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..........

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..........

Idling.Nothing wrong here.

Your thermostat is stuck open. Put a new thermostat in.

Your thermostat is stuck open. Put a new thermostat in.

have the same problem with a new tstat. just replaced the radiator cap and i think it stopped. truck has been down for a while so i want know until tomorrow. could be either one. radiator cap is much cheaper. mine was 9 years old:duh:.

At idle the engine produces almost no heat, this is because there is no throttling on the intake, it can suck in as much air as it wants all the time. This creates an ultra-lean situation when idling since there is hardly any fuel going in but plenty of air going in. When running like that, the engine cools down. It does create some heat though. If your heater is on, the temp will drop. If you take your fan off and leave your heater off, it will overheat. Now if the engine has not reached 190 when you get to the station, heater is off (engine fan still on), it will cool way off when the thermostat does open. After it cools off, it will hardly get any hotter until you start driving again, it takes a lot of heat to raise the temp, maintaining it is simple. So if your heater is on and the radiator is still really cold (tstat hasn't opened yet) and the tstat opens during idle, it will drop the engine down to 140-160 and the heater core will be taking away any heat that the engine can produce. I don't run a fan on my engine and at idle, I can turn my heater on full blast and watch the temp drop, otherwise it overheats.

Like here in Idaho's cool weather I got to leave me exhaust brake and high idle going to keep the coolant up to 190-195*F with the heater going. Other than that it will drop on me as well... Like ISX pointed out there isn't enough heat generated by the engine to feed the heater and fight off the cold outside temps so the coolant temps fall. Suggestion is to turn the blower down to lowest setting. But leave it idle... 5-10 minute stop won't hurt a thing...

I sleep in my truck all the time in all kinds of weather. I don't drop to 160 unless my thermostat is going bad. That is with the truck idling all night long.

I sleep in my truck all the time in all kinds of weather. I don't drop to 160 unless my thermostat is going bad. That is with the truck idling all night long.

Mister Torture here...

I sleep in my truck all the time in all kinds of weather. I don't drop to 160 unless my thermostat is going bad. That is with the truck idling all night long.

I have noticed once the engine is at operating temp, it has a better chance of staying there. I am wondering if it has to do with the temp of the block. I think it takes a long time for the whole block to heat up, but once it does, it retains the heat. Little jaunts might get the block to 160, so when you stop, the temp drops to 160 since the engine is no longer feeding it any heat from engine load. I noticed if I leave the heater off and let it get to 210, then turn the heater on, it will only drop to 190 then stay there. However, if the engine barely made it to operating temp (as in the block probably didn't make it at all) it will drop in temp when I stop moving.

Hey Caj, are you fussy about what brand of tstat you use? I just put in a new Stant brand and it is junk. Worse than the old one. I priced out from Dodge and Cummins and they were both in the $100 range so I went with the $40 Stant. My old one was not OEM but i forget exactly where I got it from.

I sleep in my truck all the time in all kinds of weather. I don't drop to 160 unless my thermostat is going bad. That is with the truck idling all night long.

That is not so bad, but have you ever noticed the white stinky smoke that you get out of the exhaust after sleeping with your truck idling? That smoke is there because you don't have enough cylinder temp to burn all the fuel. Unburned fuel can and will form a gunk that collects on the exhaust and seeps out of the exhaust manifold that is easily confused with engine oil.

hey guys up here in northern minnesota it is in the single digits alot and below zero my truck will heat up to 190 when i drive it but if i come into town and hit a couple of stoplights or stop signs it will cool off quickly. The other thing I manage a NAPA and they do make a good t-stat like I tell guys that only want OEM parts do you really think cummins or dodge make there own parts NO they buy them from outside manufactures and renumber them and re box them that is why the cost so much more.

Hey Caj, are you fussy about what brand of tstat you use? I just put in a new Stant brand and it is junk. Worse than the old one. I priced out from Dodge and Cummins and they were both in the $100 range so I went with the $40 Stant. My old one was not OEM but i forget exactly where I got it from.

I like the Napa ones. The autozone and advanced are junk.

--- Update to the previous post...

That is not so bad, but have you ever noticed the white stinky smoke that you get out of the exhaust after sleeping with your truck idling? That smoke is there because you don't have enough cylinder temp to burn all the fuel. Unburned fuel can and will form a gunk that collects on the exhaust and seeps out of the exhaust manifold that is easily confused with engine oil.

That really just started happening. I had no idea what it was. I have to rev the motor a few time to get it out or I will white out a few blocks. It does not happen every time though.

If you keep your EGT's up to about 400*F it will help in keeping the fuel burning... You shouldn't see the exhuast slobber... (Kind of like him) Just turning on the exhaust brake is usually enough to keep the pyrometer up...

If you keep your EGT's up to about 400*F it will help in keeping the fuel burning... You shouldn't see the exhuast slobber... (Kind of like him) Just turning on the exhaust brake is usually enough to keep the pyrometer up...

My exhaust brake is too loud. Sort of like you.

Thanks Caj I feel the love... Now I'll go to my corner and cry...

this thread keeps getting better every day. the radiator cap i put on mine made a very big differance. it actually warms up on short trips. only about 30* here though.

It was +7*F this morning. Didn't plug it in last night, because it keeps (since recently) tripping the GFI circuit in the garage..Was a little tuff to start, but she fired up. about 30 secs later, high idle engaged. 10-15sec after that, 3cyl engaged; that's the first time I've heard it trigger on it's own! :ahhh:I left it idle for about 10-15min in the driveway. Came out to leave, and the temp needle was still buried on 140 (cold).Oh yeah, I failed to turn off the heater blower when I started the truck.

Well,I'm glad to hear that it's normal.The truck did heat right back up once I started moving again.I'm pretty sure it's not the thermostat as I bought a NAPA one and I've only got 12000 miles on it and it does go right to 190* and holds when running down the road.I got the winter fronts made and installed, and I'm stopping at the Dodge Dealer today to see when I can get the high idle turned back on.

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.