This is one of the most common questions we get from new owners. If you're new to the 107 and you're wondering the same thing, please read on.
First, is your car actually running hot? Modern cars tend to have the needle come up to one point and stay there, no matter what the road conditions (uphill, downhill, desert, snowstorm), and no matter what the engine is doing. They're designed that way, to keep your peace of mind. In reality, your engine temperature is always going up and down, that's why there's a thermostat in the car; to help stabilize the temperature and keep it in the range where your engine operates best.
Mercedes didn't do that. Your temperature gauge shows what your engine temperature is really doing. That means it will be higher on hot days, going uphill, in stop and go traffic, than it will be on cooler days, running on level ground at a constant speed. It will raise when you're sitting at idle when the temperature is 95F, and it will cool down when you start moving again. This is normal. If the only way you'll be satisfied is having your temperature gauge settle on one spot and stay there, no matter what, you might want to visit your local Toyota dealer, or superglue your temperature gauge. Short of that, it's always going to be on the move.
The thermostat in the 107 is designed to open at 80C, or about 175F. Your car should run at or slightly above this temperature, under most conditions. However, your engine is also designed to operate as high as 110C, or 230F for short periods. Under heavy operating conditions, don't be surprised to see the temperature creep up. You should only be concerned if it's always much higher than where the thermostat is designed to open, even on cooler days, even when you're running on level ground, even when you're not sitting and idling for long periods.
If you've gotten this far, and you're still convinced your car is running too hot, this is the nuts and bolts of how to diagnose and repair the problem.
Cooling system components, and accessories, starting from the front and working back:
Auxiliary cooling fan: This is the electric cooling fan you should have on the front of the radiator, behind the grill star. It came standard on all US spec cars, but was optional for some other countries. If your car came equipped with AC, it came with an auxiliary fan. It's designed to come on under two conditions.
1) If the temperature of the engine goes above 100C (212F). This is not to say when your temperature gauge says 100C. We'll get to why later.
2) If the AC compressor is on, and the low side pressure exceeds a set point. It doesn't automatically come on with the AC compressor, so don't expect that.
Radiator: The early models used an all brass radiator. The radiator can clog or corrode, reducing the cooling performance. Power flushing can correct this, or getting the radiator re-cored, or replacing it completely. Later models of the 107 used an aluminum core, with plastic reservoirs. The neck where the upper radiator hose connects can crack, causing a loss of pressure and coolant, and overheating of the engine.
Cooling fan, shroud, and clutch: The cooling fan is metal, and pretty rugged. Problems with this are very unusual. The shroud helps channel the air, and increases the efficiency of the cooling fan. Your car should have a shroud. The fan clutch is a viscous coupled clutch, and it's designed to work harder when your engine is warmer. When your car is cold, the fan should be easy to turn. If you've just shut your engine off after a good drive, it should be difficult to turn. If yours does anything else, you should replace the fan clutch. A failing fan clutch can cause hot running conditions, especially at idle.
Thermostat: Mounted on top of the engine, in the housing where the bottom radiator hose meets the engine. A bad thermostat can fail open, which means your car will never fully get up to operating temperature, will idle high, foul plugs often, and get bad mileage. It can also fail closed, which means your car overheats fast and hard. You can destroy your engine quickly if you drive it with a thermostat that has failed closed, or slowly if you continue to drive it with the thermostat failed open.
Water pump: This circulates the coolant through the engine and the radiator. Because it's run directly from the engine, if you're at idle, the pump is running slower, and your coolant is flowing slower, so your engine temperature will go up slightly. At freeway speeds, when your car is running at 3K RPM, it's working fastest, helping to funnel heat safely away from the engine and into the radiator. At the front of the water pump, below the pulley, is a weep hole. When your water pump seals start to go, coolant will leak out of this hole, and drip down the front of the engine. That's your signal to get a new water pump installed. Now.
Engine: Source of power and heat. Throughout the engine are cooling ducts, which funnel the water past the cylinders, through the heads, and everywhere heat accumulates. The coolant soaks up the heat and delivers it to the radiator. Use the right coolant. Xerex G05 is a lower cost alternative to the MB branded coolant. Never use the green stuff, it's wrong for this engine and will cause problems when used long term. Also, don't mix coolant brands. The engine and cooling system in your 107 require a specific mix of anticorrosion and anti-electrolytic compounds, and using the wrong stuff won't protect your engine the way it needs to be protected.
Auxiliary fan thermoswitch: Mounted on the top of the engine, toward the front, this senses the engine temperature and kicks on the auxiliary cooling fan when your engine is running too hot. If you want to test your auxiliary fan, pull the wire off the end of this and short it against the engine block with the key on. Your auxiliary cooling fan should kick on. If it doesn't, you might need a new one, but I'd trace the wiring and check the fuses, first.
