I have a 2000 F-150 with a 4.6 engine that has developed a rough idle. The over all truck has 329,000 miles on it. The motor was rebuilt about 23,000 miles back. It has thrown no codes and seems to be inconsistant or in other words...it does not do it all of the time. When it does idle rough, it seems to have a slight power problem and makes a clicking sound when going up a hill or pulling a load. I tested the IAC valve according to the Haynes book and it test out OK.
Where do I start looking?

 

plugs and wires...or do you have cops?

 

I have COPS and when I rebuilt the engine, I went in with new Autolite double platnums. Speaking of wires, how do you diagnose the spark plug wires on these engines? They have never been changed.

 

I think you have COPS instead of spark plug wires...I usually recommend that you stick with motorcraft parts only for the engine. It may be that either your COPS or plugs are bad. Are your COPS motorcraft?

 

Some of them are NOT Motorcraft. Everytime I have had a coil go bad it has thrown a code...in this instance a code has not been thrown. Usually it will say "missfire cyl 8" or something like that. The coil is the first thing I swap out. Is it possible that I have a coil messing up WITHOUT throwing a code? What if I pulled one coil wire off at a time....would that be a way to check that? The wires leading to the coils have 329,000 miles on them. Do they wear out?

 

did you clean the IAC?

 

No, I have not cleaned it. What would you suggest, maybe removing it and running some Chemtool carb cleaner through it?

 

OK, It has finally thrown a code. I am reading PO308- Cylinder 8 Misfire. I replaced the COP with a new Motorcraft and there has been no change.

The freeze frame info reads as follows:
fuel system 1 CL
fuel system 2 CL-Fault
Calc Load % 0.00
ECT 'F -40
STFT B1 % -100.00

I have never replaced the fuel injectors...they have 329,000 miles on them. Could this be my problem? How many miles do fuel injectors usually get before they need to be replaced?

What should I try now?

 

Exhaust leak. sometimes they arent obvious untill under a load. explains loss of power, clickin', and rough idle. i just fixed mine and it solved all these problems. something to check

 

Could this cause the Cylinder 8 misfire? I checked the vacuum at the intake and it is holding a steady 18#.