Yet another variation on overheating

[jimmo62]

Went on a long drive yesterday and after 3 hours of fast A roads and motorways the low coolant warning alarm went off. Checked the temp gauge which had gone past the normal 11 o'clock to about 12 o'clock. Stopped immediately in a cloud of steam. Waited until it had cooled down and then opened the expansion filler. Put in about 6 litres over the course of about an hour as the level kept going down - but no signs of a leak (just lots of coolant in the engine bay from where it had boiled).

Green Flag came out and we bled it for about 30 mins including checking the bleed pipe for bubbles etc. - pretty much as per the Mazda manual described the process. The heater blows hot air within about 2-3 mins of starting from cold so we think the pump is circulating and the bottom hose gets hot so we think the thermostat is opening. I was then able to drive 3 hours home, checking coolant level every 30 mins and no visible drop in the level and correct temp on the gauge.

This morning the level was low again so I added another litre to get back to the right level.

So I guess the questions are
Why did it lose coolant and overheat yesterday? Could it be related to using the aircon for the first time (e.g. aircon pump faulty affecting the water pump?) - as that is the only thing different about the drive?
Why did the level go down overnight after being fine all the way home? I understand that can happen after bleeding sometimes?
Are these symptoms of the dreaded cylinder head failure or is some other problem more likely?

TIA

Jim

[Diplomat]

Check the radiator along the top join between the plastic and metal parts.

Leakage there gives the impression of boiling as the escaping coolant steams on contact with the hot exterior of the radiator. If you don't have a low coolant alarm the first sign is usually the smell of hot coolant entering the cockpit!


Frank

[mikeonb4c]

Check the radiator along the top join between the plastic and metal parts.

Leakage there gives the impression of boiling as the escaping coolant steams on contact with the hot exterior of the radiator. If you don't have a low coolant alarm the first sign is usually the smell of hot coolant entering the cockpit!


Frank

Very good suggestion. This is what happened with me only less dramatically.

[jimmo62]

Thanks I will have a look now. Forgot to mention that the Green Flag guy did a coolant pressure test with a small pump and pressure gauge. It held pressure fine at 5 PSI and 7 PSI - not sure if that makes any difference as the system was cold at the time so maybe whatever is leaking only does so when hot.

I'll have a look at report back!

[dave_aber]

Check the radiator along the top join between the plastic and metal parts.

Also check the bottom joint on the rad. I've had 2 split, one at the top (original factory one) and one at the bottom (first replacement, pattern part)

I guess that if it's the bottom joint you will have less visible coolant when you open the bonnet. The UV due kits (eBay) are an excellent way of finding leaks.

[GreenBongo]

My rad has just started leaking from the top joint as well.

[jimmo62]

Had a good look, no sign of any leaks on any of the hoses.

I started it from cold (has not run since last night) and there was just a small puff of white smoke from the exhaust then within less than a second it went clear - I have heard that if the head gasket is blown then lots of while smoke sometimes happens, but it was not obviously that.

May be a trip to the local Bongo specialist tomorrow....

[GreenBongo]

On mine I removed the air intake and then I could see obvious signs of coolant having leaked down the back of the radiator.

[Simon Jones]

Have a look from under the van. This was taken looking at the rear of the radiator where the gearbox pipework goes and you can see the coolant leaking from where the matrix crimps to the plastic end cap.