I had some errands to run today and drove the truck. As normal the temperature came up and held steady at 190*. I stopped at the local station to top off the fuel and left the truck running.It was in the low teens here in Michigan today. I know it's colder in other states but.............By the time I had topped off and went in to pay the temperature had dropped to about 160*Why..........

Some High Idle Fooler kits would go good in this thread wouldn't they Prowelder !

I have Winslow's high idle box but it doesnt help while driving down the road.

Mine has been doing the same thing but it is very random, maybe every other day or so. My brother works at a parts store and is friendly with the parts guy at the local cummins shop and asked him about a t stat for my 98 and gave him the engine serial number, well he didnt get back with my brother before he ordered it and it turned out to be a 180. I told him I thought it had a 190 in it because it runs around 190 (when its running right) and he said that the engine serial number said it used a 180. I dont know if I want to put this in or not.

190 is the standard. I run the 180 in the summer. I like it for towing.

Been using cheap ol' NAPA 190*F thermostats my max temp is 202*F typically.

Mine usually runs about 186 or so, do you think that is a 180?

I run a 180 also and generally sit between 180-185 ...

I run the Cummins tstat 192 and really pulling heavy loads it'll get to 204°

My temp is still all over the place, usually get up to 184-186 and drop to 16-170 and never get bback up to 180+. I just replaced my new thermostat with another new 190 from napa and its doing the same thing. Is it possible that my fan never stops spinning? I felt it earlier and it will move while the truck is off but it doesnt spine freely.

My temp is still all over the place, usually get up to 184-186 and drop to 16-170 and never get bback up to 180+. I just replaced my new thermostat with another new 190 from napa and its doing the same thing. Is it possible that my fan never stops spinning? I felt it earlier and it will move while the truck is off but it doesnt spine freely.

mine has been doing the same thing for a long time. but itseems to be working better now after pulling my rv for 800 miles. the actually went up to 215* briefly coming over the blue ridge mtns. i have used everybodies tstat with the same results you have. it does seem to hold better after the trip at 185* with less frequent dips in a smaller range. we sill just have to wait and see if it stays that way. as far as your fan it should move when you push but it want spin due to the clutch thats in it. if it were staying on all the time you should be able to hear it. it sounds like a turbo prop when its on. when i was pulling the mtn in 5th gear around 2300 rpm you could still hear it come on an off. it is quite noticable. when daily driving mine hardly ever comes on, especially when your temp is running low as you are experiencing.

I dont know if I have ever heard it come on or turn off either way because it always sounds the same. Im going from Tx to Tampa Fl with a small trailer this weekend and maybe it will get up to temp and stay there.

you should hear it if it comes on. but it only comes on when the temp get high enough under the hood. the coolant temp by it self does not make it come on. of course if your temp gauge is reading high it should get hot enough to make it come on. but if is cool out side it might not come on. like i say when i am daily driving it hardly ever comes on, i guess thats why it has lasted so long, it never gets used.

Have you tried replacing the radiator cap? I know it sounds stupid but when I put a new one on my temp stayed way more consistant.

Have you tried replacing the radiator cap? I know it sounds stupid but when I put a new one on my temp stayed way more consistant.

did that too a while back. it helped for a little while and then went back to fluctuating all over the place.

I will get a new cap tomorrow.

I will get a new cap tomorrow.

ill be interested in how that works out for you.

Pulled the cap off of my 03 that never fluctuates, will see how that works tomorrow.

i dont know what has got my tstat but it is still working fairly well. since the trip over the blue ridge towing it seems to be holding around 183 and 188. maybe because it got worked hard on the last trip. might be some truth in working it.

The new cap seemed to help it for a while but it still wont get up to 190, it acts like its got a 180 t stat in it and then its back down to 155ish. The new cap seems to make it more steady, instead of 150 then 170 it just sits at 155 now and then all of a sudden it will jump to 183 and sit there for a while. I drove it an hour home last night and it road at 153 for 45 min of the trip. The old radiator cap had 15pounds on it and the new one says 16 so IDK.

you gottta be driving my truck. mine acts the same way, except for the last week. the cap i put on mine is a 16# and it worked for a while then went right back to being all over the place.

Wild temp swings are typically thermostat issues...