Coolant mysteriously vanishing

[VT Owner]

For some reason, several minutes after I've stopped the car and it's been a hot day (or the engine temp is over 95°C), I hear the radiator coolant boiling in the coolant overflow bottle and then it begins to leak all over the ground (overflows I assume). Now, there is no leak in the bottle I'm aware of (I've filled it up and waited but no leaks) and after several days I always find my coolant overflow bottle bone dry and the radiator needs topping up (couple of litres).

I did replace the radiator cap with a new one from Repco but I'm beginning to suspect that it may not be making a tight seal and that the radiator is simply open to the coolant bottle all the time and I'm loosing coolant that way. Does this sound plausible (it's a problem with the radiator cap) or is it indicative of another problem?

P.S. No milky oil and the exhaust isn't overly damp or smelly.

[Sean880]

Don't mess around looking for cheap fixes. No one can reliably say what the problem is until they fully inspect the car

Get the car into the mechanic saspo to inspect the cooling system for leaks and have a pressure test carried out. 2 litres is a lot of fluid to be losing in a couple of days. IF you have needed to add that much to the radiator then you probably also have some air in the cooling system now unless you bled it out after adding the fluid.

Have it inspected now before you overheat the engiine again and cause some very expensive damage.

[VT Owner]

Thanks for the heads up but it's been inspected and pressure tested fine. It's been flushed, had new hoses fitted, new waterpump, new coolant, new thermostat and gasket, new heater tap and a new radiator cap (which is a different shape than the original one and the reason I suspect it's the wrong type).

I always bleed the air out when I refill it.

[codeunknown]

I will have a stab at this...cause i had the same issue....here is the list of could be's. No car should be overheating is point one ! however some cars are prone to it depending on driving conditions.eg..some cars are fitted with alum rads very good at heat transfer..but love fast air flow...the others need air flow but discharge heat slower. First part to test is fan and at what temp it kicks in. Second all cars need a working pressure, make sure the rad cap is good with a good seal. All car rads need a coolant and inhib additive..make sure yours is not old ..and is mixed 50 % 50 % water and coolant. Make sure your water pump is fully working ..any damaged fins means less pressure..which means less volum of water pumped to cool.Check hoses/check heater in car /flush the system to make sure no chunkies. Test and adjust the fan belt--or replace. if you have a clutch fan...try the tube test to see if its slipping.
the tube test is getting a hunk of old 2inch rubber rad pipe and tring to stop the fan from turning.while the engines runs..over time the clutch fan can get 2 issues..faulty temp active or slipping clutch..easy fix..replace unit..or checp fix remove oil and fill with glue and bolt outer case to clutch thus stopping slippage and making the clutch fan unit into a solid fan active unit ! Check the themostate..and replace it...and drill the over pressure release hole bigger! the tiny whole is great for pressure..but stuffs your engine or head if it cooks. Any one or more of these things is the reason for the build up and boil...It could be if auto or man transmission..slippage as well..clutch or auto slip pads.Check also your wheels for binding dragging of brakes etc.or bad bearings..cars overheat with temp/or load/ or slippage. There is your check list...check each one...cars that boil...from use...and not erors i mentioned..so age ...often have signs of failing water cooling system elsewhere..like faulty or pitting welsh plugs..brownwater stains or colour of dye in the coolant...anywhere shows other issues. Every week if you use it ot not..you should turn on the air con for 30 mins for seals..and turn on the heater for 30 mins for water to flushed through and thus the junk ending up in the bottom of the rad..but you need to clean and flush a rad every 12months! and do the coolant overflow and filler remove all that brown sludge and scale...Hope thsi helps ya...

[VT Owner]

Looks like I was right - it was the radiator cap. Went to Holden and got a genuine one and no more overflow problems.

Took the car on several trips and it stayed under 90°c for the most part. It even got up to 104°c idling in traffic but the fans came on and dropped it to sub 100°c temps then back to high 80's when on the freeway.

Let it idle at home and get up to 105°c and switched it off to see if it would boil in the overflow bottle but not a peep. Hopefully, this is the end of my heat related woes.

Who would have thought a brand new aftermarket radiator cap bought from Repco was the cause of my latest series of woes... (and yes, it was the correct one according to the packaging).

[s_ikari2015]

makes you think now, the price difference between a genuine and aftermarket radiator cap and the amount of issues it could/has caused.

Not really worth it with these little things. Also, if the radiator cap had been genuine and completely failed, causing engine damage, Holden would have covered the repairs.
Repco would have covered the radiator cap. LOL