I am in need of some assistance.

I am having a coolant pressure on my 1.8 16v clio. (Renault), coolant, cooling-system, overheating (car maintenance) In that when left to idle and engine gets up to temp and when revved the car will hold a lot of pressure in the main rad pipe.

I have replaced the thermostat, new rad, new header tank cap, new water pump, re-bleed numerous times the coolant system, bypassed the turbo and heater matrix. I have no mixing of water in the oil and no mixing of oil in the coolant, I have not lost any coolant other than when I open the cap to relieve some pressure I loose a little. I have also replaced the head gasket and had the head skimmed. And torqued all down in accordance with the manual.

I am still having the same coolant pressure issue and am rather stumped as to what the cause could be as clearly combustion gases are getting into the coolant system somehow. I have done a compression test also on all 4 cylinders and have a range of 148-150psi across all 4 which again I am happy with.

Now from cold the car is fine the pressure seems to only build up once warm? If I was to leave the car running for an hour or the fan will kick in and out nicely but the top rad hose is overly hard. If I was to hold the revs say at 2500 rpm for a few minutes the top rad pipe looks like it will explode and thus unable to use the car until sorted. Any thoughts?

  • Also, in the past, I have seen the lower rad hose collapse when hot if it does not have a coil of wire in it to help prevent this from happening.
    – Ed S.
    Feb 9, 2019 at 16:22

1 Answer 1


As long as you have replaced the radiator cap, then there should not be excessive pressure in the cooling system. The system is designed to hold 16PSI. If it does exceed this pressure, it will release pressure into the overflow tank.

When the engine is cold, the thermostat is closed, not allowing coolant to flow through the upper radiator hose. Once the engine warms up, the thermostat opens, allowing coolant to pass through the hose at a high rate. The coolant also expands causing higher pressure in the system.

As for combustion gases getting into your cooling system. If there is a passage between your coolant and the combustion chamber, your exhaust would be smoky, smell like coolant, and you would have bubbles in your coolant. It would be hard to check the coolant since it only happens hot.

If this hose is deforming (expanding like a balloon), then you should replace the hose.

Is the car overheating? Has the coolant boiled over?


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