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Discussion Starter · #1 ·
Hello. Let me start by telling you when and how the problem occurred. Half a year ago, I changed the coolant for the first time on the Astra H 1.7 CDTI 81 kW turbodiesel. This engine has nowhere to vent the cooling system. On the advice of some mechanics, I left the coolant tank open when I refilled a new one and started the engine as this was supposed to bleed the system (I squeezed the rubber hoses). Since it was winter and the car did not reach the operating temperature at idle, I drove approx. 6 km slowly to approx. 60 km / h, with unscrewed coolant tank. The cabin heating was set to max, however, cold air was blowing all the time, so there was air in the system. After about half an hour of running the engine, I see that it will not be vented, the cabin heating is not working at all, so I screw the coolant plug (later I find out that the instructions say that the fluid plug must be screwed to bleed the system ). At that time, however, the system actually quickly began to vent, it was also necessary to top up approx. 1.5 liters of coolant.

After changing the coolant, when the engine is idling, a relatively loud, metallic "knock" appears randomly, about once every 30 seconds - 1 minute. So a rather rare occurrence, but still every time the engine is warm. After googling the symptoms, some suggest that the head gasket between the two cylinders could be leaky, and that this sound represents “escaped” compression from one cylinder to another. Let me mention that the car does not consume coolant, the coolant is pure without oil, no bubbles are formed in it, and the oil is also clean without traces of mixing with the coolant. So when it comes to oil and water all is good.

That's why it amazes me. It is true, however, that this never happened before the coolant change and it started right then. I hope that someone is familiar with this experience and can give their opinion. Thank you all for your help.
 

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Discussion Starter · #5 ·
My guess was escape of compression gases from either one cylinder to another, or to rhe outside of the engine, due to the damaged head gasket / wraped cylinder head. But I hope this is not the case. Maybe if someone knows if this sound could be that or not ...
 

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Did you also replace the thermostat valve? Do you know the condition of the valve if replaced recently?

You said that it only happens when the engine is warm. It's a long shot but maybe a piece of the thermostat valve has come off and is floating around and the sound comes when it it hits somewhere in the cooling system or against the water pump.
 

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Did you also replace the thermostat valve? Do you know the condition of the valve if replaced recently?

You said that it only happens when the engine is warm. It's a long shot but maybe a piece of the thermostat valve has come off and is floating around and the sound comes when it it hits somewhere in the cooling system or against the water pump.
Clearly bro.
 
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