Temperature fluctuates and alarm

[jhlasher]

Have a 2001 mercruiser 4.3L. The temperature fluctuates and alarm goes off. Put in new impeller this spring, and just changed the thermostat but still does the same thing. Temp. goes from a normal 175 to about 190 than suddenly goes to 120/130 and than back to 175ish and again to 190, all while going at full throttle. When running at idle stays constant at 175. If there is a clog or blockage why would the temp drop to 120 or so? Any suggestions what to look for? Since the temp readout and alarm are separate sensors kinda rules out problem with them.

[mmwjr]

Welcome aboard

I agree seems real as two separate sensors. None the less have you taken readings with an IR temp sensor? T-stat housing and exhaust.

I would think a clog would not fluctuate. The biggest concern to me is the 190 which is too high for an open cooling system.

Do you boat in salt water? How old is the exhaust?

Has the bellows and water hose of the transom assembly been replaced? Concern is the hose. Old hoses can delam in side and collapse under suction.

[jhlasher]

Strictly freshwater. Never replaced exhaust. Not sure what exhaust you are talking about. Has only 500 hours on boat and motor. When I did the impeller this spring the bellows and water hose looked good. At least on the inside. Another thing is when I turn corners at high speed the temperature drops off a lot. I am baffled.

[mmwjr]

The exhaust manifolds and risers are water cooled and on salt water boats they need to be replaced about every 5 years due to corresion.

At what temp is the alarm sounding? Guessing when it its 190 which is normal.

How often and long is it 190?

How often and long is it at 120?

What is the temp of the water you are in?

Just to clarify this was happening prior to changing the impeller and t-stat, correct?

[jhlasher]

Thank you for your reply.
The gauge reads about 180 when the alarm goes off. It stays on even when the temp goes to 120. The other thing is that I hear a popping sound when this is happening. Like a misfire but could be something else. The alarm will stay on even when the temp is fluctuating from 180/190 to 120. It will stay at 120 for about 10 sec than go back up. I do notice that when I make a turn the temp drops about 30 deg.
I changed the impeller this spring just for normal maintenance, and had been running good for about a month. Just started this temp fluctuation, and I changed the thermostat hoping that was the problem, but didn't have any effect on the problem.

[mmwjr]

Ok only being at 120 for 10 seconds tells me it's not real as the mass of the engine cannot loss/gain thermal heat that fast. I suspect it's so hot that air pockets are forming near the sensor which throw off the reading.

IMO you have a water flow issue and the engine is overheating only.

As I stated before use an IR temp sensor and get real reading on the engine and exhaust. You can also try removing the hose from the water pump at the t-stat housing to check flow. Should pump water out the hose to shoot across the boat.

The engine circulating pump may also be bad, the impeller on the pump.
However this is usually a salt water issue.

[jhlasher]

Mike,
I tend to agree with you about hot air pockets. I am going up to the lake today and will try disconnecting the water hose to the thermostat housing and see what I get for water flow. I am trying to locate a IR sensor to check the temps. Will let you know what I find.
Thanks for all your input.
John

[jhlasher]

Mike,
Just got back from lake. I am further confused. I took it out twice and got the same result both times. First I sat at the dock and let it idle for a long time. Temp stayed steady at 175. Put my hand on thermostat housing and felt like maybe 150. If it was truly 175 I would not be able to hold my hand on the metal. Revved it up and left it at 3200Rpm for a while and still stayed at 175.
Took it out on the lake and cruised at 3500 rpm for awhile. The temp went down to about 120 from 175. After about 10 seconds at 120 the buzzer went off. I idled back and the gage went back to 175. I touched the housing and still around 150. The buzzer stayed on. Disconnect the temp switch and buzzer stayed on. I than disconnected the oil pressure switch and the outdrive oil level switch and the buzzer stayed on. These are supposed to be the only things that sound the alarm. Shut off engine and restarted. Buzzer quiet.
After awhile I did the same thing and got the same results. I never saw the temp gauge go above 175.
I am puzzled why the buzzer stayed on. Also puzzled why the temp is going down to 120.
I did disconnect the water hose to the thermostat housing and started engine. Ran at idle and water came out about 2-3 inches. Revved it a little and flow increased. It is a 1.5 hose. You mentioned it should shoot across the boat. That would be a lot of water pressure.
Anymore suggestions?

[mmwjr]

Cannot explain the alarm sounding with the sendors disconnected.

It only acts up under way, sounds like the water hose on the outer transom assembly may be leaking or you do not have a proper seal from water raw water pump housing to the water tube in the upper half of the drive.

[jhlasher]

I was trying to avoid pulling it apart to check the impeller etc, but looks like that is the next step.

[jhlasher]

I fixed the problem with the temp fluctuations. It was exhaust being sucked in by the impeller once I got over 3000RPM.
What I found when I pulled the water pump apart was a hole between the exhaust passage and the water inlet to the pump. The hole was from corrosion. Cleaned the area real good and put some EP200 epoxy putty on it.
Tried it out today and ran great for over 2 hours of running, pulling the grandkids in there tubes.
Thanks for all your input, it was very helpful.
John

[mmwjr]

Originally Posted by jhlasher

I fixed the problem with the temp fluctuations. It was exhaust being sucked in by the impeller once I got over 3000RPM.
What I found when I pulled the water pump apart was a hole between the exhaust passage and the water inlet to the pump. The hole was from corrosion. Cleaned the area real good and put some EP200 epoxy putty on it.
Tried it out today and ran great for over 2 hours of running, pulling the grandkids in there tubes.
Thanks for all your input, it was very helpful.
John

That makes sense but that's a new fault for me. Got any pictures?

[Moose]

Have you tried a laser thermometer to test actual temperature of water hoses and exhaust while cruising at least you could tell if the gauge is reading correctly