I've just finished changing the head gasket on a friend's 2003 Astra 1.7TD (Y17DT). I had the cylinder head pressure tested and skimmed. I've fitted new head kit including bolts, timing belt kit and adjusted the valves. I also fitted a new thermostat as the old one did not open when immersed in boiling water. The engine turned over fine by hand and started as soon as I'd bled the injection lines. The car ran fine for about half an hour on my drive and the ECU did not register any fault codes. We took the car for test drive and it ran fine initially, but after 25 miles the car appeared to smoke excessively, revved to the limiter and then lost power. There was a thick oil and water mixture in the expansion tank and lots of watery oil in the sump. The ECU is still not registering any fault codes. It would seem like the head gasket has gone again, but I am considering getting a compression tester that I can use with this engine to check this. I would welcome any ideas about the best way to move forward with this.