Hey everyone. I'm new to the forum and to the cummins. I bought a great 97 Dodge, 5.9L, automatic about 2 months ago with only 58k miles. It's a great buy and a great truck. I love it.

I've been having some thermostat issues lately. It seemed to run a little cold when I first got it, like around 160-170. Now it's started fluctuating from 190 down to around 160. It'll slowly rise up to 190 and then cool to 160 it does this about every minute. I know it's from the tstat opening and letting the new water in, but is this normal? I've heard different opinions from a couple of different mechanics. It seems like it should run a little hotter also. I think I can change it myself, but if there's any tips please let me know.

Thanks a lot for all of the great info.

 

very normal with your truck, and welcome to the site...

 

thats normal they all do it cummins sells a newer thermo that suppose to stop the fluctuating but i have one and really don't see much difference....
but check on the bypass line there is a jingle valve on the block side iv'e seen if it gets cloged ,the fluctuations seem to be alot worse because the block holds the hot water in and does not circulate any water untill the thermo opens.........

 

Nothing is wrong

Cummins says,

THE COOLING SYSTEM USED ON THE CUMMINS DIESEL ENGINE EQUIPPED VEHICLES PROVIDES THE CAPACITY AND PROTECTION OF THE HIGHER GVWR AND GCWR RATINGS AVAILABLE ON THESE VEHICLES. OWNERS SHOULD BE ADVISED OF THE NORMAL SLOWER WARM UP AND LOWER ENGINE OPERATING TEMPERATURES ASSOCIATED WITH DIESEL ENGINES.

THE COOLANT TEMPERATURE GAUGE WILL FLUCTUATE ON DIESEL ENGINE EQUIPPED VEHICLES. IT IS INFLUENCED BY VEHICLE LOAD, OUTSIDE AMBIENT TEMPERATURE AND EXTENDED IDLE TIME. THIS FUNCTION IS CONSIDERED NORMAL

 

Welcome.
Just like clutch said its normal.
Just some of the joys of owning the Mighty Cummins!!