Intermittent Overheat 4.3 MPI Closed system?

[Intruder 4.3]

2004 Mercruiser 4.3 MPI Alpha 1 drive Engine serial number OWOO7177 - model -MCM4.3L MPI
Drive serial number OL9566Z3
- Last couple of times out I have had the alarm go off for over heating (above ~200 degrees). This is a closed system with the through hull intake. Seems to over heat under a greater load. When I turn the motor off and restart the temp seems to drop as if the thermostat is intermittently sticking. The temperature range I am seeing is anywhere from 160-200+ which seems to wide. Last season I replaced the fresh water pump and thermostat so I am relatively sure this is not an impeller issue. My first thought is to replace the thermostat, as it is a cheap fix. I assume this a 160 degree Thermostat? Do thermostats fail intermittently? Any other suggestions on what could be causing?

 

[harleyman1975]

Do you have seawater strainers? Could be a sticking thermostat. Check the hoses and coolers/heat exchangers for blockages/debris. Make sure coolant level is up. Just someplace to start looking. Your time is free...parts are not.

 

[Intruder 4.3]

Where are the sea water strainers normally located? I assume low in the system but I have not had this apart below the seawater pump.

 

[Scott Danforth]

pull the end caps off your heat exchanger and clean out the debris. you should be doing this every year.

 

[tpenfield]

Is this a FULL closed system or a HALF closed system?

I'm thinking a slight restriction that is showing up at higher speeds and not able to handle the heat . . .

Or a head gasket issue that is filling the cooling system with exhaust gasses at higher speeds

Did the issue come on suddenly or work itself up to the point where it became an overheat situation?

Is there anything that you can tell us about the coolant reservoir when it overheats?

 

[Intruder 4.3]

This is a half closed system. I pulled inner tubes all weekend with no issues then on the way back to the dock with a full load (boat full of camping gear dogs and kids) the alarm sounded. I shut the motor off, there by relieving the pressure and restated and the temp quickly came back down to normal range. The more I think about it, it does sound like a blockage issue. The reservoir is at normal level and does not move significantly one way or the other. I will pull the end caps tonight and see what I find.

 

[Intruder 4.3]

Removed the end caps last night and found very little debris in the starboard side. The port side was a different story. Removed a hand full of crap including rocks, tape, and found the remains of my old impeller that had disintegrated 2 years ago. At the time I was perplexed as to where the remnants of it went, so mystery solved.
There were some small rocks that had lodged themselves into a couple of the tubes, do you have a suggestion for installing a screen to prevent this from occurring again. I often beach the boat and am at high risk of sucking sand and rocks even though I try to wait until I am well away from shore to start the engine it is sometimes unavoidable.

Thank you for pointing me in the right direction.

 

[harleyman1975]

That is what seawater strainers are supposed to do. they are part of the water pick up and serviced from inside the boat. make sure to close seacock before opening the cap. ours had a replaceable cover with a rubber gasket/O ring. strainer was stainless steel. sucked up part of a plastic bag once.

 

[Scott Danforth]

I would pull your power steering cooler as well. only items that made it thru the power steering cooler made it to your heat exchanger. unfortunately on I/O's the power steering cooler is the sea strainer as the holes in the leg pickup are larger than the passages in the power steering cooler and the heat exchanger

 

[harleyman1975]

This is a closed system with the through hull intake.

 

[tpenfield]

I would backflush the Power Steering cooler as Scott suggested, as that is one of the first orders of business in diagnosing/correcting sea water flow.

 

[Intruder 4.3]

I had previously cleaned the power steering cooler when I replaced the fresh water pump and I did recover a few pieces of the impeller. This is an easy check however and I will be taking a look tonight. Again I appreciate all the knowledgeable input.

 

[Scott Danforth]

This is a closed system with the through hull intake.

I understand that, I also know who designed the heat exchangers and what is in each section of the raw water path, including the ID of the internal tubes in the bundles and the diameter of holes in the strainer.

 

[alldodge]

do you have a suggestion for installing a screen to prevent this from occurring again. I often beach the boat and am at high risk of sucking sand and rocks even though I try to wait until I am well away from shore to start the engine it is sometimes unavoidable.
.

So long as you beach the boat you have problems waiting to show up. Only way to install a screen is to add a sea strainer as mentioned previously. Finding pieces of previous impellers means system was not cleaned out from another impeller replacement.

Suggest even if a sea strainer is installed, the impeller is replaced every year, and check the wear plate.

 

[Scott Danforth]

Heat exchanger ends need to come off every year for inspection. PERIOD

 

[achris]

Install a seacock immediately after the water pickup, then one of these between the seacock and the raw water pump.....

Chris.........

 

[Intruder 4.3]

While on the subject, I had to replace the original fresh water because it had significant wear (grooves) on the bottom and top side. Before replacing ($600+) I milled the top flat to try and recondition, but the bottom grooves continued, I believe, to cause the pump to not prime. Is there a way to recondition the bottom? AllDodge mentioned a wear plate, what is this?

 

[alldodge]

The wear plate comes as part of a kit for the Gen 7 pump. Its a stainless plate and gasket to place against housing.


Don't know of a fix for the inside of the pump. It still gets me why they even went with that style pump, lots of folks are having trouble with them. The style pump does allow for single point drain for winterization if equipped.

Hardin marine makes a solid stainless steel pump, but note, if you use this pump it uses the older size impeller. Explains on their site
http://www.hardin-marine.com/c-132-mercury-sea-pumps.aspx