1999 350 5.7L Mercruiser Overheating

[youngfamilyboater]

I bought my used boat last year knowing nothing about boating. I've taught myself along the way and taken on some projects. Most recently, three times out ago, my engine started overheating. My temp gauge wasn't working so, I didn't get a warning. After getting towed back, I did some digging. The smoke and burning smell came from melted upper and lower exhaust bellows. I replaced them, the alternator, impeller, and gear oil. Two times ago, I had no problems. Yesterday when I went out we cruised for 8 miles or so, stopped to eat, drink, and swim. Started up and headed out. 5 miles later I smelled smoke again and shut it down. It looks like the lower exhaust bellows are starting to melt again. The exhaust risers sizzled when I dripped water on them. My next to-do is change out the bellows again and install a new water pump and thermostat. Is there something else that could be causing the overheating issue? Thanks for you help.

 

[Scott Danforth]

Welcome aboard

Alpha or Bravo?

If alpha, replace whole pump assembly

Find all the missing impeller parts

Get your gauge fixed

 

[achris]

Hello youngfamilyboater, and :welcome: to iboats.

It would be helpful if we knew exactly what engine and drive you have. We prefer members post engine year, model and serial number at the beginning of a thread, so we all know what we're 'looking' at. Also what drive you have (Alpha, Bravo etc).... '1999 5.7' covers quite a few variables.

What maintenance have you done on the cooling system?

AS well as getting the gauge working, 1999 should have an alarm system for overheat, low oil pressure and low drive oil. That also needs to be looked into.

Chris...

 

[youngfamilyboater]

Thanks guys. It's an Alpha drive. It looks like the serial number on the engine cap was scratched off. The drive has a number though. I drained the water out of the starboard side plugs of the motor to see if there was any build up, I could not find access to the port side. Just some water drained out.. Other than that, I haven't done any cooling maintenance. What would you suggest?

 

[Scott Danforth]

Start with the manual

Pull the drive apart. You can use Chris's vids in his signature. That will get you to the water pump

Then pull the plumbing from the trans shield to the thermostat housing and back flush it

Then put it back together

While at it, with the drive off, inspect the bellows, gimbal bearing anf u-joints. Replace as needed

 

[alldodge]

I could not find access to the port side. Just some water drained out.

There is a drain in about the same location on the port side. Not sure if "just some water drained out" is for the port or starboard.

Remove the thermostat and place in a pan of water with a thermometer. See what temp it opens sitting on the stove.

Are you in salt or fresh water?
Is the motor running alright, with same amount of power?

If power has dropped off a bit maybe you have a blown head gasket and this is the cause of over heat

 

[alldodge]

Check your alarm under the helm. There should be a purple wire going to it from the gauges. On the other side of the alarm it will connect to a Tan/Blue wire going to the motor.

First make sure there is 12V on one side of the alarm. If there is take the Tan/Blue wire (with key turned ON) and ground it. When the wire is grounded, the alarm should sound. If it doesn't need to replace the alarm.

If alarm does sound, go to the thermostat housing and find a sender with the same Tan/blue wire. Remove the wire with key ON and see if the alarm sounds. If it does not, verify there is continuity between the wire at the alarm and the sender

 

[kenny nunez]

Also check the the plastic cover that is inside the upper gear housing. If it is melted then you will lose water pressure from the pump. The hose from the bell housing to the gimbal housing is also a place to inspect.