manifold guage on 5.7 volvo

[bobbert]

Just went down memory lane to diagnosis problem on 3.0 Mercruiser I/O. Ran great for 20 minutes and then the brown stuff hit the fan. Would not run under 2500, white smoke (steam) out the back, etc.
Compression test cold 150 in all holes.
Vacumm gauge showed a drop from 15" to 5" when things went wrong. Compression check indicated one hole at 120.
Took head off and took it to the head shop. Explained reading to the Head Head guy who said either head gasket or exhaust valve.
Turned out to be both a hot head gasket break and a exhaust valve with a crack AT THE SEAT.
On the 3.0 just in front of the riser is a BRASS plug this is the vacumm tap.
Answer Vacumm gauge is big indicator of engine condition and a problem solver.