Yesterday I drained the fuel filter and saw nothing truly bad in there. I was getting ready to replace the lift pump and noticed that the priming lever was all the way down, so I lifted it back up and took it for a spin - no dice, same problem.

I replaced the lift pump, then took the truck out this morning - although the truck seemed to run better in town (probably it benefitted from a new pump anyway), truck had same problem.

I've noticed that the truck loses power above 70 mph, and once it does, it is a real effort to get it back over 55-60. I had an idea (going back to a thought I had when it first happened two weeks ago), so when the truck was running fine at 70mph, I turned the overdrive off, and OILA! it lost power and dropped to 55mph. I turned the OD back on and OILA! it gained the power back.

I noticed while doing this that when I hit the OD button to turn it off, that it didn't want to stay off - I had to hold the button down.

I think my overdrive is cutting out - maybe a bad switch. Anyone had this problem before?

 

I guess it depends what you call losing power. Mine would drop in and out of O/D on the freeway, and I guess it felt like I lost power,as I could not get the truck past 65 mph, and it felt close to topped out at 60-65 mph and when my O/D would kick back in I would take right off. I guess for me I pretty much knew it was a tranny problem. If you think this is your problem,I fixed mine with a new TPS (throttle positioning sensor) It is the little square electronic device mounted on top of your injection pump (2"x2" square)! I'll try to follow this thread and chime in when I think I can help,I'm sure you'll get more response from others.And check other threads on the (O/D ,Tranny stuff) and see if its your problem! Good Luck!

 

ya, maybe we need a definition of 'losing power' Denis? At the top of the rpm range in a gear the truck will feel weaker as it starts to defuel, but I wouldn't really call it 'losing power' You do lose the ability to accelerate tho =) If you keep it locked in direct, you'll get up to around 65mph and it'll feel like it won't go any faster. But it should still feel powerful, meaning you'll have no trouble going up hills at that speed in that gear

 

Furious, maybe you are more correct in your terms than I. It definitely feels weaker at the top of the RPM range. It does feel like it won't go any faster, but it's not bucking or doing anything weird.

This morning I drove it to work and whenever it dropped, I turned the OD button OFF, then ON, and it picked up every time.

Bad OD switch?

 

There are a couple things it could be, I was told when mine was acting up,it could be the speed sensor,brake switch or the TPS,As I said it was the TPS, for me.No one hinted towards the switch. Kinda sounds like your switch is working thou,as you get a response when you turn it on or off?Also from the threads out here the TPS is a trouble spot.