When the truck has been running for a half hour or so the temp guage keeps sweeping from 190 down to about 150 and back. I assume its a 190 degree therastat and the temp drop is from the cooled fluid coming from the radiator. The part that concerned me is that it kept doing it for a 3 hour trip pulling a small 5x8 utility trailer with a motorcycle in it (probibly no more then 1000 lbs). I assumed the fluid in the radiator would equalize in temp over time, but it didnt seem too. Is this normal for running with just about no load in 85 degree temps?

-Matt

 

Nothin to worry about. A 5x8 trailer isn't working it hard enough to get the coolant in the radiator that hot.

 

Unfortunately, this si what 12v's do. I notice my 96 fluctuating much more than my 99, but everyone I have talked to says it is normal, due to the big cooling system on these trucks.

Chris

 

It is the T-stat doing that. It is very common on these. If it annoys you, you can replace the T-stat, but the new one may do the same thing. If you find one that settels it down, get the brand name and part no and post it here. The only time mine does not fluctuate is when I am pulling my trailer(20 ft flatbed 14,000 lb cap or my 34 ft RV) in 100+ deg with the A/C on running 70 or so.

 

THe problem w/ the updated stat that eliminates the fluctuations is that it takes forever to come up to temp, and never even gets near 190*. Not cool when it's 15* outside and you freeze your butt off for the first 10 miles of you commute to work.

 

Yup, you either have the up and down with fair heat in the winter, or you have the steady temp gauge and no/long warmup in the cold. The underlying theme here is these engines were meant to work, not commute little 8000 pound trucks around. Be happy for that 900 pounds of cast iron under your hood---and flog the dog snot out of it every now and again to build some heat in those cylinders

g

 

Mine does it all the time ... and I like to see it go from 190 to 150 and back up. I know that the thermostat is opening and flowing coolant through the engine. On very hot days especially when towing the fluctuations become smaller and smaller until the temp gauge holds at 185 to 190 (like my VW does all the time at 190), on the highway. When I pull off and drive on the back streets and hit stop signs, the engine cools off again and back to fluctuating.

Nothing to worry about ... actually its a good thing ... you know its working properly.

 

Kinda scary that we don't work these things hard enough (most of the time) to even keep the thermostat open Pulling my 5er the gauge levels right out

g

 

It's always been my feeling that if you drive a CTD around empty that you don't even need a radiator, the heater core will be enough to keep it cool.

 

Infidel's probably right. Diesels, by nature, run cooler. Not only the coolant, but also the oil temps as well.

 

pitr,

This is a quote taken from the TBS site.

Wide fluctuations of the temperature gauge
the result of a large radiator, it is relatively harmless and normal
Cummins has released a revised T-stat that does fix the "problem". The revised T-stat Cummins part number is 3934373. (The old T-stat Cummins Part number is 3928499. )
This solution has been reported by Gerry Willomtzer <dax@yukon.net>: 195 degree t-stat Dodge PN 3418459 intended for a 1973 or so gas engine (From Gene's Chrysler in Fairbanks www.geneschrysler.com).

 

My '95 4wd auto fluctuated as has been described here, it was most noticable on long interstate trips.

96_12V holds steady. Actually in the winter, I have heat once the temp gauge moves off the 140F mark. It will run just inside the "normal" range around town, raise just a bit with the A/C on at 70 on the interstate, and on a 100F degree day on the interstate with some load, it may reach 1/3 of the scale. I recently replaced the radiator, as the old unit had a fairly good leak in it - and even then in 95+ degree weather, the temp gauge acted perfectly normal - had to see coolant flowing out to realize the problem.