454 mag mpi blew out spark plug and now has top end taping!!
 

I've got a 99 Mercruiser 454 MAG MPI with about 350 hrs on it. I changed all the plugs a week ago and ran the boat all weekend without any problems. This weekend it blew the center of a spark plug out (the porcelain) and left the metal in the head. The diode off the porcelain part was missing though. I easily removed the leftover metal (it was all intact) and fished a magnet around inside of the cylinder and also started it without the plug to blow anything out. Once i did this I put a new plug in it and started it. It runs fine but has a taping noise coming from the valves on the same side the spark plug came out of. After getting opinions, alot of people said they think something is hung in the exhaust valve. .....i question that b/c it runs fine. At high rpms the noise tends to go away. (may just be engine noise covering it)..? Other people say possible exhaust leak...bent push rod....bent valve...and so on. Has anyone ever experienced this before??

Simple/cheapest test would be to get an adapter for a spark plug to an air line.....rotate the crank till both valves are closed and pressurize the cyl with compressed air and see if you can hear any blow by out the exhaust or the intake. If it holds air then at least you know the valves are seating properly....then you have to dig deeper into what the issue is...could be a bend push rod...loose rocker....bad lifter....who knows..

Good idea! I'm trying to get my hands on an inspection camera today. If I can't I'll definitely try that. I just don't see how it could run fine with a stuck valve..I'm hoping its something up top. Guess I should just start pulling it down..

Compression check, check for exhaust leak then pull valve cover to look for a bent pushrod.

+1

Well it was worst case scenario...exhaust manifold leaked water into a cylinder and the rest is history. Bought a used 2001 454 mag mpi and got it in the boat. Took it out this past weekend and something is off with it as well. Seems weak. Boat will only run 48mph @ 4100rpm. :( ganna hook it up and check for codes and check timing. Any help is appreciated..can't win for losing..