My '91 F-150 (EFI 5.0L/AODE) has recently developed a habit of downshifting from overdrive to 3rd gear at 70-80 MPH on the interstate. I don't have this problem from 55-65 MPH, but once I'm up to 70-80 MPH (depending on traffic flow), it wants to downshift. This is really killing my gas mileage, as the last 2 times I've been to Indianapolis and back (roughly a 120-140 mile round trip, depending on where in Indy I'm going) I've used 7/8 of a tank.

Any thoughts on what I might get checked, that's causing the downshifting at interstate speeds? Within the last 2 months, I've had the EGR tube, right exhaust manifold, idle air control, fuel pump relay and fuel filter replaced. Since January, and including the above parts, the water pump, alternator and radiator have been replaced. I figure I do need to get a tune-up done/do a tune-up as soon as I have the chance, as it's been last December since that's been done. Still, even with a proper tune-up and fluids change, I doubt that it'll help my interstate gas mileage, if it's gonna be downshifting on me. The transmission was rebuilt in September 2004, so I'm a little baffled as to why it's suddenly giving me problems again.

Matt in Indiana

 

Driving 70-80 is killing your gas mileage to.. How much thottle are you giving it when it down shifts?

 

I'm probably at 3/4 throttle when it down shifts. I had the truck at the local Ford dealer yesterday, for a diagnostic on a stalling problem I was having, where it'd stall when I put it in reverse and start to back up, but it's also stalled on me in Drive, when taking off from a stop. They found that the idle air control, fuel pump relay and fuel filter were bad, and they were replaced this morning, prior to me going to Indianapolis. When I took off from the Ford dealer, I'd been running on my front tank, which had 3/4 of a tank showing on the gas guage, and I switched to the rear tank that was full. By the time I got back to Connersville, from Indianapolis, the gas guage was down to 1/8 of a tank.

As for the speeds I'm driving, both I-74 (the way I went to Indy today) and I-70 (the way I went earlier this month) are posted 70 MPH. On my return trip, earlier this month, I went U.S. 40 which is now posted at 60 MPH through the rural stretches. Today I took U.S. 36 to S.R. 234 to S.R. 109 to U.S. 40, of which everything leading me toward U.S. 40 is posted at 55 MPH through the rural stretches.

Your asking about how much throttle I was using, when it down shifted, got me to wondering if the throttle position sensor might be the culprit, but wouldn't that have shown up on the computer, when I had the truck at the Ford dealer for the diagnostic, yesterday?

Matt in Indiana