I have 1991 302 HO.After about 6 miles when you come to a stop.You have to put one foot on brake and other on gas. I believe after 6miles the engine is in the closed loop.The idle will roll up and down.When you are at cruise it will buck.Sometimes it shifts back in forth from OD to normal drive.At stop sign it will die if you dont keep foot on gas and break.
It is not the EGR nore the IAC.Ther are no code or check ingine light.
HELP Jan

 

If its not throwing any codes and still has the severe condition you're describing it may be a bad ECU. Have you actually checked for codes with a scanner as the CEL bulb could be out.

 

If you didn't know already, a CEL wont always show when your Bronco throws codes, a lot of people don't know this. There are a number of things that can cause this in my experience so far, but more knowledgable people definitely hang around on these forums, still wont stop me from trying to help.

I'd say it sounds like the MLPS but it might throw codes.. well, might, I got an occasional "neutral safety switch out of range" CM, no CEL on this one, ever.. when I say occasional, I mean every couple of weeks.. yet my MLPS was almost completely shot. It is called an MLPS, Neutral Safety Switch, "That darn Ford transmission switch that always breaks and causes people to junk their trucks", or just shift positioning sensor.

Could also be a throttle positioning sensor, but that one should throw a CEL.. I'd blame FPR or something along those lines but they wouldn't cause you to shift from OD to 3rd..
or when you say shifting, do you mean the light flashes on and off? If so, that's completely different, it's not shifting out of OD, it's flashing because something is wrong. If that's the case then I would definitely look into your MLPS or TPS.

These motors, 302 or 351w will not run right if your automatic tranny isn't running right, throttle sensor, iac and a few other critical sensors depend on signals from your transmission. There are two plugs on a 91, the MLPS (give shift lever signal) and the .. uh.. the other one, which basically relays gear position and controls TC lockup as far as I know. If either harness, plus or sensor in the case of the MLPS is bad, it will show up as these exact issues you're having. It's common for people to have to at the very least pull both harnesses, clean them and regrease them, sometimes reseating fixes issues like this.

You also have the VSS or Vehicle Speed Sensor at your transfer case that might be the cause..

 

Please clarify for us. "Ther are no code or check ingine light."

Do you mean 1. The CEL does not flash or 2. Your codes are 111 (passed) in KOEO, CM and KOER ?

If 1. Edgethis's suggestion for a code reader is my suggestion also.