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Originally Posted by Ginger Teddy
I have always had the hiss of pressure release when I check the coolant level after the vehicle has been left overnight in almost 7 years of ownership and that includes since I had the head gasket (MLS ) and water pump replaced in April 2018
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How is that pressure created? It's impossible for trapped air to change pressure when tested at a fixed temperature. This is especially so where the header is opened on an almost daily basis.
In fifteen years of ownership, I have never had a significant hiss when opening the cold system. I suspect this 'common' 1.8 system hiss is due to equally common issues with the head gasket/liners. The factory fitted elastomer gasket was never 100% reliable from day one to the day MGR closed shop. And the much lauded MLS wasn't all it was expected to be for many 1.8 engines.
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I also replaced the coolant temperature sensor 3 months ago ( again on recommendation of club members ) after the ( unreliable ) temp gauge went down to the 6 o`clock position and fan ran on high speed and it corrected that issue, regards Paul
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If the gauge drops to the 'off' position. the high speed fan is automatically tripped from the loss of signal from the head sensor. It's almost always due to faulty wiring/connector. The temperature sensors are usually bombproof. Of course fitting a new one means you reconnect the bad connection. It's often caused by grime and muck in the plug.
TC