Random metallic knock after coolant change 1.7 turbodiesel

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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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Air was trapped in the head and when you drove it you basically drove it little or no coolant flowing through the head. Everything here is a guess but I suspect an injector(s) got overheated in the cylinder that had the least amount of coolant in the head and is randomly sticking.
Usually when a diesel injector overheats it get scored inside and anything you put in the fuel tank will probably not help but you can try it before seeking out a competent Opel diesel mechanic, not you tube video's or the same clowns that advised you about filling it.
Those and many other engines are best refilled using vacuum filling.
 

boxerbeemer

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Thank you for your reply. I really hope it is not an injector, but on the other hand it is way easier to replace than the head gasket. I am thinking about also doing a leak down test, the kit is available to buy for relatively low price.
 
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If the coolant level is not going down, no white cloud coming out of the exhaust, no visible external leaks and coolant is not blowing out of the bottle with the cap off with the engine running chances are the HG is okay. Diesels have much higher compression than gas engines so a blown HG exposes itself quickly.
 

boxerbeemer

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Luckily my engine doesn't seem to have any of these symptoms. Would you suggest that I still do a leak down test?
 
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If it runs well I probably would not on a diesel. Sourcing an adapter for the gauge then pulling injectors or glow plugs is a bit of a pain if no symtoms are present or the engine is experiencing no issues.
 
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