2001 Ram 1500 5.9 Magnum. 130,000 miles and completely stock. Been very reliable, but just threw me a curveball. I was driving at a steady highway speed at 70mph for about an hour. 90 degree day. No load in the truck and level road. I realized that the engine was suddenly racing and watched the tach fluctuate rapidly between 2000 RPM to almost pegging at 5000+ RPM. Rapid fluctuation from one extreme to the other for about a minute as I pulled to the side of the road. I did not feel dramatic surging despite the RPMs, it did not over heat, and no engine codes were displayed. I turned off the truck, waited a minute or so and then restarted it. Drove it another hour + at highway speed with no problem. I see lots of forum information about 200-300 RPM surging in Rams, but not this problem. I'm not sure where to begin on this, but I certainly don't want it to happen somewhere I can't safely pull to the side of the highway. Appreciate any insight.

 

Welcome to DF!

Think I would plug a scanner into it in any event, and see if there aren't any pending codes. Sounds to me like you are dropping to second gear...... which really isn't a good idea at 70. That's the default the trans goes to when it falls into limp mode..... So, you probably have a sensor failing intermittently somewhere.

Does the speedo remain steady?

 

Thanks or the quick response. I'm getting no code on the OBD-II scanner. The speedo remained constant...truck was not accelerating as the RPMs were going to redline.

 

Ouch. Not good.

Given that you have to drive it for an hour in O/D..... the quick/easy test isn't going to help..... I suspect you have a sensor getting hot, and failing. Trouble is, which one.... could be either the governor pressure sensor, or solenoid, or output speed sensor. (speedo gets its cue from the rear wheel speed sensor)

If your scanner will feed you live data, take a look at what those three sensors are telling you when the problem presents itself. If one of them is obviously wrong, replace it.