Last year I purchased a (repainted) black 1998 Ford F150 4X4 Lariat with a 4.6 Triton engine to transform into a show truck. My midlife crisis project. Now it has a 6 inch ZONE lift and 37 inch gripper tires. It does not win any awards for fuel economy or being environmentally friendly!It is not running at the moment. The last time I drove it, 2 weeks ago, it was running great! That night the factory installed anti theft system went off 3 times. I don't have a fob, nor a chip in the key, so when it goes off I have to go to the truck, put my key in the driver's side keyhole and turn it to stop the alarm. The next morning when I tried to start it, it made a clunk noise and wouldn't turn over. I put a brand new starter on, solenoid wire, cleaned the battery terminals and put new terminal "nuts" on it Tried to start it and still just a clunk. I did some more research and came across a thread discussing what it "looks like" when the anti theft device engages. Finally found out how to reset the anti theft system without a fob or chipped key. Wallah, it worked! But when it turned over the engine was shaking and making a fast banging noise so I shut it off quickly. Several of my "weekend (you tube) mechanic" friends are telling me the engine is frozen up. One even offered to exchange titles, he has an SUV still under factory warranty. I am having a very hard time wrapping my mind around the fact that the engine froze up when it was running great. Recently new oil (5W 30) and oil filter (motor craft). The oil was, and is, registering where it should on the dipstick. PLEASE OFFER your advice and thoughts! THANK YOU!

 

Strange story. No ideas about why the alarm went off or the weird way you fixed it. I don't think that 1998 was very sophisticated as far as anti-theft. But, if you're worried about internal damage you might remove the spark plugs, inspecting each one for signs of problems, then spin the engine with the starter. See if it still makes the banging noise.

The symptoms sound more like a hydraulic lock, either from fuel or coolant. There should be clues on the spark plugs. It might just be a conicidence that it happened after the alarm problems. Good luck.

 

Remove the serpentine belt and see if the engine will turn over. Won't hurt to run it for a few seconds without the belt.

If the problem persists, see if you can turn the engine by hand. If it's frozen you won't be able to budge it. If something is internally broken, you should be able to hear it.