1987 Merc Black Max Overheating - Maybe

[Pat Crowley]

Took my bay boat out today and had a little problem. At first it ran great but after a few miles it -maybe - started to overheat. At least the alarm started to sound. The water pump has recently been re-worked and the tattle-tale has plenty of water coming out of it. I live in Louisiana so I have removed the thermostats. It can't be that. I have changed the poppit. I don't trust the water temp gauge but it shows about 150 degrees F.

First question. When I ground the output of the water temp sender, the gauge only goes to half scale. Shouldn't it go hard right (max temp). I have 5 volts coming out of the gauge.

Second question. Does anyone out there know of any wierd problems that could cause my motor to overheat or at least cause the alarm to sound.

Tomorrow I plan to purchase a new temp sender, temp gauge, and water temp switch. Maybe the switch is going bad. What do you think. Also, when I replace the temp sender and temp switch, should I use heat sink compound?

Any suggestions would be appreciated.

 

[CharlieB]

Re: 1987 Merc Black Max Overheating - Maybe

Replace the T-stats, they are there to speed warm up to prevent a 'cold siezure', at the very least, install washers to control the excessive flow.

Inspect spark plugs and piston domes for 'steam cleaning', tho it is possible for a head gasket to leak combustion gasses into the cooling system and cause an overheat at high speeds and not show any evidence of water in the cyl. In these cases cooling is adequate at slow to moderate speeds but when approaching WOT the gasses then displace cooling flow and the temp climbs and alarms.

It this is what's happening, pull the heads.