What could cause sudden acceleration?

[Laura C]

We recently bought a 1997 4.0 4wd explorer. The only major problem that we knew of was a bad transfer case. It whines when you let off the accelerator and it also makes a whining sound in reverse and won't move about 30% of the time in reverse. My husband was moving it into our barn to start all the repairs we wanted to do. It was a tight spot so he pulled close to a fence and then went to back up to get straightened out. Of course when he pulled up against the fence is when the reverse wouldn't work. So he kept putting it in drive and inched forward then reverse but reverse wouldn't engage. So he tried putting it in 4wd to see if it would get reverse to engage. As soon as he switched from reverse to drive with 4wd on the engine revved wide open. He never even took his foot off the brake. It lunged over the chain link fence. Amazingly it didn't really damage the fence or the vehicle and there was a field on the other side. It moved a good 30 feet before he could get it to stop. He had to turn the key off to get it to quit. Afterward he drove it around the field for a good 20 minutes trying to see if it would happen again and it never did.

Does anyone have any idea what would cause it to rev wide open? I'll explain it one more time. It was in 4wd and he tried reverse and it wouldn't move because of the transfer case. So he went from reverse to drive. The engine was at normal idel and the second it went into drive it was like you put the pedal to the floor. It launched and wouldn't quit until he turned it off. Thanks in advance for your help.

 

[koda2000]

I don't know whether a bad shift solenoid might be the issue with you not being about to get reverse. It does seem to be a fairly common complaint with the V6 transmission though. Someone else will probably offer an opinion on that.

In my experience, engaging 4WD has no bearing on engine speed, neither does moving the shift lever.

With my Gen II Explorers/Mountaineers I've found that I have to be careful when stepping on the brake pedal, as I have managed to simultaneously step on the brake and gas pedal. This has happened to me at least a half dozen times, over as many years (I have big, wide feet) and it always seems to happen when I'm turning one of my trucks around in my driveway. It's happened to me enough times that I now immediately quickly slide my foot to the left w/out even thinking about it when it happens. I've never experienced this with any other vehicle I've owned. Is it because the accelerator pedal and the brake pedal are closer together in the Explorer... ? IDK. I've never read of anyone else complaining about this.

The only thing I've ever heard of which can cause this is a broken engine mount. Even if you were to floor the gas pedal while in gear from a dead stop, your brakes should be able to prevent the vehicle from moving.

Do you recall the "unintended acceleration" issues with the Audi vehicles back in the 80's? I believe the final conclusion was that there was insufficient space between the accelerator and brake pedals. More recently Toyota had a problem with the accelerator pedal getting stuck under the floor mat. Someone on this forum reported unintended acceleration recently, it turned out to be his floor mat.

 

[Laura C]

The first thing I asked him afterwards was did he hit the gas by accident. He said he never moved his foot from the brake. He had it in reverse and it wasn't moving. Put his foot on the brake then switched it to drive and it took off right away. I was standing next to him talking to him threw the window. I was paying close attention to him switching gears since we were having issues with it. The instant he put it into drive the engine reved up and it launched. He was also being extra careful because he was about 2 feet from the fence.
The brakes aren't the best on it. That's actually what he's working on right now. I drove it earlier and the brakes are bad enough I wouldn't drive it on the road. I would imagine it could push threw the brakes.
When he get done with the brakes I'm going to take it out in the field and try to see if it will do it again. I'm a bit nervous to drive it on the road now.

 

[FF_RSQExplorer]

Mis adjusted or damaged shift linkage could be the cause and the source of your symptom before the incident.

Dirty throttle valve (carbon can keep the valve hung momentarily)

Bad Idle Air Control (carbon can cause the printle to be hung and apply too much air at idle)

Bad EGR or EGR control by applying EGR purge at idle