hey new to the forum i have a 99 f150 with a 4.6 i was driving the truck tonight with out any problems. i went to park shut off the truck realized i needed to move went to start the truck and it wouldnt the motor just spun really fast. i checked for spark at the coil packs and fuel at the rails and both seemed fine. this is my first mod motor so any ideas would be great. also truck is 5 speed in case that might change anything.

 

Are you sure the starter was actually turning the engine?

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If an engine turns over real fast, (if the engine is indeed turning over), it has no compression.
But so strange if was just driving ok without any problems. Especially if a modified motor, typically even more compression.
Unless starter bolts loosened up & the starter gear is not engaging with the flywheel. But that is an altogether different sound than an engine turnig over.
Be interesting to hear what the problem is. Wish ya luck bud

 

^^^ I think he means modular motor....but yeah maybe lost compression? Timing jumped? You have to make sure you have fuel and spark on all cylinders ...did a spark plug possibly blow out? Any funny air blow/sucking noises?

Have checked the inertia switch?

Possible fuel pump bad?

Also have you checked your fuel pump relay? It may be sticking and not transferring voltage to the pump. I believe there are 2 relays in play when activating the pump. I believe the PCM relay has to activate before the fuel pump relay will. And the pump won't stay on long when you first turn the key on. It has to get a good signal from the crank position sensor to continue pumping fuel.