my 1996 4.0 V6 auto trans 160,000 miles normally runs great. but when I get up about 50 miles an hour it will start to shut off and back on real fast. only takes a second...driving faster means more bucking. it's like the electrical signal is removed to the plugs or injectors and started again...real fast off and on..the ford stealership said everything is OK, they tested it and did a full service and tuneup on it. but the bucking continues. it is not so bad, if any, at lower speeds. also when filling the fuel tank it MUST be filled very slowly...because it keeps shutting off the pump nozzle. like the vent or something won't let the air out. could this also be the cause of the bucking ? the fuel leaves the tank to be used by the motor at high speed and the tank can't vent...would that stop the fuel to the injectors for a second ? would hope someone has some input on this as it is making me crazy trying to figure this out.

 

I pulled the tank and checked all the fill and vent lines and they are clear....also I replaced the fuel pump while I was in there. still have the bucking issue at higher speeds. does anyone know of a sensor or electrical connection that I should check that might be causing this problem ? I have noticed that when I stomp down on it it shifts to low and takes off no bucking at all...I only notice it when it is in O/D crusing down the road....could it be the transmission ? I hope someone will read this. I have noticed not to much activity on this forum.

 

I'm sure there are lots of us that would love to sit at the computer all day and help with vehicle problems we might be able to solve, but work kinda gets in the way and spoils your whole day ! lol
The vent lines may be open , but are the purge and vent valves working as required ?
The only way to be sure is to energize them and see if they are actually working and that requires to be able to get air through them .
The electrical part may be working , but the valve internally could be blocked.

 

thanks for the reply. and yes I understand about having other things to do. sorry if I hit a nerve. I am just used to my jeep forum where there's alot more people on it and answers are flying all over the place. Thanks for the advice about the Purge and vent valves...now I have to show my ignorance about my new explorer....where are the purge and vent valves located and what do they do ? thanks in advance for your assistance and no rush...when ever you have time. Thanks again

 

Lets hope it is of some assistance !
Generally,,,,,,,,,, the vent valve in this case could be mounted on or near the charcoal canister. A refresher, the vent valve is a normally open valve that is involved in the Evap system. When fueling, the vapors must go to the charcoal canister where they are captured and hydrocarbon laden air is screened and only clean air is supposed to exit to the atmosphere via the vent valve. The only time the vent valve is energized is when the Evap system does a vacuum test for air leaks.
When conditions are met, as programmed in the computer, a purge valve opens and the gas vapors in the charcoal canister are taken into the intake and burned.Following a line from the canister to a valve then from there to the intake manifold should help locate the purge valve generally located in the engine compartment.

There can be a problem if , whenever fueling the operator continued filling the tank after the nozzle shut off to really top it off. What happens here is the charcoal starts flowing through the system and plugs the valves. Hope I provided some insight into the system.

 

well this turns out to be the fact that my speedo was not constant. it would jump up and down and be very erratic. I thought I had a bad speen sensor but it turned out to be the gear inside the transfer case that drives the speedo sensor. it was worn out. I couldn't find a gear so I found a used transfer case with a good gear, and just changed the transfer case, steady speedo now and no more bucking....guess it was very confusing to the computer as to what speed I was doing and it didn't know what gear to tell the tranny to be in ...all good now.