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I replaced my radiator tonight. With the car on jacks at the front filled up the radiator slowly till full.
Then with the radiator cap off started the engine and let rise to normal operating temp.
Very few bubbles but literally had a ton of coolant flooding out the top!
Eventually the fans kicked in. No bubbles at all in ages and still coolant overflowing the top.
Turned the engine off and the radiator sucked all the coolant down and made some lovely loud gurgling sucking sounds!
Wtf did I do wrong? I Let it cool down and probably put another litre of coolant in before the radiator is full again, but I think all the gurgling sounds it's sucked a heap of air into the system? Do I bleed it again?
Help!!

Did you have the heater on? (It should be)

How steep an angle of elevation on the front? (It doesn't need to be very much, just a bit)

You'll probably find it settles a bit for the second run......

Gahh no heater wasn't on

The cars a decent height at the front. High enough to get under to access hoses etc
Will give it another go tomorrow night

Heater dosent make a difference in a mx5 its always in the coolant circuit. There is no valve to turn it off when your hot, the slider just shuts the door to prevent airflow over the heater core.

My tip works with the stock rad neck.
get a plastic coke bottle (600ml is fine), cut the base off, take the lid off and put enough electrical tape around the neck that it wedges in and is reasonably water tight (which isn't really that tight) and self supporting.

Add enough coolant mix that you can see it at the base of the coke bottle when cold. Your effectively making the top of the coolant in the coke bottle the highest point in the cooling system.

Go through your warm up procedure. You should see coolant level rise in the coke bottle then it will drop as air bubbles come out then rise again.

Once everything is hot and your fans have cycled a few times your coolant level should be steady in the coke bottle.. If is still going up and down leave it till it steadies out. That should now be the air out of the system.

I'd bet most of the air is already out, which caused the coolant overflows you mentioned.

You can use a water bottle rather then coke but some start to get a bit soft as the coolant gets hot.


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I got an old rad cap and drilled out the centre to the same size as a tube about 5 inches long with a tiny funnel stuck in. Same concept as Sailaholic but it looks a bit more sophisticated.

Nice tips sailaholic and gslender.


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Cool will give those tips a try tonight.
So it's normal for the coolant to spew out the top? Or is this just the fluid taking the path of least resistance?
If I loose much more tonight I'm going to have to buy more coolant!

As air gets hot it expands. Therefore there is less volume for fluid and it pushes it out the radiator. Once that air makes its way to the rad neck it's no longer is the system and the fluid level drops again.

By having a column the fluid can expand into with out overflowing you conserve your coolant. That's why you don't fill the coke bottle only just top it up a bit.

If you were doing it for the first time you may find you need to keep topping the coke bottle up as the air comes out and more coolant flow in. However in your case I would guess the change will be minor.

If you use this method you should only loose 100ml? That is above the neck of the radiator once everything is hot.


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Out of interest, how many litres of coolant did it take?
Mine took an astounding seven litres (Nissan engine conversion).

I park mine on my stupidly steep driveway and just let it idle with the heater on until the bubbles stop and the fans have cycled 3-4 times.

Ahhhh, pebble-crete...... Is yours as much fun in the wet as mine?
(NB. my drive is even steeper and narrower than yours, Rolley!)

Nevyn,
Not so much in the MX but our front wheel drive cars struggle as tyres get older...
Mine is 2 cars wide at the bottom and one car wide over the footpath. It rises 5ft over 15ft, I'm going to be reworking the whole bloody thing when I put my shed in in the next couple of years...

Does make bleeding cooling systems easy though!

If u don't bleed the system, won't it slowly bleed itself (coolant level will drop).

The issue I believe is if you get gas blocking causing a localised hotspot in the engine while operating at high load.


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Just out of curiosity, when you run the heater (realise you dont need to do this in a mx5 as mentioned) do you have the a/c button on or just put the dial on heat and turn the fan on? Always wondered if the a/c button needs to be on. Apologise if a dumb question.