So I have a 180 thermostat (failsafe). Since I installed this mod I have had nothing but great results staying at 175-182 even on high steep climbs and stressing my rig on 4 low. Hot day, Wheeling with AC on no problems. Went wheeling this weekend to Lake Tahoe to play in the snow, all day no problems no issues. On the way back home since it's all coasting downhill with very little acceleration I noticed my temp starting to drop drastically. Went all the way down to 145. As soon as it flattens out and I have to give it gas the temp climbs nicely back to its 175-182 normal operating temps. Start to coast downhill again same things starts happening. Mind you this is a long hour 45-1 hour of coasting downhill then flats. Anyone else have this problem?

 

That's odd. I'm no expert but I think the coasting had your water pump turning very slowly and the cold air was causing the sensor to read a lower temperature than true.

But that really doesn't make sense because if you did the traditional setup your thermostat would be at the front and would be closed at that temp, holding hot water toward the temp sensor.

I'm stumped. Or maybe I'm close. I can't tell which.

 

Thermostat is stuck open.

Long periods of low engine load coupled with a large amount of airflow across the radiator with a thermostat that's stuck open and constantly flowing coolant into the radiator, and you'll definitely get that effect.

If you were here in the northern states, the cold weather alone would keep you well below 180ºF.

 

That's what I thought but if it was stuck open it would have the same effect at idle right? Also take a long time to warm up. None of which are happening. I'm pretty sure the thermostat is working because the temp still did its normal climb... 173-181 then Goes down slowly back to 173... rinse and repeat.

Either way it's also what I was thinking but I have not even put 3k miles on the mod.

 

Correct. My thermostat is right in the middle of my fan shroud. Also one thing to add as even the slightest acceleration will get it back up to temp. Like 25% throttle for a minute will put it back to perfect.

 

That behavior is exactly the same as on my D1 when I coasted it down 4th July pass from Idaho into Washington in 20 degree temps. Operating temp went from 179 to 145ish, climbing back once I hit the gas. My D2, on the other hand is in desperate need of the inline stat mod.

 

What you experienced going downhill in cold temp is very normal, and it will happen in any car, I have experienced it with many different cars, & it's due to radiator getting cold & engine not making much heat, because there is no load on engine, basically not burning much fuel because weight of car is pushing it downhill, your T-stat is not stuck open yet,!! & nothing wrong with it,
being a failsafe it has a built in feature, if it overheats once it will open fully & the small extended arms in T-stat will lock it so it won't have a way to close again, but your obviously works fine since you get normal temp in other conditions once not going downhill any more,
(Failsafe being a MOTORAD does really need this feature, because they are known to fail often,! LOL,,)

 

This just goes against anything that I thought I knew about cooling systems. I always went with the fact that a cooling system prevents over cooling not overheating. So in theory regardless of the speed or weather conditions the water should stay inside the block circulating until it reaches for my application 180 degrees. Now I get that the truck is not working hard so it won't really make temp but for the temps to go down when the sensor is on the engine (hot side) of the thermostat is what boggles my mind. Even at idle when water stays in a block long enough it will only build up heat wouldn't it not cool the water?

Thanks for the reply and letting me know this has happened in other cars. Being from San Diego and recently moving to colder climates I've never had these types of issues before.

 

You likely are thinking of your thermostatic control knowledge.
......

 

My 200tdi behaves like this too. Back home, a lot of owners put radiator muffs on in the winter to get them up to temp. It's only with the final versions of the RV8 that changes were made to have the block run hotter.