My temp gauge seems to be fluctuating on my '97 Ram 2500 5.9L engine (122k miles) between the middle area and cooler (as shown by the pic I posted). I didn't think this normal, so I replaced the thermostat (which helped some, as the gauge used to go up to about 220 or and then rapidly go back down to close to the middle). When I am cruising at a steady speed, it seems to stay where the needle is in the pic, but when I stop and then start moving again it can go down all the way down to the lower mark I made on the pic and then go back up to the normal level.

Another problem it seems to have is the coolant reservoir appears to be slowly getting fuller too, which I think not normal also.

Anyway, some of what I have done so far is burp the system (with the passenger side of the truck raised up) and also brought it to a shop to have the cooling system pressure checked. The shop test drove it 4 times (they said) and told me it wasn't overheating and everything looked normal (I had told them it wasn't overheating when explaining the symptoms about the fluctuating gauge). They also did a cooling system flush and they did do the pressure check.

So my question is, what else could be wrong? I am running out of ideas besides radiator cap or more major things like head gasket or cracked heads. But wouldn't those have shown up in the cooling pressure system check? What is the normal temp operating range?

 

1. where the picture shows is where it SHOULD be. Our thermostats are 194*.
2. If you are getting to 220, that is a tad hot.
3. Needle going down just means that the t-stat is opening and the cooled coolant from the rad is rushing in. But that usually only happens once when the t-stat first opens on a cold day...or if the T-stat is sticking.


It is normal for it to go down then back up. However, it could also be the coolant temp sensor on your intake. You will have two of them. One goes to the PCM and the other goes to the gauge. Some years (I think) have only one coolant temp sensor.

If you are not overheating, do not worry. Be glad she does not go to 220 anymore, because if you were towing, that would easily get to 240. That was a bad thermostat you had.

I remember my 1996 5.2 would always run at 210*. When towing it would vet VERY hot. When moving from Oklahoma to California (in june) I needed to drive from 5 AM to 10 AM to beat the heat. I drove from 10-11 AM and I had to turn off the AC in 100* weather to keep her from going over 220. Not good times.

BTW, my truck, with a 180* thermostat runs between your lowest mark and the tick just to the right of the 130 mark. Could not be happier. In the 108* heat in Arkansas full AC, my truck never got over 205.

 

YOu might want to drain all the coolant and run the Napa descaler, radiator FLUSH (Not sealer). That helps with the cooling. You may have blockage.

More coolant in the overflow usually means high temps or the cap has lost tension. Might want to check that as well.

Go cap, flush, THEN sensor. (Cheapest to most expensive).

When was the last time you changed the coolant? I am assuming when you changed the t-stat.

 

Blown gaskets won't necessarily show up with a pressure test, unless they are pretty bad. Also, one of the signs of a blown gasket, is overheating... which your truck is not.

If the level in your overflow bottle continually climbs...... but still looks like coolant..... It's probably just from heat expansion. (that's what the tank is for in any event. It's supposed to do that.) if it starts blowing coolant out the overflow tank, then you might have an issue.....

What brand thermostat did you put in? How old is the radiator cap?

 

I replace radiator cap about every 2 to 3 years. Rubber starts to get brittle and they are inexpensive.