Overheating motor

[Chapparall280]

First let thank anyone for the help.
I have a 2002 Chaparrall 280 with a mercruiser 5.0 and alpha one gen 2 drives. It's a carb motor and has approx 600 hours.


I have what appears to be an overheating issue. The alarm sounds and my temp gauge acts a bit odd during this time. Goes from 160 which it stays at steady as long as I keep it below 2000 rpm. Ten seconds after I get up on plane the temp goes up to 180 then will oddly drop to 120 then stay there or come back up to 200. I shut the motor down or pull back to neutral and the buzzer will go off and temp will go back to normal I can run day at 2000 rpm just putting along.

I have read many threads on different overheating and corrected several possibilities

I have replaced
impeller and housing,
replaced risers , manifolds , Elbows.
-replaced the Thermastadt, sensor and sendor and ran without thermostat
-tested water flow to thermastadt at idle hood water flow
heat temp gun and although this motor gets 15 degrees warmer then my port motor it has the alarm sounding when the thermostats housing is 180
move also checked power steering cooler line and the exhaust bellows appear clear

I will test head gasket but no reason to think they are bad

was goong to look for air in the line by connecting a clear hose To intake line but assuming I do see bubbles at higher rpm's. Where else could air be coming from. Again not sure if their is air in the line but what else could
it be. Saw someone suggested the hose that comes from drive thru transom could collapse at higher RpMs. although others say unlikely as the water pressure is expanding the hose.

I appreciate any direction as can't use the boat like this and all mechanics are 3 weeks out and in the north east short summers. Thanks

 

[alldodge]

What's the motor serial number?

Get a short piece of clear hose, double male barbed fitting and 2 hose clamps from the hardware store. Disconnect the hose coming from the drive where it connects to the thermostat housing and install clear hose.

With boat in the water see if there are any bubbles. If you see bubbles then there is a leak between the intake holes of the drive up to the thermostat

If no bubbles, remove the hose and point it toward a 5 Gal bucket. Start the motor and it should fill the bucket in about 5 seconds or so

 

[Chapparall280]

0m328896 Yes will try to install the clear line and see if their are air bubbles. Will bubbles be obvious in the line. ??

I did take the line off and pointed it into a bucket seemed like good water flow. Of course was at low rpm now high since wouldn't want to run it with no water flowing to motor at high rpm. Attached clip

 

[Chapparall280]

See attached

 

[alldodge]

Yes low rpm with test
Started flowing at 3 sec and stopped at 17 (15 sec total) and only got about 2 gallons, I'm thinking that isn't near enough

 

[Chapparall280]

Yes I did place a garden hose on the line disconnected from Thermastadt that runs to drive and water seemed to flow thru it Of course based upon flow coming in Think a garden hose produces less water flow.

 

[Chapparall280]

I will note that water test was before I put a new impeller and housing on yesterday

 

[Chapparall280]

I will repeat the test this weekend and see if more flow

 

[alldodge]

Retract the flow test comment. Looked in the manual for Gen 2 and the minimum is 1 Gal at 1000 RPM and you were idling and got more then that.

Have you checked the exhaust Y pipe?
The Y pipe will have exhaust shutters to keep water from back flowing into the motor when stopping fast. If these broke off they will restrict water flow leaving the exhaust causing a back up

 

[Chapparall280]

Ok thanks on flow. So we think flow is ok at least at idle

Yes I did peak a camera in there and again having no expertise it looked like the flaps were good to me ( see attached )

 

[alldodge]

Yes flow is good
If this isn't a new to you boat then flappers are also good

 

[Chapparall280]

This is fourth summer with boat. First time having this issue.

 

[alldodge]

ok all good, so no worries. My guess is your sucking air somewhere

 

[Chapparall280]

Also u happen to know size of the clear hose and connections I need if go to hardware store

 

[alldodge]

Should be 1.25 inch ID
Pull the hose off and measure inside diameter

 

[Chapparall280]

Ok thanks

 

[QBhoy]

Hi. All great advice above. Most will know by now…I like to come up with the odd oddball answer to some questions, haha.
So here goes. By any chance, do you have. Domestic water system on the boat with hot water tank ? If so…is it heated by a feed off the engine (teed in at circ pump and upper section near t stat) ? If so…is the engine that has this issue, by any chance, the engine that provides a feed to the hot water immersion tank ? Just a thought.

 

[Chapparall280]

I am open to odd thoughts. And yes has dometic with hot water tank with feeds from top motor. Unfortunately that's not the motor with the problem. As I was reading I was like yes yes and yes but then of course realized it's on the other motor but thanks for the idea. Def a place to check if issues in that motor.

 

[Chapparall280]

ok all good, so no worries. My guess is your sucking air somewhere.

I am def getting air in the line. Without even taking the boat out I could see air in the line when rpm taken to approx 2000 or little over. When backed down to idle still had mostly air but when bumped to 1500 rpm had consistent water flow. When went up again large amounts of air not just bubbles but air sitting there. So my question is where all can I look that it could be sucking air in ?? I had thought since drives are under water and not on plane does that eliminate the chance of the air coming from the lower unit as I would think all areas would be filled with water. ( I may be wrong with that of course.). Just trying to determine where to look. Thanks again

 

[Chapparall280]

:
I am def getting air in the line. Without even taking the boat out I could see air in the line when rpm taken to approx 2000 or little over. When backed down to idle still had mostly air but when bumped to 1500 rpm had consistent water flow. When went up again large amounts of air not just bubbles but air sitting there. So my question is where all can I look that it could be sucking air in ?? I had thought since drives are under water and not on plane does that eliminate the chance of the air coming from the lower unit as I would think all areas would be filled with water. ( I may be wrong with that of course.). Just trying to determine where to look. Thanks again