My Sierra DOHC has recently made some problems with the cold start. It sounds like it’s only running on three cylinders and takes only a bad gas. In addition, it is partly very heavy. I have the feeling that the problem only occurs if I have driven the engine in front of it really warm. I changed the head seal two years ago / ~40,000km. In total I have now 220,000 km down. Can it be that the head may have a crack?