I just got back from a long roadtrip and noticed my temps climb to 190 on the gauge and drop down about 1/8th inch, then climb back up and drop down again.....

I figure this is normal thermostat operation, but never really noticed this much gauge action before.

Any opinions?

It has a Cummins 190F as rec'd by Cummins....

 

Do you have a winter front on the truck ? If you do you will notice more m ovement with the ECT gauge as the thermostat is working more.
When I installed the winter front on more truck the gauge reading fluctuated as it should - the temperature sensor is right next to the thermostat housing. Thus movement by the thermostat is immediately indicated by the small temperature change shown.

 

No winter front, but it was cold outside.

I replaced the thermostat about three summers ago. I just hadn't noticed it working before. Everything else checks out fine.

 

my HO this last week only has moved approx 1/8th-2/8th from normal range. it has been abnormally cold in this part of Oregon. around 11-19 degrees. I noticed my truck would dip on the needle as soon as I started to go slower speeds or sit and idle. but would go back up as soon as i was driving higher rpm's.
my neighbors didn't like my truck in the wee hours of morning when the high idle kicks on about 90 seconds after i started the truck.

I'm assuming I too have the 190 degree stat
I see the part stores show a 180, or 190 for our trucks does anyone know what truck models takes 180.

 

Pre 24V take a 180. There's plenty of opinion on here that a 180 is better because its cooler but I run the 190.

My gauges, oil and temp, have also seemed to be more sensitive since I replaced the senders.

If my temp is reaching 190 the stat should open to circulate cooler water thru so I guess it is doing its job.

 

Ive never tried this but if there is a quesion on the thermostat and if and when its actually opening. take it out put it in water on the stove and use something that is accurate with reading temp in the water and watch when the thermostat actually opens. I read a article in past where a guy did this to a variety of part store 190 stats and the results showed they all opened at diff temps?

 

^^^I have done this in the past when my Pontiac 400 was overheating.

The stat was fine, cracked head was the cause!!

 

When it gets cold like it has been mine does the same thing. Especially with a clean radiator. Once I put my plow on, its blocks much of the air flow to the radiator do I see a steady water temp guage..
If I don't have the plow on or a winter front mine will run about 180 on the freeway and drop down to 160 by time I coast to a stop on an off ramp.. I just installed a 190 degree T stat from Cummins about a month ago... I don't think it takes much water to flow thru the T stat when its -20F to cool the engine down with no load on it...

 

That's what I was thinkin, I have good heat flow in the cab so I am gonna leave it alone.

My "winter front" consisted of a piece of heavy cardboard that I cut to fit between the rad and intercooler. I don't suggest doing this anymore as it doesn't allow enough air past the A/C cooler and caused it to get too warm and blow off my coolant for the A/C.

Lesson learned....

 

Mine does that as well, has done it since I bought it with 4,000 miles on it and still does it with 47,000 on it. Normal to me!!

 

SIXSLUG I'm kinda surprised you've never heard much about this topic because it has been heatedly discussed for years. Some think the fluctuation is normal but yet there's plenty of people who've never experienced any engine temp fluctuation. So why the difference? Well its been thought that it was the quality of the thermostat or that the thermostat itself isnt strong enough to support the actual 190* which its rated for. Meaning that this is a problem only for the 24v engines, as you so correctly pointed out that the 180* thermostats are only for the 12v trucks. This is because the 24v heads flow so much more air than the 12v heads that Cummins ran a hotter thermostat to offset the drop in cylinder head temps because of the added air flow. So a theory is that because of the fact that 24v heads tend to run cooler, the chances of temperature fluctuations become greater by means that if the thermostat isnt capable of maintaining a solid 190* then it could allow the engine temps to drop a little as the cool head isnt making the heat and then as the heat comes up from load the overall engine temp moves around. Weird to me as this is something I've only heard about on these trucks. I have a NAPA 190* thermostat and the engine temp will always run with a gauge needle fluctuation, unless I'm towing in the summer whereas the temps will run a little hotter.

And yeah you're right about your winter front. Placing cardboard between the radiator and the intercooler is NOT what you want to do.

 

I didn't notice it til I changed out the Therm with a gennie Cummins part a few winters ago.

It seems most noticeable in the winter when the airflow is a much lower temperature. And as mentioned it gets over in the summer when towing or climbing, but not overheating..

Yeah, >$200 to recharge and fix A/C..

 

After reading my post again, I dont think I was clear in pointing out that my engine temp does not fluctuate. What I meant about the gauge needle fluctuation I previously said was that if the gauge does move then the movement is only about the thickness of the gauge needle.

I did read in the FSM that the regulating part of the thermostat is wax. Not sure if this is the common way for thermostats to be built but I've heard good things about the NAPA versions. And in noting that.....I'm sure that NAPA uses different manufactures too which is why other people have different results. If there's anything that someone should make sure of is that the correct thermostat has to have the two jiggle pins on the sides.

 

Yeah, the Cummins stat has the little pins.

I have also read that the stats are controlled by wax...

 

I have the 180 in mine, not much heat on cold days.