98 4.0, 4x4

First time this has happened. While driving the temp gauge would drop to almost 100 and rise back up to normal temp. Sometimes it would start to drop and then return to normal, while other times it would do the opposite, start to rise then drop to just above 100.

While on the highway it seemed to consistently stay just above 100 and would only start to rise when letting off the gas or slowing down for an exit.

Then at times it followed no rhyme or reason. Moving up and down at will.

The heat output in the vehicle was normal. I don't have an infrared thermometer to check the engine temp but my first thought is a bad temp sensor. Any thoughts?

 

My 99 is doing the same thing. The gauge is jumping up and down. You're right, it's likely a bad coolant temp sensor. The one that plugs into the thermostat housing.
I'm going to try to track down the mopar part number. I'll let you know what i find.

 

Great, thank you sir!

 

Yes it is very likely your coolant temp sensor.

Mopar 56027873

 

For what it is worth, you could try removing the coolant sensor and clean it. When I first got my Jeep, it had 60,000 miles. I noticed the same thing you are complaining about, so I bought a new sensor.

I decided to flush the entire coolant system, and when I removed the sensor it was covered in crap. I just cleaned it off, and installed it figuring, "let's see". It worked, and has continued to work to this day.

That was years ago....Of course, now that I posted this I fully expect my sensor to not work tonight when I leave work.

 

Coolant level.

 

Radiator was full, but the reservoir was low.

 

Did you get this fixed? I just swapped mine out for a new sensor. So far so good and it seems to be problem solved.
I had to go out and buy a new deep socket 19mm though as the one I have didn't quite fit. The old one still didn't really want to come out so I sprayed it with some pb blaster and wrenched gently each way to loosen it up, and then it came out. It squirted a bit of coolant out, but I quickly got the new one in so it didn't make too much of a mess. I put a little teflon tape around the threads. And put some rags all around it to catch any coolant.
Good luck with yours!

 

Depending on ambient air temperature, this could just be your thermostat cycling. I would recommend changing your t-stat as a part of regular maintenance. Its cheap too!

 

No, haven't got to it yet. Glad you got yours worked out. Where did you buy the part?

 

T stat was replaced around 3 years or so ago. And this doesn't appear to be just the t stat cycling and it has never done this before.

 

I ended up just going to a local parts store and buying what they had. Ordering would have taken too long and I really needed to just get it fixed. It's also the warmest day it's been in weeks (Canada) so I bit the bullet and just did it. It's a "standard" brand or something. But I drove around for a while after and let the thing run while I put air in the tires and cleaned it up and the gauge didn't drop at all.