1995 Mercury 40 HP 4 Cyl. Oil Injected overheating only under load

[robm1]

1995 Merc 40 HP 4 cyl. S/N 0G6210460 turns on overheat alarm about 30 seconds after planing off or when on trailer at WOT in gear for about a minute.. When backing off, buzzer stops. Replaced leaking exhaust gaskets (slight external water leak & water intrusion to #3 & #4 cyl) and see no external leaks, yet #4 plug is "clean" as if water is getting into combustion chamber. All other plugs show some slight carbon. Should I pull exhaust cover and baffle plate and deck both to insure flatness then reassemble? Could this be associated with powerhead to midsection gasket? BTW, this motor has no thermostat.Thanks for any help.

 

[Texasmark]

Ping Faztbullet, Racerone, or Fastjeff on this site.

 

[robm1]

Exactly how do I do that? I have searched and came up with this: Type @ and member name and when it turns blue the member is notified to check out post? Or must I hit blue "Post Reply" button?

 

[racerone]

Replace the impeller in the water pump.-----Are you saying you run this motor wide open in gear on the trailer in your driveway ??------If so that is a mistake !

 

[robm1]

No sir! On trailer in river in plenty of water tied down to trailer. Water pump is good with excellent telltale and pressure.

 

[robm1]

Thanks for your reply racerone, just trying to figure this thing out. It doesn't make much sense to me with no external indication of leakage that internal leak would occur. Do you have any suggestions? I've read many of your posts and really appreciate your knowledge.

 

[racerone]

If #4 plug is clean ( like new ) it could be no fuel to #4 -----No spark on #4 -------No crankcase compression on #4 -----or low cylinder compression on # 4----Too much fuel on #4 cylinder.---Bad plug on #4 etc , etc.

 

[robm1]

Not clean like new, just much cleaner than the others as seen in picture of the plugs. This motor idles fine in gear after 20-30 second warm-up. All plugs have fuel on them and jump 1/2" gap in tester easily; compression is: #1=140, #2=139, #3=136, #4=136. All cylinders are combusting as noticed by miss when disconnecting one plug at a time (while not running) and running disconnected plug wire through spark tester to ground and starting. Compression of crankcase would be acceptable if #4 is getting fuel and firing? I've seen before where a little water gets in combustion chamber and causes a somewhat cleaner looking plug. This must be slight leak at exhaust baffle plate to water jacket to cylinder where combustion gas is getting into water stream? Thanks for your help.

 

[robm1]

Ideas anyone?