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Ok, not sure but I think I am having issues with my OEM thermostat. The truck warms up to about 194° in the morning on the way to work then settles down to 188+/-2° while driving 65mph with an ambient temp around 40°. Thermostat is about 4 months old and when it was new it held above 190 consistently. The ambient temp was warmer at that time maybe 50 to 60°. Also, now on downhills of any length of a 1/2 mile or more the temps drop to about 183° which didn't happen before.
Now let me get nerdy for a minute, let's say I removed a large source of BTU's in the cooling system, could it just be the lower ambient temps are allowing the cooling to remove too much heat and the thermostat can't react fast enough. Or should I look into a third thermostat in 18 months.
The Motorcraft t-stats are sad in that I have not been able to get one to last more than one winter.
That is why I now only get the IH ones for my truck.
I have found that my T-stat responds slowly at threshold, so like in your case mine would be similar on a colder day unless I had a decent load on it. It doesn't STAY low, it will come up eventually to that 188+ temp. And like you in warmer weather it isn't an issue. The nature of how thermostats work and our ability to monitor temps in real-time, I believe has led many to replace perfectly good T-stats. It is another story if your temps STAY low or HIGH. I tested 2 new thermostats in boiling water, Stant and Gates, and kept the Gates one (it is my spare), still have the 15 year old original in the truck... Here's the thread:
I just put in a Napa 192°F thermostat in just a little while ago on my 2004.5 6.0. I was running 186 to 188 ECT I had a 2° Delta at idle. About 6 to 10 at 70mph. Now a am 192 to 195 Ect with a 4° Delta at idle and 195 ECT with 200° at 70mhp. Not sure what is better. Will fill in how it does over next week or so.
That sounds pretty normal. Diesels make almost no heat when idling so I could see the thermostat no reacting quick enough to prevent the drop. 183 isn't that bad. During the winter when I head down the mountain from 6k feet to sea level my engine will get down to 160-170 as I'm litererally going downhill for 30miles straight. On the long downhill sections you can hear the injectors actually shutoff and the engine goes quiet, at which point its making no heat. Once on the flats it goes back to 190F.
That is pretty much what it has been doing. It goes up normally until the T-stat opens, then because it has been cool (it doesnt really get cold in SoCal) it drops down in temp on the rolling hills. Once my drive levels out the temp comes up slowly to about 188°. All this is with temps below 40° F in the morning. I was driving around today in the mid 60s and the truck ran at 190+.
I think that I am just not getting enough BTUs into the system and around 40 to 45° the cooling system is just to efficient.
The cooling system is designed to remove heat added by the block and heads, the oil cooler and the EGR cooler. Remove any one and the system is oversized and I think I found my tripping point of too much cooling. At this point it sounds like my system is running normal.
Anything over 186.0°F on an unloaded truck as far as the PCM is concerned, is a warmed up engine. Below that, 185.9°F & lower, the PCM thinks it is cold and will dump extra fuel to try to make heat.
So 188 on an empty truck bebopping around town is fine and expected.
As far as thermostats, obviously they do not make heat, they can only try to retain it or dump it to the radiator. So coasting downhill, foot of the throttle, there is minimal heat from the combustion chamber. But the coolant is still flowing through the radiator that also has air being pushed through the fins cooling the coolant. Of course the coolant temp will drop.
There will always be a struggle with coolant temp fluctuations especially if you rarely use the truck like it was intended, loaded and working making heat.
If you look at the 6.4, 6.7, & the 6.6 Duramax they have dual thermostats each set to a different temp range. Now you can guess why. Coolant temp management.
