Melted piston #4 on the way home

[MikeSak]

I bought a nice 89 Ranger XLT 4x4 2.9 manual everything a few months ago. I had to drive about 350 miles each way to get it. It ran perfectly fine during the test drive and for the first 100 miles on the trip back home. It started off just losing power at highway speed so I would just pull over for 5 mins and then it be at full power again for like 20 mins before having to pull over again. Well on about the 4th time pulling over to let it rest it was down a cylinder and shooting unburnt oil out of the exhaust and was obviously really down on power then. Eventually I had to get off the expressway and take the last 200-250 miles home on back roads, but it made it back on the 5 cylinders and a extra 5 quarts of oil. The next day I pulled the motor apart and found a piston melted through in one spot right on the very edge by the rings, the head cracked right between the valves, and the exhaust valve seal popped up all on cylinder #4. The engine never got hot and oil pressure was always good. It didn't even have the 2.9 tick. My theory is the fuel injector on cylinder #4 got clogged and caused the cylinder to go lean and run hot. What do you guys think happened?

 

[adsm08]

A lean run will usually burn the piston near the middle. How was the plug? If the tip broke off and it laid against the edge of the piston it would cause a hot spot, burn the piston, and crack the head. It would also account for the initial good running and then the progressive power loss, but not necessarily cause a miss.

Shouldn't be hard to check either theory. You can pull the injector and look at it easy enough. The plug should just require flipping the head over.

 

[MikeSak]

Everything looked good. The plug was intact and just looked oily from what I assume was oil coming up through the hole in the piston. The injector also looked good. I already did a full rebuild on the engine and bought a complete set of reman injectors for it. The engine is sitting on the stand ready to be put back in. I've just been way to busy at work to get it done. I just would really hate to put it back in and have it melt #4 again.