Hello. I'm looking for some advice on what steps I should take next to fix a milkshake problem. The engine is a 2003 4.3L Mercruiser in a 2003 Stingray. I've been told by a reputable boat repair store that the problem is either the head gaskets or the engine block. But I didn't have them rip the motor apart to find out. Last fall, I did a compression test, and the two rear cylinders failed (3 & 6). The others passed. I have a few people helping me out for this repair. We're just looking for some opinions on where we should start, or a general consensus on what the problem is. Should I start by changing out the head gaskets to see if that solves the problem? Any advice on this would be very helpful.
See pics below.
See pics below.