My 2000 F150 4X4 with 5.4 will run fine in the morning or cool days (which we haven't had in a while around Texas) but when the outside temp gets over 90 or so (everyday by noon here) it will start running bad. When it starts acting up, it will run fine while you have throttle applied, but if you let off, its starts running rough and will sometimes die. It continually gets worse until you can hardly keep it running. If you let it sit for more than an hour it will run right again for a while. I have cleaned the throttle body and the MAF, didn't help anything. Using higher octane fuel seems to help a little, but still having the same problem. Any suggestions on where to look?

 

Do you have any mods, such as an aftermarket intake?

 

You are describing a heat related power loss. Chasing this down can ofcourse involve a lot of rabbit trails.

Something as simple as inspecting your spark plug gaps,conditions,etc.....I would start here.

Assuming no coolant loss? No oil in coolant or coolant in oil?

 

Just a guess here. Could it be something to do with fuel vapors expanding in the tank and something in the purge system not working correctly. Not to familiar with the fuel system on these but sounds like it could be a possible issue relation with heat.

 

No coolant issues, no mods, haven't checked plugs yet. Had codes read and have the codes for being lean. As far as fuel vapors, I thought that too and disconnected the external vent system, didn't help.

 

ive been havin a sorts same problem musiman. i got a 00 4.6. ive been havin to crank it another second or more than usual for it to turn over. i let the fuel pump prime itself and i still gotta crank it. it also stumbles a little when wantin to take off slowly. my friend says the fuel pumps goin bad. could that be the case, the pump beginning to fail and lose pressure?

 

Thats a big thing to leave out-the lean codes we cant help you if you dont give us information. What are them codes EXACTLY? the numbers? Lean conditions are generally an indication of a vacuum leak..

 

how old is your fuel filter? start there.

 

I didn't get the numbers from the autozone guy that read it for me, and I got the codes after my original post. All they told me at autozone was the codes were for lean on left and right bank, 2 codes. I can go back tomorrow and get the actual numbers.

 

in the mean time check your vacuum system especially the PCV lines to the elbow at the back of the throttle body. it (the PCV elbow) commonly rots and creates lean conditions like you have.

 

ill give it a try. if thats the problem, itll save me from droppin the tank. thats always a bitch to do