After decades of relative reliability, the lowly thermostat has become a hot topic due to frequent failure. Last century, thermostat failure was sporadic and uncommon. But the first decade of this century has turned it into a buzz word.
Basically, an old school thermostat has a chamber filled with wax or ether that expands against a spring when heated. New school thermostats since 2005.5 perform the task electronically. When the coolant is cold, circulation is limited to the engine itself, the coolant fill bottle, known also as the expansion tank with its blue fill cap, and the heater core. At a calibrated temperature, an upper plate begins to lower away from a hole, allowing the coolant to circulate through the radiator. While driving above the lowest speeds, the coolant is held at a constant temperature by the thermostat’s self regulating characteristics. At very low speeds or while stopped, the thermostat will open fully, and the radiator fan system works to keep the coolant from overheating. As the thermostat opens, a lower plate blocks off the recirculation passageway back to the engine block, making sure that all the required coolant is going to the radiator and heater core.
Last century, a failed thermostat generally meant an overheat situation, maybe with steam, blown hoses or radiator, stuck by the side of the road, tow truck, etc. Modern thermostats however tend to over cool, often by just some small factor. The thermostat must precisely control the warm up faze of an engine for emissions control reasons. But cooler is not better. Too low a temperature means poor mileage and excessive carbon deposits, especially in a Tdi diesel.
Although thermostats may commonly fail, diagnosis must always be performed. It’s pretty basic, but at Karmakanix, we see other problems causing the cooling system codes about 10% of the time. As luck would have it, the same cars that are blessed with the faulty thermostat issues also have an unreliable coolant temperature sensor (ECT). Early VW Mark IV and Audi models came with a black coolant temperature sensor which would get updated to a green unit. These temperature sensors have two sensors in one unit, one for the engine ECU and the other for the instrument cluster (ECU 17). One needs to log in to each control unit and check the coolant temperature readings while at operating temperature. If the two readings are not the same by more than a degree or so, the temperature sensor is faulty or the wiring is damaged.
To check for over cooling, the air conditioning is turned on so the radiator fan system runs constantly. If the coolant temperature drops below the normal 87 °C level, the thermostat is at fault. We generally want to see the coolant temperature drop close to 80 °C before we condemn the thermostat. Usually, the owner is not aware that the over cooling is happening, because the coolant gauge reads exactly in the center any time the coolant temperature is anywhere close to normal. The manufacturer’s literature says that this is for “Customer Confidence”.
The instrument cluster ECU generally controls the radiator fan system. Radiator fan control systems don’t often fail, but the Fan Control Module Failure Code can be generated by the engine ECU if the coolant temperature it sees gets too high before the radiator fan turns on. Hence, a bad coolant temperature sensor or wiring can be at fault.
Different models will generate different codes for cooling system maladies. Early VW Mark IV and Audi models simple say “Cooling System Malfunction”. Models after 2005.5 record other information such as mileage, miles per hour, and coolant temperature when the code turns on the Check Engine Light (MIL, Malfunction Indicator Light). Sometimes that temperature is Alaska cold, and can well indicate an intermittent failure. Coolant temperature sensors can get cracks in their carbon cores that will open up and change the resistance value intermittently.
Plastic Parts In Your Cooling System: Thermostat Housings
Plastic parts in a cooling system are a mixed bag. Heat will eventually cause the plastic to get brittle. And plastic warps, especially if old oil gets on it, which also deteriorates the rubber seal. Plastic was heralded as being the solution to corrosion. And plastic shaves a few grams off the weight of a car. Really, it’s all about cheap. We do acknowledge that sometimes the only way that old coolant gets refreshed, is when some plastic part fails.
Not all plastic cooling system parts fail early. But when one changes a thermostat, it is advisable to change the plastic outlet housing. Reinstalling an really old one is courting failure. Some thermostats come permanently installed inside a plastic housing, and of course it gets changed as a unit. The housing gets made in two sections, which get glued together with the thermostat inside. On a VR6 engine, one should simply expect to change all the thermostat housing parts before 150,000 miles. Simply put, we see that job as inevitable maintenance.
Certain plastic parts are standing the test of time. Radiators don’t fail like fruit flies. Then again, that is the coolest part of the cooling system, and the technology is many decades old. Some plastic breather system parts fail early in life. May valve covers are plastic, and yes, some fail, usually during removal. One can clearly say that plastic engine parts have increased the cost for long term ownership.
It would seem important to mention that a common failure can be the plastic tee in the top radiator hose that incorporates the bleeder hose through which vapors bleed out of the main system and into the coolant reservoir. Now this part is in the hottest section of the cooling system, just after the coolant has left the motor and before the radiator. Especially in vehicles where the engine has had a history of running hot, this relatively tiny tee outlet is prone to braking off, often burning the poor soul who has brushed against it lightly. We are not kidding. The crystallization of the plastic can make this part so weak that the smallest amount of pressure will break it off. And we have seen the geyser go 20 feet when this tee broke off on a motor that was being tested for running hot, and the tee broke just from the vibration of the motor. It’s not a bad idea to test these tees while the engine is cold. Currently, the most common breaking offenders are the 5 cylinder Eurovans, just because many of them are over a quarter of a million miles, and the youngest is old enough to vote.
The Persistent Code Syndrome
One last note, sometimes a cooling system malfunction code will occur even after both the thermostat and the coolant temperature sensor have been replaced. A double check confirms that nothing else is wrong, all the temperatures and devices are working normally. The same sort of math calculations done to work the coolant gauge also apply to the warm up cycle. The computer knows what the temperature was when the engine started, how long it has run, the varying engine load and speeds. So it calculates a curve for the warm up range. If the actual temperature deviates much from this range, then the code is set. Occasionally, that range is simply too tight. We have Mark Malone of Malone Tuning in Canada rewrite the ECU’s code to expand the allowable curve range. Problem solved. Rare problem solved.