Engine temp gauge reading below normal when downshifting

When driving down a mountain in 3rd gear letting engine hold back the speed (engine braking), temp gauge goes down significantly below normal. I’d think temp would stay the same or maybe increase slightly.
Is engine’s computer telling coolant system to increase flow or pressure thus cooling engine way down during this driving method?

 

How far is significantly? Check coolant level in radiator and overflow. Thermostat could be stuck open. Fan clutch could be locked up.

Thermostat should keep truck temp the same no matter what rpm or load for the most part.

 

Interesting….
I check coolant weekly. All good there.
Significantly is gauge reads around 1st mark after C. Normal reading is close to midway on gauge.

 

We have a variable lift thermostat.

 

Is that like a smart thermostat?

 

There is a heater circuit that heats up the sensor at specific conditions to increase a flow to the radiator. Thus dropping the temp on the display.

 

Gotcha. That explains it pretty good. Thanks.

 

I’m not sure why you are surprised the coolant temp drops during engine braking. Yes you are dissipating kinetic energy as heat, but you are also not burning any gasoline. I’m personally not sure what to expect, but I’m sure it would depend on a number of factors

 

If the engine’s running, it’s burning gasoline. As the engine is braking, the rpm’s are higher as the engine is working harder holding back the weight of the vehicle against gravity and inertia. The brakes are the only system here taking a break. Pun intended.

 

Not true, many modern fuel injected engines shut off fuel during deceleration.

 

All fuel injected engines cut fuel off on extended decel so nothing is being burned to create heat, the only heat being generated is a small amount from friction and compression.

 

How does the engine run?

 

It's not running under it's own power on a decel, the wheels are still turning and the transmission is still engaged and thus keeping the engine coasting with the truck. Once engine speed drops back to near idle speed the fuel injectors are turned back on and it continues to run normally.

 

It’s being rotated by the wheels which are being rotated by gravity and/or momentum.

 

Its not running that's why you have engine braking. Try and turn your engine over by hand and you'll see how much resistance there is. That's whats happening with engine braking its just your tires are turning it instead of you.

 

I’m learning something new today.

 

Unlike a carbureted engine, a fuel injected engine injects fuel into the engine independently of intake airflow. Thus it can shut it off when it wants.

CAVEAT, the fuel injection controller takes air flow into consideration under “normal” conditions.

 

I just assumed as long as an engine is running, it’s sucking gas.

Guess it shows that my other vehicle is a carbureted jeep.

 

Just to complete the thought, regardless of the fuel delivery mechanism, the engine is burning minimal fuel when engine braking.

 

Yes it's burning gasoline but significantly less during this process add in the increased airflow of cold air through the radiator and engine bay.
I used to do this all the time with my Frontier except that it was a manual and i would put it in neutral and coast down mountain passes, the temp gauge would drop down pretty far.