I have an 87 F-150 and my C6 transmission used to shift really early, would shift to 3rd at 15 mph, modulator went out, was leaking transmission fluid, replaced and adjusted it. Transmission then shifted to 3rd at about 20-25 mph at light throttle. Lasted about 2 months, now same thing, shifts to 3rd at 15 mph. No fluid from modulator, adjusting didn't help, replaced it. New modulator did nothing, adjusting it doesn't help, and I have plenty of vacuum going to it. Any ideas? Please help, it's my only vehicle and I can't keep driving it like that, and in neighborhoods it accelerates to 15 mph in 1st gear, then tries to shift 2nd and to 3rd almost at the same time which causes a really rough shift and feels like it's going to break something internally. Thanks for any help!

 

Been busy with the holidays, so I haven't had time to mess with the truck any further, but I was reading up on some stuff and I also have a hesitation that happens when the truck is warm. Engine first cold, truck idles smooth, if I were to drive it, nothing wrong with it (except for that modulator). Then once the engine warms up, as I'm driving it begins to buck a bit, but only at a cruise. If I hit the throttle, bucking goes away, soon as I let off, it bucks. If I'm at a stoplight and the truck is in drive, it idles ok. If I shift it to park or neutral, there is a misfire when I'm idling. Read somewhere that it could be a fuel pump issue, pressure drops when it warms up, but my fuel pressure is right where it needs to be at idle. Although, I only ran the truck for 5 min when checking that, so I'm going to go after a longer test and also when driving. And just to add, my timing is suppose to be at 10º, that's where the truck bucks, if I set the timing to 0º, truck quits bucking when driving. Thanks for the help!