A loose plug would present a long, small diameter circular leak path. ( the gap between the threads. ) This small diameter won't leak much and won't pose a problem in the short term ( 200 mi ) . Long term you can get thread wear and the plug coming out but you are not there in your case.

Do a leak down test of the engine. This is where you pump ~ 90 PSI air into the cylinder when both valves are closed then see where it leaks out.

If an intake valve is hung open, the engine will have a difficult time running on other cylinders as intake operation is disrupted. It gets even worse if the exhaust valves don't open, you will get a popping in the intake as the exhaust gasses back up through the intake.

Thanks 97 SL320,

I haven't had much time to take it down again but I did finally crank it over and take the cover off. It sounded kinda bad cranking but it actually did want to start. I found a lifter contact pad/cup laying in the head next to the bad cylinder. I'm now guessing that whatever got in there to to beat up the plug and whatever else (possibly the spark plug electrode) held the valve open long enough that the pad was able to fall out. I'll see what happened by tomorrow evening

Was the disengaged part from the intake or exhaust? Is the valve spring and retainer intact?

Any news on what happened?

I had that happen once,but it was back in the days of cast iron heads,no damage.

__________________
1999 w140, quit voting to old, and to old to fight, a god damned veteran

Well, I finally got time to take it down, and the bad news is . . . . . . . .

The head of the exhaust valve broke off and hammered a 3/4" x 1/2" hole in the piston
besides beating the crap out of the piston and head. Pummeled the spark plug beyond recognition and metal got into every other cylinder, with very little damage though. there were even pieces of metal in the cats. Cylinder walls looked beautiful, even in the bad one. Motor is £v¢&ed.

I had a valve guides, a valve job and milling done on it with this job. though I couldn't figure any reason to blame it squarely on the machine shop. He couldn't figure any reason for it either. The stem was still secured in the spring cap, though I found the contact pad (little disc with a cup on one side that sits between the lifter and the valve stem) laying in the head next to the spring. He insisted on refunding my money even though I told him I couldn't put any blame on him. He's done many heads for me over the years.

All I can think of is that it was tapping a bit upon start up, which they always do until the lifters pump up. This time it had a tap that was slightly louder than usual and lasted a bit longer. I recall thinking about it some, but then it quieted down and smoothed out.
Ran fine for the 50 or so miles I tested it and for another 100 miles for my daughter.

Maybe that pad fell out early on when the lifter wasn't pumped up and the extra lash (though quiet) finally stressed out the old valve. I dunno!

Oh, the valve head was still laying in the cylinder. Looked like a silver nugget!
Think I'll look for a new 190E for the kid rather then put a new motor in this one. you usually find good one for 2500 or less.