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Coolant Temperature Concerns

OMG, my engine is running far too hot! Calm down Mr American, you are suffering from ”too much information”.

Modification Details




(I Think) My Engine Is Running Too Hot

I have had this question (or statement) quite a lot recently and I have noticed a trend. They are all from American smart owners who have recently bought some kind of OBD gauge. That's when their "problem" started.

They are obsessed by temperatures and seem to instinctively know what temperature their car should be running.

I can only assume that historically, their American built cars either run a lot cooler, bigger displacement engines
run cooler or smart engines run hotter than normal. Maybe that's the reason why smart removed the coolant temperature gauge from the American smart. If they don't have the information, they have nothing to worry about.

Proper Smart Operating Temperature

There is no proper smart operating temperature. It varies wildly depending on the climate of the country
you are driving in. The engine is only small so it has problems maintaining a hot running temperature in
cold climates but also does a very good job of keeping cool in very hot climates.

Why Is This?

The smart is touted as a city car and it is designed as such. Like a London taxi, it is designed to sit
in traffic but not over heat. They do this by using a large and efficient radiator compared to the size
of the engine and the amount of coolant in the system.

Normal Temperature Range

You can expect, depending on the outside temperature, your style of driving and the amount of traffic,
to see an operating temperature between 70º and 100º Celsius. That's 158º and 212º Fahrenheit.

At 100º Celsius (212º Fahrenheit), the front radiator fan will come on to help the air
flow across the radiator which should cause the temperature to level out or reduce.

At 120º Celsius (248º Fahrenheit) you will get a coolant temperature warning alarm.
If you hear this, turn the engine off as soon as you can to prevent any damage.

The smart coolant, if mixed correctly (50:50), the correct level and the coolant system is pressurised properly,
will not start to boil until it reaches 122ºC (252ºF). It doesn't get to this temperature and immediately
turn the whole lot into steam so you have time to turn the engine off when you hear the alarm.

What Happens If The Coolant Boils?

The coolant will only be boiling in the engine water galleries around the cylinders and the exhaust valves.
The water/coolant boils which creates a layer of steam between the hot metal of the engine and the coolant.
This steam passes its heat into the water and condenses back into water. In this small
temperature range, this process is still cooling although it's far from efficient.

As the heat increases, the water explodes into micro steam bubbles which creates a lot of turbulance in the water.
This action actually starts to corrode the metal parts that it touches and in those areas you will get nearly no
cooling at all. That's when metal starts to overheat and warp causing headgasket failures and seized engines.

Keeping Your System Efficient

Check that your coolant level is within the MIN and MAX mark on the expansion tank.
Make sure that your coolant ratio is ok for the climate of the country you are driving in.
Keep the radiator clear of bugs and mud that would block it up and reduce air flow.

Cleaning The Radiator

You don't really even need to take the front panels off. I don't expect you to sit there with a toothbrush and a pipe
cleaner cleaning out all the fins. The 2 good options are a pressure washer or one of those pump up spray bottles.

If you use a pump spray bottle, use hot soapy water (not boiling), dial the nozzle
to a jet and get in as close as you can to blast the crap out from the radiator fins.

If you use a pressure washer, don't get too close and don't have it on full jet. Make the nozzle spray slightly.
The radiator fins are thin metal and pressure washers can bend or even cut the radiator fins.
Start about 2 meters away and slowly get closer until the water cleans the radiator effectively.

More Coolant, More Cooling?

No, this isn't the case. What you add to the water is anti-freeze. The resulting mixture is called coolant.
Increasing the amount of coolant (going over the MAX mark) will reduce the pressure of the system
and therefore reduce the cooling abilities of the system.

Increasing The Anti-Freeze Ratio

A higher ratio of anti-freeze will just make it freeze at a much lower temperature. It doesn't make it better at
higher temperature. Anti-freeze is usually glycol based which is actually worse at transmitting heat than plain water.

Coolant has to transfer heat well which the anti-freeze doesn't do as well as water. Although water transfers heat
well, the antifreeze has a higher boiling point. That's why you need a good ratio of water and anti-freeze to have
a higher boiling point but a good heat transmission to help move the heat from the engine to the radiator.

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