5.7L Merc I/O Overheats at idle. Cools when running.

[Snailman]

I'm not a mechanic... I don't know anything about engines and less about boats. My biggest problem is that I own a boat. She's a 21' Wellcraft with a 5.7L Mercruiser I/O.<br /><br />The engines cools just fine at speed but will begin to overheat when idling. I had a marine mechanic change the impeller for me but the problem seems to persist (albeit to a lesser degree).<br /><br />My next thing is going to be to change the thermostat (that I think I can do myself) but I'm told that it might be the thermostat housing!?<br /><br />I run the boat in saltwater but I flush the engine after every pull and I winterized it by draining the engine and then filling it with antifreeze.<br /><br />Responses to the board are great but if you'd like to e-mail responses that'd be fine too:<br /><br />snailman22030@comcast.net

 

[Trent]

Re: 5.7L Merc I/O Overheats at idle. Cools when running.

Most likely scale (rust) has built up in your manafolds and risers. Saltwater boats should have them replaced every 4-5 years.

 

[Snailman]

Re: 5.7L Merc I/O Overheats at idle. Cools when running.

Another boater responded to me in e-mail:<br /><br />As a 5.7 owner myself you may try this........remove the exhaust manafolds with the elbows attached then take the elbows off .....you may have to use a small grinder to get the rusted nuts off, then soak all main parts in a muratic acid solution overnight,also remove multi hose connection and soak cast iron fitting that houses the thermostat ,sits on top in front of carb.You may also check condition of rubber hose water feed hose,kick out drive to the right to the max it's about a inch in dia. and 18 inches in length ,that hose supplys saltwater to cool your engine.Check the engine water pump,may have to temporarily hook up with a clear hose for a visual test but dont leave in.I am not a pro mechanic but these tips were from a pro giving to me. You may have to buy marine exhaust gaskets in putting back the heavyweights good-luck .

 

[Snailman]

Re: 5.7L Merc I/O Overheats at idle. Cools when running.

I guess I should've mentioned that this is a fairly new engine in an old boat.<br /><br />The engine has only been run in saltwater for one season and has only about 80 hours on it.<br /><br />snailman22030@comcast.net

 

[johnkc]

Re: 5.7L Merc I/O Overheats at idle. Cools when running.

snailman, do you know what year and type of thermostat hsg. is on this engine .is the thermostat hsg.for a alpha one .or for a alpha one gen II.as these housings had different amounts of water flow being supplied from the drive.and also if you have the older thermostat hsg with check balls in the "T"connecter on top of the hsg the balls may be stuck in the open position.good luck.

 

[2770]

Re: 5.7L Merc I/O Overheats at idle. Cools when running.

Check to be sure all of your hoses to and from manifolds and t-housing are connected to the right place.I crossed mine one time when winterizing and had the same problem as you in the spring.It took me a while and a lot of money before I noticed it. CC

 

[Snailman]

Re: 5.7L Merc I/O Overheats at idle. Cools when running.

This is why I LOVE this forum! You guys are right on top of this!<br /><br />I'm going to print all of this out... take it with me next time I get to fiddle with the boat and get this worked out!<br /><br />You guys are the greatest!<br /><br />Thank you!

 

[Boatist]

Re: 5.7L Merc I/O Overheats at idle. Cools when running.

Snailman<br />My boat has a flush hose connection to flush without useing muffs and a couple of times I have left the cap off. When I do that water runs out at that point at low rpms and motor gets hot.<br /><br />I would make sure your water circulation pump belt is snug and not slipping. Also when on the water pull the hose from outdrive impeller and make sure have good water flow.<br /><br />Manifold and risers normally cause to over heat at high speed and it cools down at low rpms.

 

[boater4life]

Re: 5.7L Merc I/O Overheats at idle. Cools when running.

You might also want to check the hose between the transom and outdrive for pin holes or kinks. This is very often overlooked!<br /><br />

 

[Snailman]

Re: 5.7L Merc I/O Overheats at idle. Cools when running.

Boatist!<br /><br />"Flush Hose Connection?" Is your engine a Merc? 5.7L? What year? Where is it (the connection) What does it look like? I'm wondering, of course, if I have such a connection.<br /><br />I'm going to check all of these things next time I get to play... Not until 5/16 but I'll be sure to post and let everyone know what I've found.<br /><br />Thanks!<br /><br /> snailman22030@comcast.net