High RPMs on highway

[Mosaic4077]

Howdy! I just got a 1998 4 cylinder automatic TJ last month. This is my very first jeep, and my first off-roading/adventure vehicle since high school so I'm looking forward to getting the wheels dirty! Aside from feeling like piloting a flying brick while going down the highway, I'm really enjoying it!

Anyhow, this week I noticed that for the first 5-10 minutes of driving, it maintains it's normal RPMs, but after it gets heated up (or about 10 minutes of driving time) the RPMS shoot way up! At first, I can be putting down the highway going 60 and pulling RPMs somewhere in the 2ks. After it acts up, in order for me to maintain 60 MPH, it'll sit at a little over 4K RPM!

Has anyone else had this issue? Could it be an issue with the transmission overheating or perhaps the throttle cable needs adjusting?

I've already tried swapping out the trans fluid/filter and it did not seem to help me out one bit.

Any advice ya'll can give me would also be greatly appreciated!

 

[Chris]

That's odd behavior. Is it throwing codes by chance?

 

[Brianj5600]

My guess is that the first part of the drive it is borderline being able to maintain speed in 3rd gear. As every thing heats up power is reduced and causes it to downshift. Does it do any better at night or early morning? What does the temperature gauge show?

 

[Goatman]

Sounds like it isn't wanting to stay in hi gear. I'm no auto tranny guy, but a couple things might be the kickdown linkage is getting bound up or even possible clutch pack slippage to perhaps the torque converter not staying locked up. How was the fluid when you did the filter change? Was it dark and have an odor? Was there any debris in the bottom of the pan?
My knowledge of auto's is very limited. Never had one. Never trusted them. I know that makes me old.
Check if there are any codes in the obd2. If not, check when the problem is happening. Someone with a lot better knowledge about the slush box should chime in.

 

[-T.]

You didn't mention tire size, but you say around 2000 RPM at 60 mph? That sounds like maybe stock sized tires. I've never had a 4 cyl auto, but I think it would run around 2300 RPM at that speed? Someone here will know better than I do. However, if you are running over sized tires, you're likely a little under geared. I am pretty sure the 98s had the 3 speed auto, but all the TJs have lock up torque converters. My guess is that, maybe not so much the engine temp, but just over all load on the highway is a little too much for the 4 cylinder, so the torque converter is unlocking to bring the RPMs up to make some power. Try moving to the slow lane and slowing way down, say 50 or below and run that way for a little while, as the computer tries to keep the torque converter from locking and unlocking to frequently. But see if, after a bit of time, the RPMs drop back down. You'll feel the torque converter lock, it feels sort of like a subtle gear shift. Then as you start increasing speed again, particularly when you come to an incline, or if you turn into a headwind, the torque converter will unlock again, and the RPMs will jump back up. If it happens like that, pretty consistently, the transmission is functioning normally, and your just dealing with our relatively under powered TJs. Sadly, the 4 cylinder automatics are the worst of the group.

 

[Mosaic4077]

Thank ya'll for ya'll's attention on this! My jeep hasn't thrown any codes yet, which would be helpful... Right now, it is on 31" tires and if the plate on the diffs is accurate, it is geared at 3.73. I'm new to jeeps, so I don't know a lot about gearing yet, but from my lurking on this forum, it appears to be a big deal. lol.

While going down the back roads at 40mph, when I first start it it'll 1700-1800 RPMS, but if I drive for too long, it will shoot up to 3K.

Anyhow, I've been keeping it off the highway to avoid too much wear and tear until I resolve this. On the way to work this morning, it overheated, but the temperature gauge didn't move at all! At this point I really think it has to be directly related to it getting too hot.

However, It could also be the torque converter as Gotman mentioned. When I changed the fluid/filter, it didn't look too bad, but for sure like it was in need of a change! When I start if cold, it takes two cranks usually to bring it to life. I hear that this can also be a symptom of a bad TC. But if the TC is bad, why would it work okay for 5 or so minuets of highway driving, then act up?

I guess my next course of action it to find out why my radiator overheated this morning without moving the needle at all.

 

[Goatman]

Is the water pump original? It also could be that the tranny cooler in your radiator isn't cooling like it should if the coolant dosen't cool. You may be in for a complete coolant system replacement. It's far easier to do it all at once then to just replace 1 item and then find out you need to change out something else too. If so, be sure to really flush the out of the engine and heater core. Replace everything. Hoses, thermostat, 'stat housing, heater hoses, pump and radiator. Consensus here is to be sure to get a new MOPAR radiator. Pump dosen't seem to matter so much. Don't go for any 'high capacity' type pump though.

 

[billiebob]

I'd say if the tach is correct, which it must be, you must hear the doubling of rpm, this is a "simple" automatic tranny gremlin..... and the reason I've only had 5 automatics in 45 years, tho I do love the ones I've had..... with the exception of geting them rebuilt.

At least that 3 speed automatic is one of the best automatics ever, Well worth the cost of a rebuild altho this might be the torque converter. ANY shop should be able to give an accurate diagnostic. This is not rocket science.

Cold, all the oils are much thicker than hot. Once warmed up "and thinner" likely the wrong word, sure to be corrected by the gods from hell, performance will change.

Waiting for the inevitable corrections lol.