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Hi All,

Just wondering if anyone could fill me in on what to check.
This is the second time this has happened to me.
If I turn the car on and leave it at idle, after about 10 minutes the overflow bottle start boiling like crazy and spitting out coolant.
At first I thought I had a leak somewhere as after I drove off the was a big green puddle.
But after driving for 1-2 minutes temperature came back to normal and everything was fine.
Everything seems fine when driving, it is only if you leave the car at idle for extended periods of time??
Currently the radiator is just filled with the green coolant, should I have it mixed with water rather than straight?

Thanks
Rod

Is the fan coming on?

It's DEFINITELY the radiator fan.

Temperatures are coming down when driving because air is flowing through the radiator, at idle no air is flowing through it so you're not getting any cooling and the coolant is simply boiling.

Easy to test this, just take the car for a short spin, pull over, pop the hood and see if the fan is running.

The water temperature gauge, in NBs at least, is effectively a dummy gauge so don't trust it. If your car has an OBDII port and you have a reader, check the water temperature with that since it IS accurate. The radiator fan should activate when the water temperature is about 97C (ie. prior to boiling).

If fans don't kick in at all, check the radiator fan thermoswitch, it's screwed into the radiator.

Should be possible to jam a big paperclip in the end of the cable that plugs into the switch, to close the circuit.
Do this when the ignition is off, and make sure to cover it in duct/electrical tape to be safe.
Then fans should come on, if that is the cause of the problem.

Happened to me once while I was idling in westfields waiting for parking. Turned out my fan had died and wasn't turning on.

But you should drain your coolant if it's 100% coolant concentrate. Should be 1:1 or 1:2 coolant:water ratio depending on your climate conditions.

Also check the line from the radiator to catch can - make sure it is not blocked. Mine was and i started boiling off fluid and over heating if i didn't keep the rad fluid topped up

Sent from my GT-I9100T using Tapatalk

If the car has air-conditioning, turning it on is another way to make the radiator fans work on these cars.

The additive is usually mixed in a certain %. Bear in mind that it is toxic- deadly to animals- like dogs and its attractive to them so please take care. I do not think the engine should overheat at idle even with the electric fan not kicking in but sure I might be wrong. Its probably not going to cost much to get it checked or at least talk to the radiator repair place. Overheating can be very damaging to your engine.

Mr Morlock wrote:I do not think the engine should overheat at idle even with the electric fan not kicking in but sure I might be wrong.

Car manufacturers didn't put the fans there to chop off mechanics fingers. Those fans are there to create airflow when the car is stationary.

Looks like I'll have some playing to do this weekend.
I'll first test to check if the fans are actually working.
Planning on changing the radiator as it is starting to get brown, so good chance to check everything else.

Thanks
Rod

Let me know how you go Rod, I have a spare NA radiator somewhere if you need to borrow it.

RJ45 wrote:If fans don't kick in at all, check the radiator fan thermoswitch, it's screwed into the radiator.

Should be possible to jam a big paperclip in the end of the cable that plugs into the switch, to close the circuit.
Do this when the ignition is off, and make sure to cover it in duct/electrical tape to be safe.
Then fans should come on, if that is the cause of the problem.

you can also short the GND and TEN terminals of the diagnostic connector in the engine bay

project should know better than me but cooling fans are there for extremes. Automotive cooling today is normally well in advance of normal operating conditions. Witness what users do in extreme climates like Northern Aust and the Middle East. Idling your MX5 in traffic without a fan I do not believe will cause boiling unless there is an underlying problem.

Mr Morlock wrote:Idling your MX5 in traffic without a fan I do not believe will cause boiling unless there is an underlying problem.

so whats the fan for then? just for show? the fan is there for when there is no air flow, passive cooling doesn't work very well!

Apparently the fan should come on just below boiling with no pressure in the system. (97 degrees)
I'd start by replacing the radiator cap with a new one, its possible your current cap is the wrong pressure, or faulty.

However, it still shouldn't boil if the fan is coming on when it should.

the poster can let us know what was the root cause(s) if it gets diagnosed properly.