U need to check pressure in engine.
Take next steps: 1. remove cap and hold hand so it simulates cap.
2. If u feel that under the hand is pressure then it's bad. If so, then u need to clean all ventilation tubing on engine. Then maybe it helps to create vacuum in engine. It suppose to help. If not, then u need to check cylinder pressure. There is slight chance that piston rings are causing the problem.