Saw something unusual today. My temps are varying much more than normal. Usually temps stay between 190 and 195. Today, 181 - 200. The thermostat is only 3 months old. All temps are per my scan gauge. What's up with this?Thx,Eric

99.9% of the time is a bad thermostat.

99.9% of the time is a bad thermostat.

Are you referring to me as the .01%? Yours truly, thick skull

I'm in that percentage too. My temp fluctuation ended up being my ECM.

I'm in that percentage too. My temp fluctuation ended up being my ECM.

I believe mine is in the wiring. hope so anyway. To the OP, mine is swinging wildly low and not high. I also am reading with a Scan Gauge. My signal appears right at the sender, but both the dash and SC are getting the same fouled up signal thru the wiring harness.

Like these guys are showing there is a rare chance that something else will effect weird coolant temperatures. But would rather replace thermostats that guessing at ECM and wiring unless the thermostats don't fix the problems. So make sure to do you testing before buying ECM or wiring harness.

Another possibility is a plugged radiator core. Cooling liquid spending too much time in the radiator. Another one I'm considering is a mal functioning fan clutch. Not kicking out enough could cause over cooling. Another one to consider is a faulty temp sensor.

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For those using non cummins thermostats and achieving consistent temps, what brand are you using. $60 for a cummins unit and I still have a 15 degree swing. I was more consistent with a napa one, but it was on its last leg and had physical damage.

I have a napa 190 in right now. It runs between 186-190 depending on speed and temp.

If you still have your old tstat you might try re installing it and see if it acts the same as the new one or as it did before you replaced it.

For those using non cummins thermostats and achieving consistent temps, what brand are you using.

NAPA... I've got Dripley's $10 thermostat and it works fine! http://forum.mopar1973man.com/threads/5637-Would-you-come-forward-please!?highlight=thermostat

Another possibility is a plugged radiator core.

Typically that would show up as a constant hot engine because of diminished cooling capacity of the radiator.

Cooling liquid spending too much time in the radiator.

Happens every winter with outside temps in the minus.

Another one I'm considering is a mal functioning fan clutch. Not kicking out enough could cause over cooling

Still an all the thermostat should closed and keep its heat. Once again consider my minus temperature up north here I still have ZERO problem keeping 190-195 solid all winter long.

You guys sure do jump on the bad parts band wagon very quickly.The OP did not mention the driving conditions he was in, was it steady cruise, towing, city driving, etc???The temps he posted are not abnormal for many conditions. Remember these thermostats (sounds like he has a 190°) crack at 190° ±3° and are full open at 207°. When idling the temp drops to 180°-183° very quickly.These trucks have a swing, its 100% normal. It always amazes me how much money people throw at their cooling system trying to fix what isn't broke. When you have a severely oversized cooling system and a thermostat with broad operational range it's going to swing, nothing is wrong.

Q: Why is is people like myself can have very stable cooling systems over the life of the truck? 193-195*F of coolant temp regardless of weather or outside temperature or 195-197*F while towing. At a idle it will drop to 188*F and hang (full close). No cycling up and down just a steady temp.

as long as mine doesnt overcool or overheat im fine

Mine too stayed very steady for 6 or 7 years, just a tick below 190 on the gauge. The only time I saw any changes was pulling the rv up a long grade, about a 10* increase, and in the winter in temps below 20* it was hard to keep the temp up with stop and go driving, on the highway it was stable.

Q: Why is is people like myself can have very stable cooling systems over the life of the truck? 193-195*F of coolant temp regardless of weather or outside temperature or 195-197*F while towing. At a idle it will drop to 188*F and hang (full close). No cycling up and down just a steady temp.

You don't see much of an increase when towing due to the speeds at which you drive. You are not even getting your thermostat full open. It is hard to get them full open and does take good power, but it is doable with 8K behind you depending on the speed at which you climb hills. On my old TT I would only see the thermostat full open a handful of times a year in 3-4K miles of towing. With the new heavier TT it's more common to see it go to full open, and it takes a long grade at 70 on a hot day to get it to go more than 1-2° above full open. Interesting that you only drop to 188° at idle, it must be the location of the temp sender? All the 3rd gen's I have seen drop to 181-183 pretty quick when you stop and idle, same 190° thermostat and I don't think there are any differences in the block and cooling flow. My temps are stable, but thru-out the year there is a range of about 35° of operation and it's always normal. But to many people a 188°-197° is a swing, even thou it's at different times. I have seen people chase that very swing on these trucks across the board. When steady driving produces a big swing then there are issues, and that is normally the thermostat, and should be looked at. But all too often people define normal operation as swing, just like oil pressure on diesels has a lot of movement and thanks to people not knowing what they are looking at we got a fake oil psi gauge.

I posted because this was "unusual" for my truck regardless of the type of driving. However, just to clarify, this varying temp was in town and on the highway at 60mph for over 15 minutes. Solo, not towing. I'm just going to get it all done late next week - heater core, evaporator, thermostat and water pump. I don't think the water pump has ever been replaced, so, while I've got the coolant down, I'll just go ahead and do that too. Towing season is just around the bend!

You don't see much of an increase when towing due to the speeds at which you drive. You are not even getting your thermostat full open. It is hard to get them full open and does take good power, but it is doable with 8K behind you depending on the speed at which you climb hills. On my old TT I would only see the thermostat full open a handful of times a year in 3-4K miles of towing. With the new heavier TT it's more common to see it go to full open, and it takes a long grade at 70 on a hot day to get it to go more than 1-2° above full open. Interesting that you only drop to 188° at idle, it must be the location of the temp sender? All the 3rd gen's I have seen drop to 181-183 pretty quick when you stop and idle, same 190° thermostat and I don't think there are any differences in the block and cooling flow. My temps are stable, but thru-out the year there is a range of about 35° of operation and it's always normal. But to many people a 188°-197° is a swing, even thou it's at different times. I have seen people chase that very swing on these trucks across the board. When steady driving produces a big swing then there are issues, and that is normally the thermostat, and should be looked at. But all too often people define normal operation as swing, just like oil pressure on diesels has a lot of movement and thanks to people not knowing what they are looking at we got a fake oil psi gauge.

So you were driving steady at 60, without towing, and saw 181-200?

Yes. It was on a divided hwy with very gentle (2-3%) rolling hills. I was using the cruise control.

So you were driving steady at 60, without towing, and saw 181-200?