I buy the cheap over the counter thermostat. I install it according to the FSM. It works great until I tow at highway speed. The engine gets to 205 degrees and then the thermostat will not close enough to keep the engine warm at low load speeds.

I buy a genuine Cummins Thermostat from Geno's garage. The same thing happens except the temerature of the coolant goes to 210. Who makes a thermostat that can work for a living and not lay down when it is cold?

In both cases I was running in the mountains on interstate and I was not over loaded. First trip was 9000 pounds and the second trip was 6000 pounds. Weather was cool both trips and yes the clutch fan works.

After I dropped the trailer today I could not get the coolant above 155 degrees at sixty mph.

 

Maybe you need thermostat sweet dreams!!!

 

I now it sounds silly, bu tif you dont position teh jiggle pins correctly, the T-stat wont function wont properly

 

Yup, the jiggle-pin needs to be installed UP in the bore (so as to vent air and keep the temperature sensing device submerged in coolant).

 

The jiggle pin is at twelve o clock. The bore of the Thermostat housing is bigger than the template that was sent with the thermostat. I realize that this engine needs monitoring while under load. After all the cooling system was sized for 165 hp not 265. But my main complaint is that when it reaches an unknown temperature above 200 degrees something stretches or missaligns and then the t stat won't close all the way. When that happens fuel economy suffers and soot builds up. Summer is coming and it won't be as much of a problem. Today it was 66 degrees and if I drove it like I stole it I could keep the engine above 160 degrees. Card board is not the answer down here. The thermometer swings way to far in a day.

 

Boat, it sounds like you got the wrong T-stat or even a defective one..
there is a TSB about the T-stats and the jingle pins.. since the housing is not circular, it MUST set in correctly to work properly, otherwise it can lodge in either the open, or more common, closed positions.
I will have to recheck what "hour" the jingle pins are supposed to be at. ( I think its in the thread in the Stickey)
The other issue could be that you need to "burp" the system. the way the coolant flow runs, it is common to get an air bubble just on the block side of the T-stat housing.
It can keep the coolant from circulating, but normally causes an over heat condition...
You should bleed the thing by loosening the Allen head style bolt in the block above the T-stat housing..


edit: They should be at 12 and 6 o'clock...
https://www.dieseltruckresource.com/...d.php?t=146039

 

Due to the manner in which these thermostats are installed, it is an easy mistake to install one backwards.


I have had good results with my plain old Stant thermostat that was installed with no thought to jiggle-pin orientation; it warms to 180 and stays there, rock solid, up hill and down, loaded or empty, winter or summer, without the erratic gauge fluctuations that I see in most of the other 1st Gen./Cummins trucks that I have occassion to drive.


I, also, agree that Dodge was pretty stingy with the cooling system on these trucks; I could remove a section of my Ford radiator, equivalent to the size of the Dodge radiator, and still have more cooling than the Dodge.

 

I installed the thermostat just like the picture. The sensing bulb is towards the head. I admit it is a juggling act to get the seal, lifting eye and thermostat all in the right place at the same time but this is the third thermostat I have cooked. After seeing over 200 degrees the thermostat seems like it will not close all the way and in the cool mornings the engine struggles to get above 140 degrees as long as I am running unloaded and doing the speedlimit. I guess I will take it back out this week and redo it again. I feel confident that the thermostat is aligned correctly because I don't have a leak. Does anyone know of a bigger radiator I can get that will keep the temps a little lower in the future? I plan on making some more trips but I don't like changing thermostats after each venture.

 

I had this same problem on a 2 gen truck the dealership swore up and down there was nothing wrong . Took it to cumimns. The tech took it for drive come back said you have bad thermostat pulled it and changed it. afterward it was rock solid (I Had huge swings in the temp to just like you are describing He told me the thermostat was installed incorrectly (Edit note) he said my choice replace it or reinstall the old one correctly I made the choice to buy new one and have him install it)

 

It really sounds like the T-stat just isnt in there correctly or is bad.. also, you need to "burp" the air bubble that tends to stick right behind the t-stat by cracking the hex-bolt in the block right above the t-stat housing.. air there will cause erratic operation..

 

OY VAY!!!! you do mean the spring is aimed into the block correct???? otherwise its backwards.........
Check against the pictures here:http://dodgeram.info/tsb/1994/07-07-94.htm

 

Fellows, I took the t-stat out for a look see last week. It appeared to be installed correctly. The spring washer is toward the head. One of the three jiggle pins is at twelve o'clock and no leaks. After I reassembled the housing this time I took it for a test drive using the same almost new t-stat. It works. I towed a 6000# tractor on a 4000# gooseneck for 150 miles. It still works. I can only summize that I didn't get the t-stat square in the hole of the housing the last two times and when it got hot it bound up or the spring loaded washer didn't seat properly. What I did differently was I realigned the ground wire off the altenator so it wouldn't be in my way. I left the lifting plate and the seal undisturbed. This way I could actually hold the thermostat housing with one hand while evenly snugging up the three hold down bolts. In short the finger points directly at the mechanic, not the thermostat. I hate eating crow. The feathers tickle going down.

 

Happens to the best of us, glad you got it back to snuff.

 

Hey I "installed" mine 5 times before I got it right when I was doing the write-up on changing it!
Its one of those things that has to be "just so" to work correctly and can be very frustrating...

 

Well, where is the "hat is off to you" smilie when you need it?

Good job!

If that's the most embarrassing thing you have done lately, you have a lot of catching up to do . . .