I'm having trouble with my 2003 F150 5.4. I'm getting P0171 and P0174 codes. I'm a contractor and use the truck a lot. It runs fine most of the day then usually on my way home it's starts to big down and seems to have a miss around 1500rpm and shuts down. I can restart the truck after a few minutes and can drive for a few minutes before it happens again. When I start the truck the next day it's fine until I come home again. I have changed the plugs and coils as well as fuel filter, cleaned the mass air flow. Any ideas? I was thinking it may be the fuel pump?

 

Have you thoroughly checked for vacuum leaks?

 

I have checked the elbow on the back of the intake and used propane around the engine but couldn't find anything.

 

The 171/174 codes tell you the fuel tables are shifted RICH.
This will not show up on cold starts until the engine is fully warmed because the engine is normally set rich by the program for cold starting.
This causes poor idle and stalling due to the excess richness after full warm up. You can even hear it in the exhaust note change during warm up from a cold starts that cover up the leak..
You cannot hear/ detect air leaks with propane. It adds even more fuel the system tries to change it's settings for..
Only way to use propane is watching a Scanner for changes in the fuel tables showing the computer has detected the propane by changes in Oxygen sensors output back to the computer..
Check the PCV line and hose going to the rear of the intake for softness, a hole sucked into the hose, splits etc.
Anywhere else there is air getting in besides the normal intake tract will cause the issue.
Do not change parts until a proven need.
Find the cause first.
Good luck.

 

Speaking from experience, when I had those two codes thrown I had to replace the whole intake. Over time mine developed a leak and my computer was reading more air than fuel ratio. A small leak is not too bad because the computer will compensate for it using more gas. But obviously yours has gotten to the point that is beyond barrable for the ECU. Hence you are now seeing noticable symptoms and codes being thrown. What I did to find out it was the intake was sprayed some carb cleaner around the intake and observed the engine Rpms. If the engine Rpms went up then I knew it had sucked the carb cleaner in and hence I knew where my leak was.

 

Be awhere Propane is in gas form,... 'combustible' ….type cleaner is a liquid.
Gas goes right in,.... the other being a liquid blocks air for a short time through the same point..
Result are a different short term action.

 

Does it have the plastic intake manifold on that year? MIght need to look at that?

Mitch