operating temp not steady

[tr220]

I have a new to me 2006 220 with the VP5.7Gi-E and duoprop. The engine seemed slow to reach operating temp and the temp would not stay steady. It would warm to about 165-170 and then fall clear to 120. This cycle would continually repeat. I changed the thermostat thinking that it was most likely the culprit. It did seem to warm to temp quicker, but it does not remain there. It falls to 130 or so and repeats. Occasionally it will only fall to 150 or so.

I have also changed the output hoses on both the seawater and circulating pumps, as well as the impeller.

My previous boat was a Sea Ray with a Mercruiser 5.7. Once it reached operating temp it remained there like cars and trucks do. I am new to the Volvo Penta engines. Is this normal? It would seem that the engine needs to remain at temp so that the ECU can do its job better. If the temp falls that much I would think the mixture would be richer. My main concern is that it may be indicative of another issue.

Any ideas? Anyone have a similar experience?

[cmattj]

The fact that the temp falls to 120 is odd. As besides ecu enrichment , the oil temp remains low and moisture won’t get a chance to be evaporated.

Go boil a pot of water with a meat therm to drop your old thermostat in and check it’s open
Do you have a infrared heat gun to see temp at the therm housing?
That would be a start as I’m sure your lake fresh water is high now.
Let’s determine true temp first then we will wonder about the gauges and senders .

Hey at least it’s not overheating right?

So get us some pics and a report!

[Big Block Power]

Right at least it's not over heating. Too much water. Lol
Just kidding. Yep you really need to see the real temp. What is the ecu seeing? If you have a 165 Stat it will go up to that then open and cold water will go past that and lower the temp. How cold is your lake water? Then if it's that cold Stat costs to it warms up again and then opens. In the winter here we will see our cars do that. Newer cars are much better but the good old oldies I could watch my Stat working like a charm. No mine does not do that to that extreme. I have a 140 Stat and it's always on cold.

[tr220]

Sorry I haven't posted a follow up. It has been a crazy year. Here is the Paul Harvey version. I mentioned in my first post that I had replaced both pump output hoses. What I didn't mention is that both were replaced after splitting while we were out for a cruise. A great way to instill confidence in a new boat. I knew that simply replacing two hoses that split on nearly back to back cruises was not the final solution. I had begun to suspect sabotage. What it turned out to be was the thermostat housing. At some point in the past the impeller was destroyed and they failed to clean out all of the debris which was tightly packed into the thermostat housing. A new thermostat housing solved the unsteady temperature issue. It never fluctuates at all now. And the hose failures were caused by over pressurizing the output hoses. I have had no further issues with cooling.

Thanks for helping debug the problem. I have found several helpful posts here and I wanted to post my experiences and solution so someone else might benefit.

[akorcovelos]

Thanks for closing the thread out! Glad you’re all fixed up!