I have a 2002 4.6L V8 F150 with 135,000 miles on it. Truck is experiencing rough idling and occasional stalling upon initial start up only. Problem occurs on nearly every cold start, and occasionally on restarts when the truck has been sitting for an hour or less. Problem started about 8 months ago and has become more and more frequent. Only trouble code is for a o2 circuit malfuntion, which intermitantly appears about every 100 - 300 miles without throwing a CEL (always appears on start up and clears itself on the next start up, wont reappear for some time). In the last month, I have replaced the IAC, PCV, TPS, fuel filter, and 2 of 4 O2 sensors (I have the other 2 but have not found time to install them yet). Upon start up, if I lightly give it some throttle input (1/16" or so) and hold the RPM at around 1,100 for 5 seconds and release, it does not have this issue and will idle fine. The problem usually lasts for 5 - 10 seconds, bouncing from 300 rpm back up to 1,200 rpm and then returning to the correct high idle if it does not stall first. Once started, the truck runs perfectly and idles/drives without a hiccup. Fuel economy is running around 16.5 combined, which is pretty normal for this truck. Does not have the "pep" it once had, but power seems adequate for a truck with this mileage. Truck is also equipped with a cold air intake, aftermarket exhaust, and Edge Evo (which is only ever used in the trans only setting). Engine oil gets changed every 3,000 miles, the air filter is cleaned/reoiled every other oil change, and the MAF is religiously cleaned when I clean the filter. Spark plugs were changed about 20,000 miles ago with new MSD ignition coils all around. Also, truck uses very little, if any engine oil between changes.

I'm considering pulling the throttle body and intake elbow and giving them both a thorough cleaning, and I will be replacing the last 2 O2 sensors asap. I'm also wondering if the MAF could be having issues, because if you have every forgot to replug the damn thing in and start the truck, it will run up to 1,500 rpm, then down to 300, then back up, etc etc. If anyone can help me diagnose this issue, it would be greatly appreciated. I know I listed a bunch of stuff that has nothing to do with this issue, but I wanted to give as much information as possible. It is starting to become both annoying and a little embaressing at times.

Thanks in advance
- Teal

 

What brand plugs and what brand O2 sensors are you running? Does it do it if you return the tune to stock?

 

Check the Vacuum line that runs from the back of the throttle body to the passenger side valve cover for cracks. Mine was cracked where it connected to the throttle body. Only when I pulled it off is when I could see the crack in the hose. Once I replaced it, it idled and ran perfectly.


David
1999 F150 4.6 V8 214,000 miles.

 

Thanks for the replies. I will check that hose for leaks, I noticed one of the hoses going into the TB was looking pretty worn.

I am running Motorcraft plugs and O2 sensors (they are rebranded Bosch sensors if I remember correctly).