[gr8shot]

Have had this problem for quite a while on my chummins. Well basically always since I did the swap 6 years ago. The engine won't get up to normal temp. I have a 40 minute drive to work, mostly highway 60mph, temp gauge barely moves by time i get to work. Driving around town it really won't heat up unless it's 90*+ out and I'm in stop and go traffic. Then it'll maybe get to normal temp once in a while.

I've put 3 genuine cummins tstats in it over the years thinking one may be bad. Last time was last spring after a head gasket & studs install. Same results. I can temp the engine all over with an IR gun and maybe get 140-150 after pretty much any amount of normal driving. Temp gauge in the cab reads similar. I run it the majority of the time with the engine fan off the engine because even without it, it still won't get hot. The engine has the normal tiny little bypass hose that goes around the tstat. And other than that just the normal hoses going to and from the heater core. Water pump is a couple years old. Problem was there before and after changing the water pump.

The only time I've gotten it to heat up and run at normal temp was earlier this summer in 95* heat pulling way way too much of a load. A 14k skid steer on a 7k trailer, running 4th gear at about 50mph. Even then it only got up to normal temp about 190*.

Truck has a first gen radiator and first gen intercooler in it. Not sure what else is relevant. Thanks for any ideas. Would be nice to get it to warm up a bit since winter will be here before we know it. And it barely gets warm enough to heat the cab even with no fan and the radiator blocked in the winter.

 

[ofelas.5z]

Are you using a first gen or second gen stat housing and stat ?
Edit - just saw where you mentioned the bypass hose, so you’re using a second gen housing and stat.
What are the part numbers of the stat and the 2 rubber seals you’re using?
The wax pellet end of the stat goes towards the cylinder head, and the domed top goes towards the rad.
Pull your current stat and drop it into a pot of hot water on simmer. It should crack open at just below 180f, and be fully open by 190f or so.
Heat and cool the water a couple times, ensuring the stat closes fully below 178f or so.
The skinny seal sits in the stat housing, the stat goes into the housing next, and the thicker seal goes into the stat housing last, with the smaller stepped end against the stat. The other end should barely protrude out of the housing so it seals against the head.
All this assumes you’re using the lift plate between the housing and the head.

 

[gr8shot]

Are you using a first gen or second gen stat housing and stat ?
Edit - just saw where you mentioned the bypass hose, so you’re using a second gen housing and stat.
What are the part numbers of the stat and the 2 rubber seals you’re using?
The wax pellet end of the stat goes towards the cylinder head, and the domed top goes towards the rad.
Pull your current stat and drop it into a pot of hot water on simmer. It should crack open at just below 180f, and be fully open by 190f or so.
Heat and cool the water a couple times, ensuring the stat closes fully below 178f or so.
The skinny seal sits in the stat housing, the stat goes into the housing next, and the thicker seal goes into the stat housing last, with the smaller stepped end against the stat. The other end should barely protrude out of the housing so it seals against the head.
All this assumes you’re using the lift plate between the housing and the head.

Not sure on the part number. Got them from our local Cummins store. Told them it was for a 97 Dodge/Cummins and they got it for me. The most recent time it was installed by a pretty reputable shop when they had the head off. So I feel pretty confident it's installed right.

 

[GAmes]

Rather than shooting all over the engine with the temp gun I suggest you just shoot the radiator hose where it attaches to the t-stat housing. That is where you will determine easily at what temp the t-stat opens. Keep in mind that when the engine isn't working it is creating very little heat and that the temp probe is the length of the engine away from the radiator. As a result, the temp at the t-stat housing will vary somewhat from what is shown on the gauge. In addition, if you are using the Dodge probe it could be different than a Chevy probe ECM signal. Not long ago I installed a custom 4 row all metal radiator because neither the OEM or Spectra replacement 2 rows (the original OEM was a 3 row) were up to the task of towing in the mountains. I'm confident that when winter comes my heater will work just fine.

 

[alanack6795]

Heat works better when the engine is up 190 or so. Heat output really takes a beating when the temperatures drop and the engine is running at 140-160. Usually my heat discharge is only around 120-130, where most other vehicles I've driven put it closer 150+

 

[dd453ford]

Just a really silly question, but........ are you using the stock Chevy temperature gauge with the stock Cummins sending unit or are you using an aftermarket gauge assembly???

 

[gr8shot]

Rather than shooting all over the engine with the temp gun I suggest you just shoot the radiator hose where it attaches to the t-stat housing. That is where you will determine easily at what temp the t-stat opens. Keep in mind that when the engine isn't working it is creating very little heat and that the temp probe is the length of the engine away from the radiator. As a result, the temp at the t-stat housing will vary somewhat from what is shown on the gauge. In addition, if you are using the Dodge probe it could be different than a Chevy probe ECM signal. Not long ago I installed a custom 4 row all metal radiator because neither the OEM or Spectra replacement 2 rows (the original OEM was a 3 row) were up to the task of towing in the mountains. I'm confident that when winter comes my heater will work just fine.

