I'm stoked I found this site!
My '96 Cummins runs fine, but it may be running too cool most of the time. Regardless of the weather it will not warm up at all if left idling, and most of the time will only warm up to between 140 and the first line of the "normal" range. Sometimes it will warm up to near the middle of the guage, but if I shut it down for even a few minutes it usually drops down as soon as I start driving again. I have replaced the thermostate, but that didn't seem to have an effect. I thought the guage may be reading wrong, but I have noticed that the heater doesn't work as well when the guage is showing low. Is this normal? If not, any ideas?

 

They take forever to warm up ideling. Driving, it should warm up to 180 or so then "dive" down to 140 only to cycle back to the 180 and so on.

 

What brand thermostat did you use when you replaced it? A lot of people have had problems with non cummins thermostats. Also, it is possible to install it so that the seal interferes. Seeing that you are in Galveston, the weather should not be cold enough that you would need a winter front.

These engines are fairly thermally efficient and they will take forever to warm up by idling. It isn't good for them and you burn a lot of fuel so try to avoid it if you can.

 

I'm not sure of the brand since it was replaced while the truck was having some AC work done. The shop has always done good work for me, but I guess it's possible they didn't swap it. I should probably get a Cummins part and do the work myself to be sure.

I'm not sure about a winter front, but I have the same problem in the summer when it's 95 out. The outside temp doesn't seem to have any effect on the engine at all (at least the temps here in Texas). The only time I have ever seen the guage above about 180 is while driving in the east Californian desert during a very hot summer (about 120). Even then it was still well within the normal range.

I don't usually let it sit and idle for more than a couple of minutes because I discovered it doesn't do any good. The truck will never warm up by just idling.

 

Sounds perfectly normal... They fit a medium duty cooling system in our trucks. The thermostat will fluctuate and you'll see the needle dropping but on hot days or if you're towing you'll notice the temp. will stabilize in the middle range.

 

if you ever find a fix i would like to know my 97 will not warm up on a 90 degree day driving it hard. 3 cummins thermos. 134 degrees the highest ever recorded with infra red. cummins tech says it cant do that. have never owned a vehicle that i couldnt warm up. this one has me stumped.

 

The cause of the "problem" you folks are having is that you're not using your trucks to the extent they were designed for. The large cooling system is designed for hard work towing or hauling, not cruising around empty.
Since the engine isn't working hard the stat closes, coolant in the radiator has lots of time to cool way down, stat finally opens allowing a gush of cold water that makes the gauge dive.
As long as you aren't running below 140° this is totally normal and won't hurt a thing.
Replacing the stat to eliminate fluctuation usually just results in the engine running even cooler.

 

you sir are dead wrong. i have a 90 model that very seldom tows. in the coldest day in tn. it will be up to temp within 6 miles. i am not the sharpest knife in the drawer but i have over 40 years of experience on most kinds of automobiles and this truck has me baffled including cummins tech support.

 

Guess Cummins is dead wrong too.

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.

 

I wish they had equiped them with overkill transmissions as well.

 

i dont care if the radiator is as big as a garage door if the thermostat does its job the engine will operate at a steady temp. there are temps cold enough to cause a problem but there aint none in tn.

 

I think that there are a few different interpretations of the original post going on. I took it to say that the coolant temp barely ever gets over 140F. This is why I suspected the thermostat. I think a lot of you are talking about the temp gauge fluctuation but I don't think that is what the original poster is talking about.

The temp gauge does fluctuate without a load but it will get to 190F or so before it swings down to 140F. If it never gets over 140F, then something is wrong. The truck is in Galveston, Texas so the ambient temps there never get cold enough to explain the low engine temp and lack of heat.

 

Maybe it's time to change the coolant.

j/k...

 

Who said the gauge was correct? If you are bound and determined that your peers and Cummins don't know operation of engine, then get a mechancial gauge and plumb into system. The infared test is not conclusive. You could have a sending unit, a minor wiring issue or gauge issue (doubt). Then you can put dog to rest. We the unknowing would like to be taught how to keep system from diving up to 40 degrees without major changes.

 

i submit that i have done something wrong on my 97. i have had 2 techs that i consider much smarter than i am to try to figure it out. one thing we are sure of is the water temp. if i live long enough i will find the problem. i talked to 2 cummins techs and both guys agreed with me that this truck cannot do this. now this discussion is going nowhere so i am going to bow out.