1999 f150 4.2 swap with 3.8 mustang block

Robert N

New Member
Swapped out f150 4.2 block with 1999 mustang 3.8, had timing issues, then found it was plug wire order was out of order on 2 cylinders, switched them then it fired right up. after several starts without any problems.. it suddenly stopped cranking up. no spark on any cylinder. replaced coil pack with the mustang one, same problem. bought new coil pack.. still same problem. can hear the fuel pump come on from underneath.. when i press the purge valve on the fuel rail with the key on.. fuel comes out for maybe 2 seconds and stops. have switched out the crankshaft sensor with the one from the mustang.. no help. i know i have the timing correct because it was running great.. and i used the tool to set the cam synchro with it at TDC. I have swapped the fuel pump relay with the pcm relay.. no help. tried other relays from the mustang on each and still no help. i have ran another ground to the fuel pump.. to the frame, just in case it wasn't getting a good ground... no help. there's no theft light on. when cranking the engine over... the service engine light goes out as it should... from what i read it should go out when cranking, indicating the crank sensor is working. I have tested every fuse under the hood and in dash, all are good. The only thing i can think of is cam synchro.. I don't have another to test with.. hoping someone may have an idea on what else i can try to get the truck firing again? or what would stop it from firing? would fuel pressure stop it from firing if is too low? any help appreciated.. ran out of ideas.

Edward S

If not mistaken, theres 3 coils in the coil pack, take a ohm reading across all of em, if there all within each other, check the pins on the plug into the coil pack


Active Member
The crank sensor plug may have came loose. Make sure the crank sensor connection is good.
Aside from that, do what Edward S said.

The spark system is pretty basic on the 3.8. It consists of a VR sensor, reluctor wheel, coil pack and wires.

Assuming the balancer didn't fall off and the coil pack didn't melt. The only other point of failure would be to do with the crank sensor and or wiring connections.

Any rpm signal on your scanner while cranking?

Robert N

New Member
on the advice from a friend mechanic, there's two diodes in the fusebox under the hood.. sometimes they don't get a good connection. they're black and look like small fuses, basically took them out and put them back in the same place and the truck started up. really a weird fix. but it worked.