I have a 86 F-150 that seems to be possessed. It runs great when it is cold out, but when the outside temp is above 60 degrees it starts acting up. It looses power, back fires, and acts like it is starved for fuel.. I have changed the fuel pump (it needed it), changed the fuel filter. Has a fresh tune up, and I have changed the coil. It has the 5.0 fuel injected engine. When it acts up, it struggles going up hill, but if I shut it off for about 10 minutes it will start right back up and run Ok for about another 20 minutes. I am about out of ideas. Anyone have any other suggestions? I love this truck and it is in great shape otherwise. Thanks for any input.

 

Sounds like it might be over heating.

 

I thought that too, but it isn't running hot. The temp gauge is normal, (but who knows). I believe the thermostat is broken, because the heat barely works. I haven't seen any other signs that it is overheating. I replaced a hose about a month ago, so it has new coolant. Drove it today, and it ran great. No issues, but its in the mid 50s outside today.

 

If. it is injected it should have a coolant temp sensor in addition to a sending unit for the gauge. It is a cheap replacement and will cause weird conditions if it is sending the computer the incorrect information.

 

More signs pointing to over heating. My 79 wouldn't​ warm the cab at all. But if it was warm out it start giving similar issues. Lack of power. Sputtering, even loose my power steering. But my tempature gauge would stay at normal. Because it was broke. It would rise just to the R and nothing more finally figured that out when I put an aftermarket gauge.

 

Yeah this sounds like an over heating problem, I have a 4.9L 86 f150 and mine did the same thing to me on the interstate, turns out my thermostat was stuck shut, so after a new thermostat (160 degree) and a new thermostat housing because the original one cracked haven't had the problem again. If you havent fixed the problem yet replace the thermostat (around $10-$15) and see if that helps, and if you have an infrared thermometer see what temp its running at. Hope this helps!

 

If you know your thermostat is broken fix it first.....