I don't know if this has been covered, but here goes. I bought a 1990 F-250 XLT Lariat 460 about 6 mo. ago. It has been running fine (Except for being a Dog.) up until a couple of weeks ago I went to Phoenix in 108-110 degree weather. On the way home It started bucking and missing and would hardly run. After stopping a couple of times it started accelerating slowly until I hit 65mph and then seemed OK, I switched tanks and it started doing it again, switched back and it ran fine the second I switched tanks back. Then a couple of days later, I switched to the rear tank again and it ran fine. Now for the weird part, I went back to the Phoenix area today and on the way home it started again, wouldn't run over 35 mph and was bucking and coughing all the time. I switched from the rear tank to the front tank at 160 mi. and it was running OK, then about 20 mi. down the road the crap started. This time it didn't help to switch tanks. I limped down the road till I hit a gas station and filled the rear tank and pulled out and it seemed to run fine. Switched tanks again and missing backfiring etc. I noticed a varnish smell when all this was happening. Think maybe seafoam is knocking loose old deposits in the tank? There is a large filter on the frame drivers side, is this factory? Sorry for the long post.

 

first thing i would do is check fuel pressure when it is not running properly. my guess is either clogged tank vents , or weak fuel pumps.