Check your 'stats
Replacing a bad thermostat can save fuel, increase oil life and make your cab warmer.
Early engine builders discovered that an engine would use a lot less fuel and run a lot better if the coolant was kept hot enough.
But when equipped with an adequate radiator for hard work in hot weather, the engine would run too cold almost all the time.
So they devised a gadget called a thermostat, which controls the flow of coolant through the radiator in response to coolant temperature. When the engine is cold, the thermostat blocks access to the upper radiator hose. This allows the coolant to circulate only within the block and head through a small bypass, and through the heater core through the two connecting hoses. When the engine reaches the thermostat setting, the device cracks open and then controls the flow to keep the operating temperature within a narrow, acceptable range.
Many late-model engines run at 180 degrees F and above, and post-2002 EGR engines often run at 185-195 degrees F and above.
Why is hot coolant so important?
Diesels depend on the heat of compression to ignite the fuel. Holding coolant at the required temperature keeps the inner surfaces of the cylinder head and liners warm enough to increase the temperature produced by compression, and this helps the fuel ignite quickly and burn smoothly. The hotter air also helps burn off the soot that forms during the combustion process. In fact, an engine running well below operating temperature will throw a lot of junk, such as soot and even raw fuel, into the oil.
Also, the viscosity of the oil used in your engine is suited to the operating temperature. When an engine runs too cold, the oil will be thicker than it should be, resulting in more friction and higher fuel consumption.
Diagnosis and Repair Basics
We consulted the North American Institute, the school run by Volvo and Mack to train technicians and owners for both brands. Dennis Schantz, service trainer, and Mike Berger, Mack program developer for training manuals, showed us all the steps involved in replacing a heavy-duty engine’s thermostat.
Schantz says truck owners rarely test thermostats after removal any more. Keeping your eye on the temperature gauge will do the job. After a cold start, once the engine starts taking load, the temperature should rise steadily until the thermostat opens. The temperature shown will then level off and remain within a narrow range unless running in extreme conditions (e.g. up steep hills) in hot weather. Carefully note the operating temperature range when the engine is new, so that you can tell if the thermostat has gotten lazy and is allowing the engine to run too cold.
A sudden overheating problem often indicates that a thermostat has failed to shut. But you should also check that there is plenty of coolant in the system, and that the fan and water pump are being driven at normal rpm by belt drives in good condition. You also must have a functional fan clutch, though that is typically not needed during level highway cruise. If the thermostat has failed, the block will be very hot, but the radiator itself will be relatively cool except right at the top. There will also be appropriate codes in the ECM.
Slow warm-up or failure to reach operating temperature when the engine is being worked are always a faulty thermostat problem.
We’ll describe and show replacement of the thermostat on a Mack E-tech engine. While procedures vary a bit from engine to engine, the basics are very similar.
You must remove anything in the way and then disassemble the thermostat housing to remove the thermostat. It is normally necessary to replace two seals on the unit itself, as well as a gasket between the housing and cover. You should pay particular attention to using a new gasket and new seals. These normally come in a kit with the ‘stat, but if they do not, ask for them. Also, carefully prepare all the surfaces both work against by removing not only old gasket or seal material, but also rust or pitting. Coating seals (other than the gasket) with fresh coolant prior to assembly to lubricate them will allow you to install them without marring the surfaces and causing a leak.