I have the GM temp probe installed right below the thermostat housing. So it should be reading about the same as what the tstat housing is. I adapted the threads to use the GM sender so the gauge would read accurate. I know it's usually in the back of the head, but I was lazy and didn't want to run new wires and extend the harness when i did the swap, and I'd read about guys putting it by the tstat housing with good luck, just noting that it isn't the "hottest" place to put it. That would be the back of the head.

The reason it baffles me is because I have a 2000 24 valve project truck and it gets up to normal operating temp within a few miles and stays there on the same drive. The 12 valve barely will move the needle despite numerous new tstats and what not. I know how the system should work, and logically I'd think it's another bad tstat. But I'm sick of buying them and putting them in to no avail. I'm just curious if there's any out of the box ideas that might point to something else I'm doing wrong.

I've driven it this way for years and it runs fine. But this will be the first winter it'll get more use than usual. Sold my duramax last January so this will be the first winter I really rely on it. So trying to iron out some of these things.

 

[GAmes]

I've read that the 24 valves temp gauge is pretty much an idiot gauge because of all the complaints about swing gauges in the 12 valve. I've never encountered a bad Cummins t-stat.

 

[ofelas.5z]

Cummins parts are usually reliable, but I've encountered 2 bad 63mm Fleetjunk stats for the second gen 12v, out of the 5 I've installed this year.

I've had better luck with the equivalent NAPA HD stats, and use a Genuine Mopar HD 440 stat in my daily driver.

Here is a post I made for reference a few months back - https://www.cumminsforum.com/forum/...stem-balance-genuine-mopar-vs-fleetguard.html

I've read that the 24 valves temp gauge is pretty much an idiot gauge because of all the complaints about swing gauges in the 12 valve. I've never encountered a bad Cummins t-stat.

 

[gr8shot]

I've never encountered a bad Cummins t-stat.

Thats why I'm not sure what to try other than to just keep throwing t-stats in it and hoping for the best. Nothing seems out of whack to my unprofessional eye.

 

[GAmes]

Thats why I'm not sure what to try other than to just keep throwing t-stats in it and hoping for the best. Nothing seems out of whack to my unprofessional eye.

Have you confirmed your t-stat isn't opening at about 180?

 

[sdnative13]

I am not expert but I bought a 97 2500 from my uncle in January. I drove it home from utah and froze my but off till I got to vegas. I immediately got online and read post after post of the temp gauge moving constantly. I know in mine it seems that it will get up to temp but once the tstat opens it drops to the 1/4 or below position. Slowly it will move back to the middle. Not a big deal in the summer but in the winter the heater is basically useless. It seems from my research (nothing proven) that this is a very common occurrence.

 

[alanack6795]

A winter front will help quite a bit. Also check your heater core floor good Flow.

Using the recirculation position will help tremendously.

The automatic transmission will sap all the heat out of the engine in cold weather operation.

Some guys run the 440 thermostat to run hotter but I'm not sure if that hurt the truck during commercial operation.

 

[GAmes]

I am not expert but I bought a 97 2500 from my uncle in January. I drove it home from utah and froze my but off till I got to vegas. I immediately got online and read post after post of the temp gauge moving constantly. I know in mine it seems that it will get up to temp but once the tstat opens it drops to the 1/4 or below position. Slowly it will move back to the middle. Not a big deal in the summer but in the winter the heater is basically useless. It seems from my research (nothing proven) that this is a very common occurrence.

It is a common occurrence for the gauge to go back and forth. It has nothing to do with the heater giving good heat. My gauge fluctuates, my heater works perfectly, even when driving in the great white north in the winter. If your heater doesn't work it is more than likely a clogged heater core or an excessive amount of dirt/leaves/dog hair inside the HVAC. Now would be the time to find the problem and fix it.

 

[alanack6795]

Gary what is your discharge temp at the center vent when it's very cold out ?

 

[GAmes]

Gary what is your discharge temp at the center vent when it's very cold out ?

About 150f if I turn the knob to the stop. The normal position of the knob once the engine is up to temp is pretty much straight up and down. I seldom use the recirculate position on the vent knob because I want more air flow to my feet.

 

[sdnative13]

question

It is a common occurrence for the gauge to go back and forth. It has nothing to do with the heater giving good heat. My gauge fluctuates, my heater works perfectly, even when driving in the great white north in the winter. If your heater doesn't work it is more than likely a clogged heater core or an excessive amount of dirt/leaves/dog hair inside the HVAC. Now would be the time to find the problem and fix it.

Do not mean to hijack this tread but if that is the case how do you explain the fact that every time my needle drops my heat output does too. When the needle returns to the 12 O'clock position so does my heat.

 

[GAmes]

...... every time my needle drops my heat output does too. When the needle returns to the 12 O'clock position so does my heat.

You measured a heat drop with a thermometer, or it just felt like it got cooler? I have a gauge in the center vent and have never seen any fluctuation in it, but then again it isn't an instant needle. The fluctuation of the gauge is only 10 to 15 degrees and it is measured a long way from the heater hose port. The heater hose port is next to the t-stat, so it would be 180 +/- or hotter whether the t-stat is open or closed.