Coolant overflow: Mounted on the passenger side wheel well on US cars (driver side if you're from one of those countries where you open the bonnet to see the engine), this holds extra coolant, allowing the coolant in your system to expand and contract as the engine heats up and cools off. Very early models used a brass tank, later models used plastic. This should never be empty. The radiator cap is designed to hold 1 bar (1 atmosphere at sea level, or 14.7 lbs per square inch). A failing radiator cap, or a leaking coolant overflow tank can cause coolant loss and overheating.
Auxiliary water pump: Models after 1977 have this. Locations vary, but generally on the passenger side of the engine compartment. At idle, remember your water pump is working at the least efficient. Also remember this car came from Germany, land of cold winters. When you have the heater on, the auxiliary water pump helps force the heated coolant through the heater core, making sure you always have a ready supply of toasty warm air, even on the coldest days. If your auxiliary water pump has failed, you'll see a noticeable decrease in heater output.
Temperature gauge sending unit: Mounted on top of the engine, it looks very much like the auxiliary fan thermoswitch, but it works a little different. Where the aux fan switch is a binary on/off switch, the temperature gauge sending unit varies the resistance, making the needle on your dash go up and down. Pulling the wire and shorting it against the engine block should cause your dash temperature gauge to peak at max temperature. If it doesn't, your temperature gauge, or the wiring leading to it, are bad.
Finally, the temperature gauge: it's a good idea to keep an eye on it, but it isn't the last word. It isn't calibrated, when it says your engine is 100C, your engine might be 95, or 105. And if you're wondering why your auxiliary fan doesn't turn on the second the needle touches 100, keep in mind that the sending unit for your temperature gauge is on the back of the engine, and the thermoswitch for the aux fan is at the front of the engine. Using a calibrated thermocouple, I've measured as much as 10F difference between the two locations. And, finally, just as the temperature gauge and sending unit aren't calibrated, neither is the aux fan thermoswitch. It's designed to kick on at about 100C, but it might happen a little lower or a little higher.
And very last: Your emissions controls. The Air Injection Pump blows air through the heads, around the valves, and reduces combustion temperature, reducing the amount of NOx. On an engine designed for this kind of cooling, disabling or not repairing a broken system can cause higher than normal operating temperatures.
First, is your car actually running hot? Modern cars tend to have the needle come up to one point and stay there, no matter what the road conditions (uphill, downhill, desert, snowstorm), and no matter what the engine is doing. They're designed that way, to keep your peace of mind. In reality, your engine temperature is always going up and down, that's why there's a thermostat in the car; to help stabilize the temperature and keep it in the range where your engine operates best.
Mercedes didn't do that. Your temperature gauge shows what your engine temperature is really doing. That means it will be higher on hot days, going uphill, in stop and go traffic, than it will be on cooler days, running on level ground at a constant speed. It will raise when you're sitting at idle when the temperature is 95F, and it will cool down when you start moving again. This is normal. If the only way you'll be satisfied is having your temperature gauge settle on one spot and stay there, no matter what, you might want to visit your local Toyota dealer, or superglue your temperature gauge. Short of that, it's always going to be on the move.
The thermostat in the 107 is designed to open at 80C, or about 175F. Your car should run at or slightly above this temperature, under most conditions. However, your engine is also designed to operate as high as 110C, or 230F for short periods. Under heavy operating conditions, don't be surprised to see the temperature creep up. You should only be concerned if it's always much higher than where the thermostat is designed to open, even on cooler days, even when you're running on level ground, even when you're not sitting and idling for long periods.
If you've gotten this far, and you're still convinced your car is running too hot, this is the nuts and bolts of how to diagnose and repair the problem.
Cooling system components, and accessories, starting from the front and working back:
Auxiliary cooling fan: This is the electric cooling fan you should have on the front of the radiator, behind the grill star. It came standard on all US spec cars, but was optional for some other countries. If your car came equipped with AC, it came with an auxiliary fan. It's designed to come on under two conditions.
1) If the temperature of the engine goes above 100C (212F). This is not to say when your temperature gauge says 100C. We'll get to why later.
2) If the AC compressor is on, and the low side pressure exceeds a set point. It doesn't automatically come on with the AC compressor, so don't expect that.
Radiator: The early models used an all brass radiator. The radiator can clog or corrode, reducing the cooling performance. Power flushing can correct this, or getting the radiator re-cored, or replacing it completely. Later models of the 107 used an aluminum core, with plastic reservoirs. The neck where the upper radiator hose connects can crack, causing a loss of pressure and coolant, and overheating of the engine.
