So I have a 2007 Tahoe with a 5.3l flexfuel engine. I have been having issues with hard start. It will usually fire right up then stumble for a second or 2 then pick back up. Sometimes it completly dies then takes a couple seconds of cranking to get back running. Checked fuel pressure and it is 55psi at key on, 60 psi at idle and less then 1 psi leak down in a minute. So I dont think fuel pressure is an issue. Also it seems to be worse when its cold out. Other day it was 60* F and it fired right up. When it gets below 40* thats when it happens. I checked ambient temp sensor and its reading within 5* of local weather. Also no fault codes. I have a CAI, MBRP exhaust and Diablo tuner. Set tuner back to stock, no change. Ive read where the fuel/alcohol sensor can be out of wack and cause this. Im waiting for a guy at work to come back so I can use his snapon reader to hopefuelly reset it. Any other suggestions would be great! Thanks!

 

let it sit and see when the pressure is. next time when its cold cycle the key a few times, turn it on wait a few seconds then turn it off, turn it back off for a few seconds then turn it back wait 3 or so seconds then try starting. what im thinking is the check valve in the pump isnt working properly and all the fuel drains back into the tank overnight.

 

I already tried turning key on and off multiple times. Doesnt change a thing. Also when I checked my pressure after sitting all day it was down to 0 psi. But as soon as I turned key on I had 55psi at the rail I would think draining off fuel pressure in 8 hours isnt so bad especially being the fuel is right back at the rail as soon as key is turned on. Unless maybe it gets an air pocket up in the injectors possibly. Still doesnt explain why it would be worse when its cold out tho. Fuel pressure is fuel pressure. Whether ita 60* or 40*.

 

the colder it is the more fuel thats required. how much does it drain after sitting for 10 min?

 

Yea I get that part but I would think a 20* difference wouldnt really matter. Especially when its getting 55 psi of fuel within 2 seconds of key on. Im not sure what the drop is in 10 minutes. I can check that to see. I just went off the haynes manual I have which only says 5 psi in a minute. Also it says my pressure should only be 45 psi at idle but im getting 60? So idk how accurate the manual is. Or maybe its my tuner making the pressure higher than spec at idle.

 

well think about it like this, over night the fuel drains out and gets replaced by air. when you turn the key on the air in the line has no where to go but to compress. so while youre seeing 55 psi it may be 55 psi of air not fuel. the truck starts and has to bleed off the air which when cold will require more fuel to run. when the outside temp is warmer it can deal with the air in the line but when cold it needs more fuel but cant till the air is bled out.

 

Yea that makes sense for sure! Well like I said when I hooked up the pressure gauge to the rail after sitting for probably around 6-8 hours there was 0 psi. And no fuel there. So i can def. Say it is bleeding off. Is the fuel pump the most likely cause of bleed off or could it also be the injectors leaking down? I only have 123k miles on it.

 

i would think fuel pump or else you should have some miss fires and the oil should smell like fuel and thinned. after sitting for 8 hours i would think the system would bleed down anyway. tell you the truth im not even sure if this isnt normal operation of the fuel system of these newer trucks (as far as bleeding down). wait till tech or one of those guys chimes in and tells you what they think

 

Awesome. Thank you!

 

Just wanted to give an update. Got my coworkers snapon ethos. Hooked up and found my alcohol sensor was reading 53%! I reset it and now its only reading 3%. Truck has been starting perfectly so far but it hasnt been really cold past couple days. So we will see. Hopefully that was my problem!