Where to start? Mercruiser Overheat / Thanks

[Pedro19]

1995 Searay 220 Mercruiser 5.7 Alpha 1 ( only 200hrs ). 2nd time out on the water and boat has started to overheat at idle. As soon as you start to up the RPM -it will cool right off and runs fine. As soon as you start to idle, the temp will slowly start to overheat. The boat had set for two years prior to the seller selling the boat and the dealer required the seller to install a new impeller since he had no record of having that done w/in the past 3 years so that should be good to go. I am no mechanic but somewhat handy. Where should I start? When moving the engine temp runs around 165 -167 degrees ( is that normal ) When it happened it actually go so hot that the buzzer went off but as soon as I gave it some gas the temp started to cool down. Thanks in advance.

 

[alldodge]

dealer required the seller to install a new impeller since he had no record of having that done w/in the past 3 years so that should be good to go.

When overheat starts, you always start with the impeller. Does "dealer required" mean you have a receipt that the dealer did replace the impeller?

Get a 1 ft piece of clear hose, a hose clamp and male barbed fitting from the hardware store. Disconnect the hose coming from the drive at the thermostat housing and place the clear hose inline. With motor running, look for air in the line. Run with it in the water and not on a hose

 

[Pedro19]

When overheat starts, you always start with the impeller. Does "dealer required" mean you have a receipt that the dealer did replace the impeller?

Get a 1 ft piece of clear hose, a hose clamp and male barbed fitting from the hardware store. Disconnect the hose coming from the drive at the thermostat housing and place the clear hose inline. With motor running, look for air in the line. Run with it in the water and not on a hose

Thanks for the direction. They did provide me with an invoice, they charged the seller $454 for the install of the water impeller and $199 for the overall mechanical / safety inspection. Would the thermostat cause this? By looking at the bolts, it has never been replaced. This is a fresh water boat by the way.

 

[kenny nunez]

There is a metered hole in the thermostat housing which will get corroded shut that can cause what you are describing. Sometime the hole is so corroded that it is not visible. Get a new thermostat with the gaskets and scrape out the housing. If you have a friend with a bead blast cabinet would be a great help.

 

[tpenfield]

I'm thinking a restriction in the cooling system and the increased RPM is enough to overcome it.

Your mission (impossible) will be to determine where the restriction is and remedy the situation.

Is this 'open' cooling or 'closed' cooling (ie Heat Exchanger)?

Places to look/check
- Water intake tube at the bell housing
- Oil coolers along the way to the thermostat housing
- The thermostat housing
- Exhaust manifolds and riser/elbows

You could use an IR temperature gun and see where the greatest temperature rise is occurring as it may help with the search.

 

[poconojoe]

I'm gonna tag along to see how things go.

 

[Pedro19]

There is a metered hole in the thermostat housing which will get corroded shut that can cause what you are describing. Sometime the hole is so corroded that it is not visible. Get a new thermostat with the gaskets and scrape out the housing. If you have a friend with a bead blast cabinet would be a great help.

I replaced the Thermostat but I did not clean out the housing very well. I was very corroded and had lots of gunk down in there. I will pull it back off as I still had the same problem after installing the new thermostat. It wants to run at about 169 degrees and I currently have a 142 thermostat. Runs like at top at 169 degrees though. Thanks for all of the help.

 

[Pedro19]

I'm thinking a restriction in the cooling system and the increased RPM is enough to overcome it.

Your mission (impossible) will be to determine where the restriction is and remedy the situation.

Is this 'open' cooling or 'closed' cooling (ie Heat Exchanger)?

Places to look/check
- Water intake tube at the bell housing
- Oil coolers along the way to the thermostat housing
- The thermostat housing
- Exhaust manifolds and riser/elbows

You could use an IR temperature gun and see where the greatest temperature rise is occurring as it may help with the search.

Well, I replaced the thermostat but I did not clean out the housing very well as I was in a pinch. There was lots of corrosion down in there. I am an open cooling system. I will take the housing back off and see if I can clean it out a little better. I did notice lots of corroded pieces of metal flakes down in the water. Hope that is not bad. We ran the boat for about an hour at 168-169 degrees Hope that is not bad for it.