Cooling fan, shroud, and clutch: The cooling fan is metal, and pretty rugged. Problems with this are very unusual. The shroud helps channel the air, and increases the efficiency of the cooling fan. Your car should have a shroud. The fan clutch is a viscous coupled clutch, and it's designed to work harder when your engine is warmer. When your car is cold, the fan should be easy to turn. If you've just shut your engine off after a good drive, it should be difficult to turn. If yours does anything else, you should replace the fan clutch. A failing fan clutch can cause hot running conditions, especially at idle.
Thermostat: Mounted on top of the engine, in the housing where the bottom radiator hose meets the engine. A bad thermostat can fail open, which means your car will never fully get up to operating temperature, will idle high, foul plugs often, and get bad mileage. It can also fail closed, which means your car overheats fast and hard. You can destroy your engine quickly if you drive it with a thermostat that has failed closed, or slowly if you continue to drive it with the thermostat failed open.
Water pump: This circulates the coolant through the engine and the radiator. Because it's run directly from the engine, if you're at idle, the pump is running slower, and your coolant is flowing slower, so your engine temperature will go up slightly. At freeway speeds, when your car is running at 3K RPM, it's working fastest, helping to funnel heat safely away from the engine and into the radiator. At the front of the water pump, below the pulley, is a weep hole. When your water pump seals start to go, coolant will leak out of this hole, and drip down the front of the engine. That's your signal to get a new water pump installed. Now.
Engine: Source of power and heat. Throughout the engine are cooling ducts, which funnel the water past the cylinders, through the heads, and everywhere heat accumulates. The coolant soaks up the heat and delivers it to the radiator. Use the right coolant. Xerex G05 is a lower cost alternative to the MB branded coolant. Never use the green stuff, it's wrong for this engine and will cause problems when used long term. Also, don't mix coolant brands. The engine and cooling system in your 107 require a specific mix of anticorrosion and anti-electrolytic compounds, and using the wrong stuff won't protect your engine the way it needs to be protected.
Auxiliary fan thermoswitch: Mounted on the top of the engine, toward the front, this senses the engine temperature and kicks on the auxiliary cooling fan when your engine is running too hot. If you want to test your auxiliary fan, pull the wire off the end of this and short it against the engine block with the key on. Your auxiliary cooling fan should kick on. If it doesn't, you might need a new one, but I'd trace the wiring and check the fuses, first.
Coolant overflow: Mounted on the passenger side wheel well on US cars (driver side if you're from one of those countries where you open the bonnet to see the engine), this holds extra coolant, allowing the coolant in your system to expand and contract as the engine heats up and cools off. Very early models used a brass tank, later models used plastic. This should never be empty. The radiator cap is designed to hold 1 bar (1 atmosphere at sea level, or 14.7 lbs per square inch). A failing radiator cap, or a leaking coolant overflow tank can cause coolant loss and overheating.
Auxiliary water pump: Models after 1977 have this. Locations vary, but generally on the passenger side of the engine compartment. At idle, remember your water pump is working at the least efficient. Also remember this car came from Germany, land of cold winters. When you have the heater on, the auxiliary water pump helps force the heated coolant through the heater core, making sure you always have a ready supply of toasty warm air, even on the coldest days. If your auxiliary water pump has failed, you'll see a noticeable decrease in heater output.
Temperature gauge sending unit: Mounted on top of the engine, it looks very much like the auxiliary fan thermoswitch, but it works a little different. Where the aux fan switch is a binary on/off switch, the temperature gauge sending unit varies the resistance, making the needle on your dash go up and down. Pulling the wire and shorting it against the engine block should cause your dash temperature gauge to peak at max temperature. If it doesn't, your temperature gauge, or the wiring leading to it, are bad.
Finally, the temperature gauge: it's a good idea to keep an eye on it, but it isn't the last word. It isn't calibrated, when it says your engine is 100C, your engine might be 95, or 105. And if you're wondering why your auxiliary fan doesn't turn on the second the needle touches 100, keep in mind that the sending unit for your temperature gauge is on the back of the engine, and the thermoswitch for the aux fan is at the front of the engine. Using a calibrated thermocouple, I've measured as much as 10F difference between the two locations. And, finally, just as the temperature gauge and sending unit aren't calibrated, neither is the aux fan thermoswitch. It's designed to kick on at about 100C, but it might happen a little lower or a little higher.
And very last: Your emissions controls. The Air Injection Pump blows air through the heads, around the valves, and reduces combustion temperature, reducing the amount of NOx. On an engine designed for this kind of cooling, disabling or not repairing a broken system can cause higher than normal operating temperatures.
