I have a 2001 F150 4x4 5.4L that will not start. The #18 Fuse under the hood blows. The theft light flashes while cranking. I followed the wire that the fuse plugs into through loom and found that it goes to the PCM. I unplugged the PCM and the fuse still blows as soon as the key is turned on. I have no wiring diagram so I am not sure where the wire goes. I did look in the owners manual and found that this fuse controls The PCM,Fuel pump relay, and mass air flow. This happened once before so I replaced the fuse and problem solved but this time I haven't been so lucky. Both times its happened I have had the 4wd engaged. Maybe this is just a coincedence. I hope someone can help me.

 

First check your fuel pump relay.
This is a wiring layout of fuel system:

 

Unfortunately, that diagram only shows one point that F18 Feeds and it's a pretty unlikely one, at that.

Inspect your O2 sensor harnesses and see if any are melted to an exhaust tube. They're your most likely culprit. If not, disconnect all 4 sensors to verify none of them have heaters shorted to the case.

However, fuse F18 covers a lot of stuff under the hood. If checking the sensors doesn't resolve the problem, I (or someone else) can compile the full list of the circuit's distribution.

Steve

 

Thanks guys. This info will get me started. I hope I can figure it out.

 

Did you install a Cold air intake? If so check the red wire going to the MAF for chaffing.

Regards

Jean Marc Chartier

 

no cold air intake but I will check that wire.
Thanks

 

If you don't have a cold air intake then the odds are that that wire is fine. Was the transmission ever removed/serviced or exhaust work done on the truck? Some times the transmission wires can get cut or chaffed when the transmission is bolted back up. Exhaust work that was not done properly can allow the O2 sensor wire to lay on the hot pipes like Steve mentioned.

Regards

Jean Marc Chartier

 

It has had some manifold work done on it. I just stuck another fuse in it and it started up. I am really baffled now. Its hard to find a problem when its not malfunctioning.

 

A wire may have been chaffed when the work was done. All the fuel injectors share fuse 18 as does the IAC and the fuel pump relay. I was mistaken about the O2 sensors.

Regards

Jean Marc Chartier

 

yeah I did notice that the theft light flashes while cranking when this problem happens. It is still a mystery where this bad wire is. The search continues.
thanks guys

 

Circuit 361 feeds all of the following from fuse F18:

MAF
PCM
8 Injectors
IAC
Fuel pump relay coil


I was mistake about the O2 sensors, they're on circuit 391..

Steve