ctuck0659

Merc 502 MPIs, closed cooling, rebuilt top end 100 hrs ago. These eng's have never used a drop of coolant. Ran 1 season after rebuilds w/ no problems. Mid-way through this season I got a temp alarm on the port eng. when docking. Coolant reservoir was almost a gallon low. Pulled the plugs- all looked fine except #3 had some dark deposits built up on it. Replaced the injector just in case it was sticking. Checked compression in all 4 outer bank cyl's and 2 on the inside bank- clearance between the Gil mflds. is too tight on #4 & 6 cyl's. All (incl. #3) were +/-10% of 170 lbs. Pressurized the coolant reservoir to 20lbs. on the raw water side and looked for air bubbles through the antifreeze fill cap (negative). Pulled the intake plenum and squeezed down the intake mfld. bolts a little although all were within torque range. Checked for air bursts in the anti-freeze recovery bottle while running at speed (negative). Engine runs STRONG and doesn't miss. Ran the rest of the season waiting for the problem to become obvious as it became worse- never happened. Every time I run for 15+ minutes the antifreeze always drops down to about a gallon low and then appears to stop leaking- the eng. never gets hotter than 170 on plane but will heat up at idle with the slower circulation so I usually have to put it in neutral and rev it for 20-30 sec's after I idle for a minute to cool it back down. Each time I went for a run I topped the reservoir, checked the bilge for antifreeze (never a drop), looked for milk or moisture under the valve cover oil and breather caps (negative), checked for increased level on oil dipstick (negative). Eng. still runs strong, no missing with the exception of the first 45 or seconds after start-up, after which it clears out. Recently I pulled the plugs again and #3 has some wet build-up on it and smells like fuel. I also ran the eng. up to temp and watched the antifreeze in the reservoir- I noticed some black specks (carbon?) floating by on top of the antifreeze. Has anyone ever experienced a head gasket leak so slight that the cylinder still fires and maintains compression? I know a coolant leak is supposed to scrub a plug clean but could a slight leak cause antifreeze residue to form deposits on the #3 plug? I hate to pull the heads off for no reason, so if anyone has any ideas I'd really appreciate hearing them!