Everybody knows what a thermostat is, right? It’s that ugly thing on the wall, impossible to decorate around, and with which everyone in the family fiddles so you are continually too warm or too cold, right? Well, almost. Actually, everybody fiddles with the thermostat’s setting. The thermostat controls the temperature (more or less) to wherever it is set.
How many of us are as familiar with an automobile thermostat?
That knob you turn or lever you slide or button you push to select the temperature for cabin comfort has nothing to do with the thermostat. Those devices on the dash are temperature selectors that manage the mixture of cooled and/or warmed air.
The thermostat controls the engine’s temperature and is vital to engine life and fuel economy.
Most wear occurs in the first few minutes after the engine is started, when it is still cold. By “cold” we mean that it has not reached its normal operating temperature, so even on a sweltering August day, the engine will be cold after sitting a while.
When the engine is cold, the oil flows slower. Fuel condenses in the cylinders instead of being burned; the cold cylinders quench the fire. So some liquid gasoline washes down the cylinder walls, taking the lubricating oil with it. Then, the gas mixes with the oil in the pan, diluting it and rendering it less effective as a lubricant.
Water is a byproduct of combustion in your engine. Though most of it leaves as vapor via the tailpipe, some water in a cold engine finds its way into the oil where it can create sludge.
Some people mistakenly believe that they can extend their engine’s life by installing a thermostat having a lower temperature rating. This may be a throwback to the days when motorists sometimes switched between a winter and summer thermostat.
In the medieval days of motoring, there were no all-season coolants. The antifreeze was usually just alcohol. It kept the water in the engine from freezing, but had to be drained and replaced with straight water in the summer to prevent boil over.
Prestone invented ethylene glycol all-season antifreeze in 1927 (it originally cost $5 per gallon), but the Big Three began using ethylene glycol as the factory fill beginning in the 1960s.
Cooling systems were not as effective, either. With no recovery bottle, expanding coolant went down a hose along the side of the radiator and onto the road.
When things got really hot under the hood, even the water occasionally would boil. Owners would replace their “winter” thermostat that may have been rated at 185 degrees with one rated at 165 degrees. The colder the temperature rating of the ‘stat, the colder the engine runs.
To replace an original equipment ‘stat with a colder one today would be folly.
Advanced coolant formulations accompanied by pressurized, air-free, cooling systems can run at about 230 degrees without boiling over. The engine control computer waits for the coolant temperature sensor to report that the engine is fully warmed up before the computer begins controlling the fuel mixture–a condition known as closed loop–and a colder thermostat might not let an engine get up to the proper temperature.
Until the engine warms up and the controls go into closed loop, the computer operates the engine based on a performance map (three-dimensional program) created by the engineers. In this time, the fuel mixture remains rich.
If the engine does not reach normal temperature, the car continues running rich, wasting gasoline and spewing emissions. If you drive this way for too long, you will destroy the catalytic converter, which tries to burn off the excessive fuel.
But a “hotter” thermostat makes no sense, either. The engine will not heat up any quicker and excessive heat can cause carbon and varnish deposits to form in the engine. Also, the oil thins and loses its ability to lubricate.
Carbon buildup increases compression, and that leads to engine knock. When detected by the knock sensor, the ignition timing gets retarded and your fuel economy drops. Varnish blocks oil flow.
Thermostats should be replaced only with ones having the same rating–and only if they fail. They can fail one of two ways: They may not open (causing overheating) or close (causing the engine to run too cold).
You can’t help but notice overheating, especially if steam is billowing from under the hood. But under-heating is harder to detect unless you have a temperature gauge. A drop in your fuel economy is a clue.
A thermostat is pretty reliable, so don’t fiddle with it.
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Bob Weber is an ASE-certified Master Automobile Technician, having recertified every five years since 1978. Contact him at MMTribune@netscape.net.
