Overheating - Coolant Reservoir bubbling after a 15min run

[No1]

Hi there,

I got a blinking red temp light on (>100C) 15min after leaving the area I had my car parked (reallyyyy slope road, for about 3h). Opened the bonnet and the reservoir had a lot of pressure inside. I let it cool down a bit and removed the blue cap. I missed a lot of coolant that spilled out right after opening it (I was aware of severe burn risk).

I called the tow truck and got my car parked at home. Then, I turned on the engine with the Reservoir cap off  and tapped some of those large hoses to release some eventually stuck air.

It took me about 15/20min with the engine running (so head gasket might be good?) to start seeing some bubbles again in the reservoir, some of them coming back from the bottom small hose. The top thin hose pours water back into the reservoir on idle so the water pump may be working properly. Also, the more I press the gas pedal, the more water it pours in. great!

I repeated the entire process again, now turning on the Heater fully on 'hot' after starting to see some bubbles 15min after engine start. It immediately dropped the temp and stopped the bubbling while it was on (cap off).

Some aspects I've noticed so far:

• Timing belt and water pump are new (1.5y) OK
• Thermostat was changed last week. OK
• Reservoir lid is free of residues. OK
• Coolant is new (1 week old). it was refilled after the old one came out due to high pressure inside the reservoir. OK
• No leaks, spills on the ground below the car, or missing coolant after every/some cycles. OK
• The radiator fan (the only one I'm aware of) only comes ON if AC (hot) is turned on (not idea if the same result with just the heater on hot). It's not turning on by itself at 100C when bubbling starts occurring (ac or heater off). Is this the problem?
• Pressing the gas pedal increases the pouring flow of coolant into the coolant reservoir. Letting the engine on idle also pours water in, at a lower flow. OK

Do i need to add distilled water to the G12 when filling up the tank? This is the one I'm using, directly into the reservoir. 

Question: Should I close the lid while the heater is still blowing? When do I know there is no more air in the system? What procedures should I take?

Any ideas? Ty!

Edited July 14, 2022 by No1
Tags added

[ObjectiveAway]

Your cabin heater can cool down the engine, then the fault is clearly in coolant flow,  eighter your new thermostat is malfunctioning, or it is any other reason that blocks coolant flow to radiator, for example sharp curve on incorrectly asembled radiator pipes that prevents flow. 

Edited July 14, 2022 by ObjectiveAway

[No1]

@ObjectiveAway the mechanic thought that the issue was in the thermostat and changed it for a brand new one last week, but the problem persisted so it cannot be the therm

Also, this problem surged from nowhere. The car wasn't intervened in the last months. But got to check those pipes

[ObjectiveAway]

1 hour ago, No1 said:

@ObjectiveAway the mechanic thought that the issue was in the thermostat and changed it for a brand new one last week, but the problem persisted so it cannot be the therm

Also, this problem surged from nowhere. The car wasn't intervened in the last months. But got to check those pipes

Then try to remove the termostat, thats 15min work -  just flush the coolant in a bucket and unscrew two screws. Also the mechanic was able to see either the removed thermostat was open or closed.

If the problem is gone then thats something about thermostat. If the problem remains, then some problems with coolant flow through radiator. After you flush the coolant you can try to test that with garden wattering hose. 

[No1]

@ObjectiveAway Just found out today the top radiator hose becomes very very hot to touch while the bottoms one stays always cool, and the 2 tiny ones entering/leaving the reservoir become very very hot, so hot the top one starts to shake due to pressure building up inside.

If the thermostat was always open both hoses had to be hot right? And if it was faulty i.e. always closed the bottom one shouldn't be as stiff as the top one. They become unsquishable after 20min, maybe because they have gases inside (from faulty hg)?

[lupogtiboy]

Sounds like you may need a new thermostat outlet, they are made with a solid piece of plastic that runs across the middle of the outlet that a pin on the thermostat pushes onto when its opening. Even with a new thermostat fitted, if the plastic piece is missing, it has nothing to push the pin onto, so the thermostat can't open

[mk2]

Top advice @lupogtiboy. So many people miss that. 👍🏻

Edited July 14, 2022 by mk2
Typo

[No1]

Amazing @lupogtiboy! thank you! I'll look into that! 
I will report later  

Edited July 15, 2022 by No1

[No1]

On 7/14/2022 at 4:50 PM, lupogtiboy said:

Sounds like you may need a new thermostat outlet, they are made with a solid piece of plastic that runs across the middle of the outlet that a pin on the thermostat pushes onto when its opening. Even with a new thermostat fitted, if the plastic piece is missing, it has nothing to push the pin onto, so the thermostat can't open

[SOLVED]

You were actually right! thank you!

I took it to a new mechanic and explained the problem. He went right to the "newly installed" thermo by the previous mechanic (1 week ago) and find out it was that plastic piece that was broken and the thermo pin was pushing into the void. The other mechanic didn't noticed the broken piece and only replaced the thermo... This one replaced the whole kit (thermo and accessories = almost same price as only thermo) 

 

[lupogtiboy]

Awesome! Glad you're back on the road!