Why Some Engines Have 2 Thermostats

In liquid-cooled cars, a thermostat is a valve that regulates the flow of coolant to keep things running safely within your engine's correct operating temperature range. Watch your car's temperature gauge — if it has one — for high and low irregularities. Overheating, that classic roadside steam event, can lead to catastrophic problems, such as a cracked engine block. Overcooling is a less-famous situation that can likewise prematurely kill your engine. Both are potential signs of a thermostat gone bad and impending cooling system failure.

Knocking noises and the color of your car's radiator fluid are also indicators of cooling system health — you're inviting trouble if you never change your coolant. Please dispose of the old stuff responsibly if it's a DIY affair. Thermostats are typically replaced when they break, not necessarily with every coolant change. However, if you're already throwing other parts at the job, adding a new thermostat probably won't bust the budget.

A single thermostat commonly manages things in most engines, but some engines use two thermostats. Depending on the vehicle and its needs, and with efficiency and wear in mind, the second thermostat can offer a variety of benefits. Subaru is known for dedicating a second thermostat to the continuously variable transmission (CVT). You'll find two thermostats in some Volkswagen, Nissan, and Dodge engines, for example, to split coolant circulation up for different engine components. And many, but not all, diesels use two thermostats to help engines adjust for the variable stresses of towing and hauling. Whatever the application, the way these small thermodynamic devices work is pretty ingenious.

Sealed inside a thermostat housing is a tube-shaped wax element. Inside of that, there's a piston that opens the main valve, which regulates coolant flow into the radiator. A pressure spring on the outside of the assembly is calibrated to close the main valve and help modulate flow. As your engine warms up, the wax element heats and expands against the piston, which presses it up and opens the main valve. While that's happening, a separate radiator bypass valve allows some coolant to heat and circulate while the main valve is closed, which enables the engine to warm up faster and more efficiently.

That's why, in addition to the usual cooling function, Subaru uses a second thermostat for its CVTs. Warming the transmission fluid helps bring things up to a minimum operating temperature, so the torque converter can lock. And that, in turn, enables the transmission to perform optimally, resulting in reduced emissions, higher fuel economy, and smoother operation.

Volkswagen engines that feature split-cooling systems with two thermostats for the block and cylinder head work in much the same fashion. The design isolates some coolant in the cylinder block so the engine can warm up faster and so the block can reach higher temperatures than the cylinder head to reduce friction on the internal crankshaft components. Meanwhile, the cylinder head can run a bit cooler for improved combustion.

Where pulling huge amounts of weight matters, diesels rule the big truck market because diesels are better for towing and hauling compared to gas vehicles. These torque monsters — and their cooling systems with gigantic radiators — have to be designed to operate efficiently both when under load and when unladen. 

The challenge here is that radiators, which help bring temperatures down by dissipating engine heat from coolant into the air, don't magically change size with engine strain or vehicle speed. One way around this is to somehow control how much air flows into the radiator – grille shutters, with opening and closing flaps, accomplish this by covering portions of the radiator on big trucks. The other way around it is to regulate how much coolant is circulating from the engine to the radiator in the first place. 

That's why big rig diesel engines often have a primary and a secondary thermostat — a setup you'll also find on General Motors' Duramax turbo diesels. The primary thermostat runs under normal conditions, while the secondary thermostat remains closed. This restricts the volume of hot coolant flowing in proportion to what's needed for low-stress, efficient operation. The secondary thermostat is calibrated to open at a higher temperature, which allows all of the coolant to circulate for full effect, whether you're hauling rocks from a quarry, or a boat to the lake.