relearn procedure...

now onto what i think is the issue but im not sure how everything works... Fuel pressure with vehicle idling is around 58psi, which i gather is normal, fuel pressure does slowly bleed away when vehicle is off, which also seems normal, however every time i cycle the key, the pump does not run... i have seen fuel pressure down as low as 20psi and its not until the engine is actually cranking that the fuel pressure comes back up... so it seems like sometimes when key is in run position the pump runs, but other times it does not.... should the pump run every time the key is cycled? is ignition switch telling the fuel pump relay to go when the key is turned?
what controls fuel pump once the vehicle is running? same relay? or something else?

i have tried swapping the fuel pump relay also with no improvements. sorry for the long winded post but figured the more info the better. i cant find much online as far as the same issue im having. thank you

These motors run at 43.5 psi. The key on pressure goes to about 50-60 psi and is not supposed to lose more than 5 psi in one minute.

Have you recently checked the plugs to see if the injectors are working properly and not bleeding down? Have you signed up for the shop manual at AllDataDIY and run through the troubleshooting guide? Without the manual and Tech-2, you're running blind.

 

[wsteele]

hello everyone. im having a issue with my 08 tahoe. 5.3L flex fuel engine
Ill try to list everything ive done or looked into thus far
engine sometimes starts just fine, other times it turns over maybe 5 or 6 times before it stumbles and eventually starts idling, other times its like it starts too soon maybe the first crank or crank and a half which also causes it to stumble and then idle normal. It seems if i hold my foot on the accel pedal just slightly. this issue is less prevalent.

Now onto what ive tried so far...
when i bought the tahoe 2 years ago i changed plugs, wires
at first i believed i had a ad fuel pump, as fuel pressure would slowly bleed off and figured it must be the check valve inside the pump. replaced fuel pump with no change... then also replaced fuel pressure reg on the fuel rail with no change...
i have removed throttle body, cleaned that, and also did the relearn procedure...

now onto what i think is the issue but im not sure how everything works... Fuel pressure with vehicle idling is around 58psi, which i gather is normal, fuel pressure does slowly bleed away when vehicle is off, which also seems normal, however every time i cycle the key, the pump does not run... i have seen fuel pressure down as low as 20psi and its not until the engine is actually cranking that the fuel pressure comes back up... so it seems like sometimes when key is in run position the pump runs, but other times it does not.... should the pump run every time the key is cycled? is ignition switch telling the fuel pump relay to go when the key is turned?
what controls fuel pump once the vehicle is running? same relay? or something else?

i have tried swapping the fuel pump relay also with no improvements. sorry for the long winded post but figured the more info the better. i cant find much online as far as the same issue im having. thank you

I didn't think your MY had a fuel pressure regulator on the fuel rail, I believe it is in the pump assembly.

Key on engine off, the pump will run for a short period to assure prime. I don't think it will keep coming on with each key on/off cycle.

You fuel pressure seems off when idling. With the 2008 and up models, your pressure is controlled by the FPCM (aka FPDM, etc.) and should be as @swathdiver indicated at 43.5 PSI.

Dorman sells a preprogrammed plug and play FPCM. The FPCM is located right under the drivers seat facing forward on a frame rail.

 

[Joseph Garcia]

Welcome to the Forum from NH.

Lots of knowledgeable folks here who freely share their knowledge, experiences, and perspectives. Knowledge is power.

I hope that you will become a participating member in the Forum's discussions.

Pics of the truck, please.

You are already receiving sage advice from the knowledgeable folks on this Forum.

 

[mxking1712]

thanks guys, when i get some time i will re check the fuel pressure, its been a while since i played with it, i could be remembering the wrong number there. my biggest concern is the fuel pump not kicking in with key on engine off all the time. i understand it shouldnt do it back to back to back. but i am pretty concious when im driving it to listen when i start it up. i do hear it sometimes, but not all the time, when i dont hear it is when it struggles to start. Im more so trying to figure out what makes the pump kick in when i turn the key.. sounds like the module..

