Hey guys, first post. I have an 07 F-150 XLT. 4X4. Truck has bad history of misfire from previous owner. I kind of had it dumped on me to figure out. Im a pretty experienced mechanic but know NOTHING about Fords. I've put 6 or 7 fuses in so far trying to diag. I also spent on all new Cats. All new o2's. I unplugged both VCT's, still popped. Plugged VCT's back in, then unplugged purge valve (on fire wall above master), popped. Should I just keep going down the list? Next I guess its going to be going back over O2's to make sure I secures harnesses out of the way. But something else interesting is I probed connector for purge valve and I have continuity on BOTH PINS to ground. Ground pin had 0.6 ohm hot side had 0.2 ohm. I was going from pin to negative terminal. Not sure, where to start. Ive never seen this many systems on ONE CIRCUIT before lol. Please abstain newbie opinions and all that junk. Like I said before I'm a pretty experienced mechanic, just clueless on fords. I've done tranny jobs but that was just R&R stuff. Any advice os appreciated. And yes before you tell me to, I've searched all threads I can. Eyes are burning. Thanks boy's.

 

Only reason it should pop is from it completing the circuit to ground by chaffed wires, or possibly a reversed wire.

Also, make sure you are using the correct amperage fuse. You can find this information in the owners manual.

One last thing... please, don't substitute the correct fuse with a larger fuse or a nail or something else. You'll have an inferno.

 

Was the fuse blowing before the exhaust (cats) work was done? If no, suspect unintended damage to the wire harness.

Go read this thread along with the two additional threads linked there in: https://www.f150online.com/forums/20...ical-help.html Same problem different causes.

The observation regarding the purge valve is interesting. I'd just leave it unplugged until you resolve the short circuit issue then revisit it once the primary problem is resolved.

 

Found this. Trans harness and left side(passenger side) upstream o2.


so I pulled out trans harness no insulation broken. Taped it up, new loom. Looks new. Got pigtail harness (BDW) wired that in taped and loom. Runs without blowing fuse, problem fixed. But misfire. From researching this ive learned these trucks have .....issues with ignition. 😆 but, this truck is really growing on me (grew up with chevy guy as a dad). But timing chain tensioners/ vct os another problem. Pretty sure I hear chain slap. Will that also make truck run like **** also? As far as I know plugs and coils were done. But I'm sure theyre autozone. By the way, the pic of sensor harness I pulled ot up over intake and disconnected battery and computer.