Help with engine temp swings, high and low

This is a puzzler for me and my 05 DCSB OR 4.0, 77000 miles. Changed coolant and thermostat 12000 miles ago. Never had any coolant temp problems up till 2 months ago. One day towing my camper home up a small hill I noticed the coolant temp was reading 194. Usually the temp stays around 185 +/- 3 degrees, as monitored with Ultragauge. Then going down a long hill the temp dropped to 177! It continues this up/down depending on engine load. Outside temp has no effect. Just changed out the thermostat this week, and still seeing this weird temperature swing, towing or not. The dash temp gauge never moves from center once warmed up. Any ideas out there?

 

You’re talking about a 5% difference. That isn’t going to hurt your engine.

I wonder if you could have air trapped in the system somewhere.

 

Normal temperature variations. My scan guage reads low 190 up hills. Downhills the engine defuels therefore it cools down some depending on the length of hill.

 

+1 - nothing to worry about.

Try climbing a hill in 100 degree weather, pulling a camper, with a supercharged engine and then come back to discuss coolant temps

 

Understand this is in the "normal" range, but truck has never done this before. Guess I will just quit watching it so close.

 

Your OE dash gauge is an idiot light with a needle, placed there to make consumers think they are getting 'real' gauges.

The needle doesn't move because the range of temps that allow it to sit in the middle have not been exceeded.

But there are probably some things that you should consider............

What is the temp of the T stat you installed?

Did you install it right side up?

Are you comfortable that all the air has been burped from the system?

As others have said, your numbers are in a reasonable range. If the above items are good, you're operating fine.

 

It's fine, it'll vary a little bit. In the winter I run 181-186 usually. In the summer I'll see 200° off-road sometimes. I'm at almost 84k miles with the original water pump and thermostat, the coolant has been changed once.

Mine will drop below 180 occasionally if it's really cold and I have the heater blasting. What brand of thermostat did you use?

 

So your saying you changed the thermostat twice. Did you go back with original stat at second change and still seeing wide range?

 

Used Toyota OEM thermostats. They all are marked 82C. Never went back to original unit as it is 13 years old.

 

I have the same thing going on in my 2005 DCSB TRD OffRoad. I have 202000 miles though...and thought it was the water pump or thermostat. I was going to do a coolant change and install the new Toyota thermostat to see if it changed anything. Now I guess it won't change the temperature swings but I'm sure it won't hurt to replace it anyway and see what happens. I think the big thing to remember (at least I keep telling myself that) is that the engine cools back down to normal temps.
I also submitted an oil sample to Blackstone Labs thinking I may have the dreaded head gasket leak. I turns out I have zero coolant in the oil. Yay me!!!