 

[wsteele]

thanks guys, when i get some time i will re check the fuel pressure, its been a while since i played with it, i could be remembering the wrong number there. my biggest concern is the fuel pump not kicking in with key on engine off all the time. i understand it shouldnt do it back to back to back. but i am pretty concious when im driving it to listen when i start it up. i do hear it sometimes, but not all the time, when i dont hear it is when it struggles to start. Im more so trying to figure out what makes the pump kick in when i turn the key.. sounds like the module..

You might double check my math on this one, but I think you don't even have a fuel pump relay (should be an empty spot in the main fuse/relay box on the drivers side of the engine compartment) as the pump control is centered in the FPCM (aka Fuel Pump Drive Module). I believe everything from 2008 on is like that.

 

[mxking1712]

So a little update. I ended up ordering a New fuel pump control module through the dealer. Had them program and install. Did not fix my issue.
I do not have a fuel pressure gauge(yet) however I have a decent Autel scan tool. When I hook into the truck, I get a desired fuel pressure of 58psi when key is in run position, once engine running this drops down to 43 desired and the pressure matches.
When i turn the vehicle off, fuel pressure climbs, I’ve seen it as high as 80psi.
If I shut Tahoe off and on over and over back to back, it starts perfect every time. It seems as though once the fuel pressure comes way up to 80psi and then starts to fall again. Takes around half hour to start slowly creeping down, is when the hard starting happens. Is it normal for fuel pressure to go that high?
If I have a weak injector, when I go to start the car after fuel pressure being so high. Could it possibly flooding one cylinder out and causing my poor start? Anyone have any ideas?

 

[mxking1712]

Last post for tonight.
I went and drove the Tahoe around for about an hour. Came home shut it off with scan tool plugged in and monitoring fuel pressure sensor.
After about 15 minutes of sitting pressure was up to 80psi
It held there for about 20 minutes and started to drop.
After another 10 minutes I was down to 55psi
10 more minutes took me to 34 psi. At which point i tried to cycle the key but the fuel pump did not run.
10 more minutes down to 28 psi. Cycle key no pump
10 more minutes 17psi, no pump.
I then tried to start the truck, which it turned over about twice as long as it should before it stumbled to life for 2 or 3 seconds and then found a smooth idle.
I do not understand why the fuel pump does not run under 58psi
The scan tool tells me the desired pressure when key on engine off is 58 psi. Shouldn’t the pump run anything below 58?
I really believe this is my trouble here and I have no idea why the pump won’t run. Sometimes it runs just fine and starts great. But this being intermittent is driving me crazy.

 

[BG1988]

Last post for tonight.
I went and drove the Tahoe around for about an hour. Came home shut it off with scan tool plugged in and monitoring fuel pressure sensor.
After about 15 minutes of sitting pressure was up to 80psi
It held there for about 20 minutes and started to drop.
After another 10 minutes I was down to 55psi
10 more minutes took me to 34 psi. At which point i tried to cycle the key but the fuel pump did not run.
10 more minutes down to 28 psi. Cycle key no pump
10 more minutes 17psi, no pump.
I then tried to start the truck, which it turned over about twice as long as it should before it stumbled to life for 2 or 3 seconds and then found a smooth idle.
I do not understand why the fuel pump does not run under 58psi
The scan tool tells me the desired pressure when key on engine off is 58 psi. Shouldn’t the pump run anything below 58?
I really believe this is my trouble here and I have no idea why the pump won’t run. Sometimes it runs just fine and starts great. But this being intermittent is driving me crazy.

so how old is the battery?

highest i have seen my FPR was 90PSI(according to the computer) normally runs 44PSI while running

I would check the fuel pump relay swap with another with the same numbers on top..


on the start up issue try cleaning the MAF sensor(could also be bad) spark plugs cables ignition coils compression