407 Sport HDI 2.2 Coolant pressure

[vortexco]

Ok, I've just had my cam and aux belt changed including a water pump as mine was leaking.

fully flushed the coolant system, and refilled with coolant. When i drive with heater off, the temp goes up, radiator fans kick in and the coolant is blown out through the expansion tank.

If I have the car running at fast idle with the cap off there are bubbles coming out from the expansion tank and I think it smells of exhaust. I have a cold so sense of smell not good.

If i replace the cap and leave the car running at normal idle then no problems.

I read it could be the head gasket, but wouldn't that blow bubbles into the coolant regardless of engine speed? I also heard i could have a EGR cooler leak which can do the same thing.

Is there a way to tell? really can't afford the Head gasket change at the moment.

Thank you