It sounds like carbon is breaking up and holding a valve off it's seat.
I would probably run a leak down test to determine where the leak is but I would bet it's a valve problem. In most engines you can remove the rings and still get a lot of compression. BUT stick a valve open on it will go away instantly.
I've seen carbon build up in ex-police cars from the way they are driven. Patrol and highway ones are usually pretty clean but the ones used locally get loaded up with carbon from the constant short trips and no real load on the engine.
It might clean out with steam but it may need to be opened up to actually remove it all.
Tuesday, October 31st, 2017 AT 6:10 PM