[jeffit]

I have 2001 F150 4.2. On cold start engine runs rough.
Autel 808 scan tool shows the following
-Fuel trims are sky high on bank 1, Reved engine and fuel trims stay the same
-Injector No.3 on live data shows faulted
-No fault codes for misfires but I have seen one for bank 1 lean. I have recently replaced intake manifold gaskets and the battery was disconnected so maybe other codes have not had a chance to come to light. I have smoked the intake and do not believe this is vacuum related.

Now for the kicker, once engine is warmed and has been running for about 15 minutes, No.3 injector is no longer faulted and engine runs great and fuel trims return to normal. Is this a bad injector? Why would It start working when the engine is warm? I plan to take check the resistance when I get a chance

 

[jeffit]

So I checked the resistance. Engine cold, No.3 injector is at 15.4 ohms and No.1 for comparison is at 13.8, Both still seem to be in range according to the Haynes manual. I found that the locking clips for the injector electrical connector was broken and thought I had found the problem. I replaced the connector by de-pinning and re-pinning with one from the junkyard. Everything seemed okay.

Next day, Cold start, Its back. I unplugged No.3 injector and plugged it back. Fixed.
Day after, Its back. Rough idle cold, good after some time. Check engine light on for No.3 cylinder misfire.

I'm now leaning towards a loose connection inside the wire harness or a bad injector

 

[85ltdcrownvic]

Try wiggling the connector while it’s running bad. The connector type for these injectors are pretty crappy. I wouldn’t be surprised if it’s the connector itself again.