Mercruiser heat alarm

[Sheriff007]

2002 mercruiser.. V8 260hp alpha..
boat runs good at any speed no problem with overheating unless I throttle to fast. If I increase speed quickly temp gauge goes past 180 and the quickly jumps over 200 and alarm goes off. When I increase speed slow no problem I can get to wot and no problem.. Alarm only goes off on quick acceleration that can be from a crawl to half throttle or half throttle to wot..
any suggestion on what I should be looking at.

 

[alldodge]

You in fresh or salt water?
When was the last time the impeller was replaced?
Did this just start happening, did it do it last season?

 

[Sheriff007]

Its in salt water and I bought boat in February and have had problem since then. I had new Gimble bearing put in last month and mechanic said impeller was ok. I dont increase speed fast to often so I manage the problem for most part but when I do it usually spikes the temp.. Also coming through the inlet it can happen when i am fighting the swells.

 

[alldodge]

Since you just bought it, then it needs a new impeller.

Don't see how your mechanic would know the impeller is ok without splitting the dive. Its an easy enough job

 

[Sheriff007]

Is that the most likely thing with what I described..
could it be the Risers or manifolds?

 

[alldodge]

In my opinion, the impeller is always the first thing to look at.

Don't see it being the manifolds because the motor is running out of water to cool it, an the reason temp jumps so high fast. If it was the mans, water would slowly keep rising and when you slowed down, temp would slowly come back down.

You can prove it to your self by adding a piece of clear hose between the drive intake hose and the thermostat housing. My guess is your going to see bubbles, and when you gun it, you see a lot more

 

[Sheriff007]

If I slow down before alarm goes off temp does come down pretty fast. If I dont catch temp going up alarm triggers and I shut engine off for a few minutes then restart it

 

[achris]

This sounds like air in the water line. Get a section of clear hose and put it in the incoming line. There should never be any bubbles in the hose.

Chris.......

 

[Sheriff007]

If it is air in line how do I get air out

 

[achris]

If it is air in line how do I get air out

It's not about getting it out. It's about finding out where it's being drawn in. THAT will be what's causing the overheat.

 

[Sheriff007]

So took boat out today to feel how hot risers were getting.. Left one was pretty hot at hose connection and temp was a few degrees hotter than normal running at idle or about 10 miles hr 175 degrees.. Im going to put it in this week for impeller and go from there.. Keep u up to date after.
question tho wouldnt riser be hot if impeller wasnt pumping enough water through

 

[alldodge]

One side is always a tad hotter then the other. Yes risers will get hotter with less water flow and so will the motor.

Your doing touchy feely for temp, go get a IR temp gun to get some readings

 

[QBhoy]

You really need to check the impeller as AD says. But usually (not always) a bad impeller will overheat at low speeds as an indication it’s on its way out. Are your exhaust flappers ok ?

 

[Sheriff007]

Trying to learn as I go.. Cant do impeller myself as I dont have a trailer. Im pretty sure I can do risers and manifolds if need replaced but what are the flappers

 

[alldodge]

Flappers are exhaust shutters and are found inside the exhaust tube. If the break off they will restrict exhaust and water flow out. If exhaust is restricted then in most cases there boat lacks power

 

[Sheriff007]

Boat doesnt seem to lack power. Just dealing with the overheating issue.. Like I said it is mostly on quick accelerations.. Almost anytime I throttle up fast temp gauge starts to increase.
im going to call Marine Max today see if I can get it in soon for impeller and go from there

 

[kulle]

Flappers are exhaust shutters and are found inside the exhaust tube. If the break off they will restrict exhaust and water flow out. If exhaust is restricted then in most cases there boat lacks power

Alldodge is 1,0000% correct, it happened to me, severe lack of power, boat was doing a wheelie. when i removed my bell housing, i found this.

 

[Sheriff007]

Picking up boat tmrw after having impeller replaced hopefully that was the problem.. Was told it was in pretty bad shape so we will see.

 

[Sheriff007]

Had Boat out Saturday with 5 people so a heavy load.. Ran great no problem with temp stayed around 175 at higher speeds so looks like it was impeller...

 

[alldodge]

Cool beans :thumb: