mx5cartalk.com

Hi all,

Recently I've had a bad run with my 1998 NB's cooling system.

First, the top radiator hose went, so I replaced this. Shortly after, there was [potentially] a leak from the radiator, so I installed a new 42mm Cooling Pro radiator.

Thirdly, yesterday, my heater hose split, messing coolant all over my hot exhaust manifolds (yay!). I knew that one of my heater hoses had a very very slow leak, and had the replacements on hand, so I replaced it this morning.

I topped up the radiator again with a 3:1 coolant mix and started her up. What I observed was:

1) After a few minutes, the OEM gauge was just under half-way, it's usual spot that I've never really seen move once the car is running.
2) I also observed that both fans eventually came on while the car was sitting still. The strange this is that:
3) I have an aftermarket water temp gauge that takes the temperature from the top radiator hose. The gauge was sitting at 40C (the lowest value it can go to), while the fans were running. Radiator was cold.

Does this suggest that the thermostat is not opening? Is it normal for the fans to run, while the thermostat is closed? My water pump was replaced about 4000km ago.

My main concern is that, while driving, my OEM gauge never moves from the under half-way point, and my aftermarket gauge never usually reads more than 90C while driving, but today, it was going up to 94,95C while driving. Eventually when sitting still for a few min in traffic, the aftermarket gauge was reading up to 105C, and the OEM gauge moved ever so slightly over the half way point. As soon as I was driving the temps went down again.


As you may understand, I'm a bit paranoid about the cooling system now... it's also quite cool in Melbourne today, but was hoping someone could confirm that this sort of behaviour is OK or not? I don't want to damage my motor

I would take a guess at either a sticky thermostat or an air lock.

Did you bleed the system after refilling it?


Park it with the nose slightly uphill (basically so the radiator cap is the highest point for the cooling system.

Take the cap off and start the car, let it idle until warm.
occationally give it a slight rev to help move things along.
As air starts to bubble out of the radiator cap top it up with coolant.
Keep doing this until there aren't any more bubbles.

Thanks for the replies!

hks_kansei wrote:Did you bleed the system after refilling it?


Park it with the nose slightly uphill (basically so the radiator cap is the highest point for the cooling system.

Take the cap off and start the car, let it idle until warm.
occationally give it a slight rev to help move things along.
As air starts to bubble out of the radiator cap top it up with coolant.
Keep doing this until there aren't any more bubbles.

I did do this when I refilled it. When there were no more air bubbles popping up and the level seemed consistently near the cap, I closed it up. Is it worth checking this again tomorrow?

Yes always.

Dann

NitroDann wrote:Yes always.

Dann

Agreed.

Especially since it's a 5 min job to check it.

Thanks guys.

I'll also probably replace the thermostat tomorrow, just in case (if I can find one at Burson, Repco, Supercheap, Autobarn etc) otherwise will do it next week.

Will also check the radiator level and top up/burp as required

The stock temperature gauge in the NB is a POS. Mine goes to halfway within a few minutes of driving and will sit bang on halfway even under competition conditions.

deviant wrote:The stock temperature gauge in the NB is a POS. Mine goes to halfway within a few minutes of driving and will sit bang on halfway even under competition conditions.

Haha same as mine, good ol NB's just like the dodgy oil press gauge.

Anytime Iv ever tried that bleeding technique (not in my mx5) the coolant/water spills out the top of the radiator since there is no cap on, is that normal? Serious question by the way.

The stock gauge is designed to have a deadband around normal operating temperature. It's not dodgy / lazy but a deliberate design. It's so people see "normal" and "pay attention I'm hot".

It's not the best design for an enthusiast / track car I agree. I think you can mod the stock gauge to read linearly again.

Another trick for bleeding. Get a 600ml coke or similar bottle. Cut the bottom off, take the cap off, wrap electrical tape around the neck until it's a firm fit in the radiator neck.

Then 1/2 fill with water and use this to bleed the system, water level should rise a fair way up the bottle as it gets hot then drops away as the air comes out.

When it drops low, top it up. When it's steady for a good 10 minutes after the fans have kicked in and the last burp just pull the bottle out and stick the cap on.

Don't use a cheap water bottle (mount franklin etc) the bottles are a different plastic and deform at engine coolant temperatures.


Sent from my iPhone using Tapatalk

help it burp along. squeeze that top hose a few time with ya hand.

also, should the coolent mix be 2:1, or if you track it 1:1?

As a general rule for these cars you could say that if one hose splits then they're probably all at the end of their life and the best thing to do is replace the lot, plus the thermostat, at the same time. It's much cheaper to buy a set and much less work than doing it one at a time. Make sure you do all the little hoses that run around the front of engine, intake manifold, throttle body and oil cooler (they're different for every model but they still all fail).

Thanks to everyone again for the great advice!

I certainly did not bleed the air out of the system appropriately before. I believe it's all good now (using the coke bottle technique sailaholic described ) - temperatures appear to be normal again now.

manga_blue - I've started ordering replacements for each hose