2005 Jeep Liberty
My Jeep will occassionally drop into passing/towing gear for apparently no reason. When it does this, if I take my foot off the gas it doesn’t go below 3000 RPM. Putting it in neutral and back into drive doesn’t help either. If I stop and turn off the car for a second then restart it the problem goes away which to me as a layman indicates a sensor or computer problem. It is most likely to occur going up a steep hill but will sometimes do it on a low (1-2%) grade and once on a flat freeway. It has only done it a few times before but on my current trip it did it twice heading up the Rockies and 6-8 times going through Cental Utah.
Forgot to add. I checked the transmission fluid and it seemed OK. Had the oil changed a few months ago and they checked fluids too. I will check today before taking off but I don’t think that’s the problem.
Is the check engine light on? Most auto parts chains will scan the codes for you for free.
I suspect a sensor out of wack. Transmissions are all electronically controlled now and it sounds like the computer thinks you’re pressing the accelerator further than you are. A faulty throttle position sensor may cause something like this. Look online for the testing procedure; it’s different for different makes/engines/vehicles.
If you search online you’re bound to find a Jeep Liberty owners forum. These type of forums are a valuable resource for these type of questions and the procedures to troubleshoot them.
+1 sensor
Someone else hadmentioned the MAP sensor. Looks like a $30/5 minute fix
That would definitely turn on the check engine light. Is the light on? Does it work?
Throttle Position Sensor (TPS) if that’s what it’s called in a Jeep.
Yep. CE light comes on every time it does this.
My 1999 Jeep Cherokee had a similar problem when it was still under warranty. (IIRC, about a year after I bought it.) It would not shift into 4th gear. It may have been a sensor, but I know I had the computer replaced under warranty at some point and it may have been that. I recall a sensor being replaced at some point as well. The replacements may have been concurrent, but I don’t think so. It’s been over a decade, and didn’t cause any hardship that would make me remember them specifically.
ETA: The Check Engine light did not illuminate.
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When you get a chance, go to Autozone, Advanced Auto Parts or some place similar and get the code read. It stays in the computer even if the CEL is no longer on. You can go to a ODBII code site and look up what’s up. Unfortunately, many of the codes are not specific about exactly what the problem is. But it will point you in the right direction for a starting point to solving your problem.
You should be able to turn the ignition on (one click before start) and off (all the way off) three times, leave it on the fourth time, and watch the odometer. It should flash a number or two like “P0120… P0124… done” then you can look up the numbers here.
It sounds to me like it is just dropping out of overdrive which is very normal in hills or in some cases a stiff head wind. If the car is in need of a tuneup or the airfilter is plugged you may be depressing the throttel harder than you are aware. Have you tried bumping the brake pedal when it does this, almost sounds like it is going into cruise control. I would hit the set resume and on and off switches for the cruise next time it acts up just to see if it makes a diff.
It’s definitely a sensor or computer issue. All modern transmissions are computer controlled. Not to sound insulting, but my oldest brother recently bought a new Jeep after having had his previous one for 10 years or so, and his was shifting on its own initially. But it turns out he was just leaving his hand on the automatic shifter lever and didn’t realize that moving it side to side a little while still in D makes it downshift manually! 
Always a Jeep!
I don’t know about this particular problem but “sometimes” removing both positive and negative cables from the battery terminals and leaving them offf for 10 to 15 minutes allows the “black boxes” to reset the FM (F’n Magic) inside. I’ve seen this work for radio problems and engine starting and stumbling. Don’t cost nuthin’ but could end up clearing the codes also. just sayin’
OK I changed out the MAP sensor and it seems to be getting a little more power and shift easier. The real test will be when we head back over some real grades.
Worked like a champ over the Rockies.