I have a 2002 F150 XLT 4.6L with about 131,000 miles. I recently worked on my radio and disconnected the positive battery terminal. About 2 days later, my truck would not start. When I turned the key to crank the motor, all the gauges went to the max. (speedometer went to 100, tach went to 8,000) and stayed there. The motor did not turn over. I checked the voltage of the battery and it was 12.6v. I jumped the truck and the motor started. I drove the truck for about 10 miles, stopped the motor and it would not start. I heard a clicking sound coming from the fuse box location and discovered the “battery saving relay” was making the noise. I replaced it and the truck started right up. I noticed right away the idle was not right. In fact I had to give it a little gas to keep it from cutting off. Once I gave it some gas, I could gradually lift my foot from the accelerator and it would idle ok. This occurs about 90% of the time on a cold start and about 50% of the time on a hot start. I read in the owner’s manual about the engine having to “relearn it’s idle and fuel trim strategy for optimum driveability and performance” after the positive battery terminal has been removed and put back on again. I have followed those instructions and it really didn’t seem to make any difference. I have tried to pull codes off with an OBDII analyzer, but I don’t think I have driven enough miles for codes to show back up after the positive battery terminal reconnected. I also think there is a slight miss traveling at highway speeds.

FYI: I am the original owner. The truck was running perfect up to the above event.

Anyone have any ideas what might be going on here?

 

Ever do maintenance on your truck? Spark plugs? Anything else besides the relay? might be time to do some. Start off cleaning the throttle body and IAC new plugs maybe a new set of coils or at least replace the one bad on it might have..

 

Sounds to me like a pure case of poor battery cable terminal connections.
When you jump start, the jumper cables are put on the battery cable ends.
If there are no other problems you have bypassed to root cause, battery cable connections on the posts or a broken cell connector.
The battery voltage will measure correct if it's fully charges but if you can't get anything through the connections the loads won't get it.
Remove both battery cables, use a cleaning tool on the post and connectors then put them back on being sure they are tight enough.
See what you get by doing this.
Removing power to the PCM reboots it back to the base tables so how the motor runs can be somewhat different for awhile.
Good luck.