Why is my boat overheating?

[evanlaczi]

98 Malibu response lx carb 350 running hot. I checked all hoses and coolers for blockage. Replaced impeller and thermostat. Still running hot. Or so the gague says 160 then all a sudden 240 spike so I turn off and right back on back to 160. Is my gague bad? Bad connection? What should I do?

[Steve B.]

Does it feel hot, or smell like its burning when this happens? Either way, I'd do two things. Pull the thermostat and run that baby. In addition, I'd clean the umbelical connection. Spray contact cleaner on both mating sides and give her a whirl.

Watch it like a hawk, dont let it over heat if it is. Could have an internal blockage, or circulating pump may be bad.

Steve B.

quick edit: while you are at it, just buy a new thermostat, regardless.

Edited June 9, 2015 by Steve B.

[OLDGUY]

Over the years I have had a couple of over heating issues. Check the suction side of the impeller pump for loose hose clamps etc. I have a fresh water strainer on the suction side of the impeller pump the old one had a hair line crack in it that was introducing air in to the engine that was causing the over heating issue. I replaced no problem since. Just a suggestion.

[GreenMan]

if the gauge is literally in an instant going from 160 to 240 (and back), it is almost certainly an issue with the temperature monitoring (probs not the gauge but a sender or wiring issue). The coolant temp won't literally change 80 deg in a second.

As Steve B. said, if the engine actually got that hot it would likely be stinking hot, stinking literally and probs some smokiness.

If everything feels normal to touch (sure, the engine and exhaust exhaust manifolds are hot but not enough to sizzle your skin straight away) it adds weight to an electrical issue with the gauge.

Go over any connections you can get to, in particular the one on the sender. If it's a push-on type, it may have lost the springiness that maintains a good connection - if so, try squeezing it closed with pliers and re-connecting. To check the gauge you can turn the key on and disconnect the temp sender and the gauge should go full scale and then touch the wire onto the block or somewhere and the gauge should go to zero - if that happens, the gauge is likely OK.

Replacing the sender unit is a pretty cheap, easy thing to do as well.

Sounds like the thing sits nicely at about 160 for a bit then goes crazy? -If so, I doubt it's the thermostat. Especially if you are seeing 'instantaneous' changes on the gauge.

[evanlaczi]

How do I know sender from wire issue. Where is sender?

[EchelonMike]

I had a 1997 Echelon LX with the Merc 5.7L carb and I believe the temp sending unit looks like this:

There is just a small wire, orange I think, that has a press-on connection to the sender. I believe the location is around the water pump housing or intake manifold, but it's been a few years and I forget exactly. I'd start with checking the connection to the sender, since it is a push on type that relies on friction to hold it in place.

Agree with previous posters that if temps instantly goes to 240, there is something electrical. Also, if you shut down and restart, and see the temp is back to normal, that is the opposite of what happens in "real life". Typically if you shut down a boat engine that is at normal operating temp, and let it sit for a few minutes, when you first start it after sitting, you will see a higher than normal temp...because the water in the engine gets heated hotter than the t-stat setting from sitting. As soon as you fire it up, you may see a temp of 200ish, then a quick cool down to 160ish.

It is not something to take lightly, however. If it is overheating, don't let it do that or you'll have some big repair bills...but if the gauge looks to be acting strangely, I would start tracking that down.

Also, I know you said you checked for debris, but did you make sure a broken up impeller or other debris is not clogging up the tranny cooler screen? That happens a lot. I would also double check that screen even though you installed a new impeller.

-- Mike

[electricjohn]

How do I know sender from wire issue. Where is sender?

This sounds like a job for your mechanic.

[Grundall]

Just replace the engine on our 1998 Malibu response. After the engine was replaced boat was idling high, ended up being a leak in the manifold, got a new manifold, and now boat is overheating. Replaced the water pump, didn’t help, removed  the temperature gauge that seemed to help. All the components on the boat seemed cool to the touch except for the sending unit. Very hot to the touch. What would cause that?

[formulaben]

1 hour ago, Grundall said:

Replaced the water pump, didn’t help

The raw water pump, or the recirculation pump?

[BlindSquirrel]

What about the trans cooler filter?