On a recent 1000 mile trip, I noticed my engine temp guage would rise up to 190, then quickly drop down to the low end of the normal bracket, rise up, drop back, etc. It never got hot (190 max.). It seems to me the t-stat was openning full, then closing full, etc. the entire cycle took only about 20 seconds from 190 to about 160 back to 190.

Is this normal? I never noticed it before. The outside air temp was about 35-40 and I was towing about 6000 lbs. Thanks

 

Totally normal

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

 

Thanks. I expected the lower temps, and the diferent temps in different conditions, but watching the needle go up and down wat sorta mezmorizing. Kinda lulled me to sleep 'round about the middle of nowhere at 4 am.

 

I don't know about the above story, but mine did it empty or loaded. I changed the sensor and it's fixed. I can't see water cooling that radical in 20 to 30 secoonds coming out of the block
Oops, my 2 cents.
Happy truck'n

 

That's because it doesn't measure the temp of the water coming into the block, the temp sensor is at the rear on the drivers side , the thermostat is at the opposite corner.

It IS possible for the water temp to vary that much , when the 'stat opens , cold liquid from the radiator is introduced into the head , which closes the thermostat , and stops the flow (gauge drops) the engine heat slowly warms the water temp up to the point that the 'stat opens (gauge rises) , and this keeps repeating.

 

When the t-stat opens, cold water from the rad floods the sensor, cooling it quickly. The overall engine temperature isn't changing that much - just the coolant in the location its temperature is monitored.

I believe the location of the sensor was changed in later years to stop the "guage sweep/wiper action."

 

Thanks folks. I forgot the t-stat is a "dumb" spring. And, I did not know about the location of the sending unit.

 

Where did they moved it ? I have same problem, I replaced tstat with updated Cummins but it still bothers me to see needle up and down, I wonder if I ever will notice and overheat in an emergency, I am used to see it going up an down with the corner of my eye.

 

*shrug* I thought they just moved it, I could be wrong.

I was about to say "anywhere that is high on the engine should be good." Then I checked my manual, and it is high on the engine. Guess it just cycles water through the engine quickly, *shrug*.

I've had my truck for a couple years, and towed some big stuff with it. It stops cycling under heavy thermal loads (e.g. hot day, climbing hill). Under those conditions, the thermostat starts staying open constantly, and the coolant doesn't get cold enough to allow the cycling. Alternatively, if your thermostat is broken and jammed shut, it won't allow the cycling. In any event, by my experience, by the time your engine temperature is passing the upper comfort range (but still 20C/40F or so from danger), it will stop wiping.

 

I bought my '95 new and the temp guage fluctuation was my first call to the service department. Now I'd think something was wrong if it wasn't moving....

 

I know this is a old post, but mine just started doing this today. It goes all the way to the high line, not into the red, but the drops fast to the colder line, then creeps back up. when I got home, idling in the drive, it stayed about in the middle, which is around 190, for my guage. The egt's never rised during this, not sure if it would affect it or not. I've only had this truck a week, but first time this has happened to me. Oh ya, it was about 1/2 gal low on antifreeze, but even after adding the fluid, it still did it, just did'nt rise as fast. What do you think, sending unit. Is that the unit, located above, like where the fuel filter is, 2 prong hookup? thanks

 

The cycling that occurs will be as others described, between 190 and the low end of the gauge, 140 on my 96. If yours goes above that before it drops, you likely have a sticking thermostat.

Chris

 

is the thermostat located on the block, where the top hose is at? thanks

 

Well, apparently there has been no firm and final resolution to this issue as this is the newest post I can locate on it.

My temp gauge never bounced until recently and then kept doing it even with a new thermostat.

The coolant coming out of the thermostat is consistently within degrees of 180 according to my remote thermometer so it is likely not the cause.

Thermostats do not generally snap open and closed allowing fluid to rush in and then stopping it completely. If they do then they are defective.

If you are in fact continually shocking your engine with 50+ degree differences in coolant temp, I imagine that there must be some negative consequence of this.

What I have not investigated yet is that I apparently have a coolant temp sensor and a coolant temp sender, at least according to rockauto... Any ideas on these parts?

 

Infidel, do you ever wish you could get a piece rate for answering this question? I hasten to add you answered it for me and I bet I have seen you answer it 15 times in the 3 years or so I have been reviewing this board. I mean no criticism of Trooperthorn--after all I asked the same question 3 years ago. After running vehicles all my life where the temp gauge assumed a position and held it, it is scary to see that sucker bouncing around like that. thanks for all you do..........