I believe my thermostat is stuck open, I've noticed the last few weeks that it takes longer to warm up in the mornings and when I watch the temp gauge, it really justs creeps up, very slowly, to operating temp. And it never gets above 170-180 degrees. I have two questions....

1. What should the optimum operating temperature for this engine be ?

2. Could the stuck themostat have an affect on my fuel mileage, because I haven't been getting my usual mileage the last few weeks. Could it be the engine is not running hot enough ?

'99 3/4 ton quad cab, 24v, 165k miles - running edge comp

 

What kind of temps. are you seeing ?
Do you plug in at night ? about 35* f & below I do .
What about winter fronts ?
170-189 is about norm. unloaded / not running hard
If they have a winter mix then you could see less mpg , also diesels run more efficant with a warm-hot eng. , so with cooler weather its going to take some warmup driving to get it to run best .

 

Kind of hard to tell because of the huge cooling system these trucks have. Mine was stuck open for probably five months. I only noticed slightly more temp pulling the 15 mile 6% during the heat, but finally in the fall when coming down the pass (cooler temp) instead of the temp gauge only coming down a little, it went to right down to full cold. T stat took care of it. If you haven't changed it since 99 it might not be a bad idea.

 

I am going thru the same thing now My needle will only go up half way

to 190 on the gauge. Changing it out tomorrow.

 

If you sit and idle for a bit, what does your temperature do?

My last thermostat failed by just getting weak so that it would open up anytime water was flowing. When I pulled it out, it was closed, but you could operate it by hand (which you couldn't do with a new one). At normal operating conditions, I was just slightly lower than normal. But if the truck idled for a while, the temp would drop to 150-160*.

The other option is that the frame around the t-stat broke so it's partially open all the time. With this, you might be a little low on temps at times, but if you load it up it will overheat due to insufficient flow. I've seen this happen quite a few times on friend's trucks that tow quite a bit.

As for the correct temp, the stock Dodge t-stat is a 190*. You can also get a 180* t-stat from Cummins. Personally, I like the 180* since it flows more and helps keep my temps down easier when I work it hard.

 

180-190 should be operating temp, i put a 190 in mine and like it better then the 180, ive read the warmer the better. and yes it will effect fuel econ.