You take a foot long hose that looks like a hose you use to refill your ac put a thread on one side that matches your spark plug, on the other end you put a pressure dial. Start the car, measure at idle, measure and mid and high. and repeat for all cylinders and see if one is getting any off numbers.
everything was fine when I did it which lead me to believe it is only under extreme load. which I can not put a dial on and measure.