www.iFourWinns.com

I don't really understand this but here is the situation. I have twin 5.0GI volvo pentas. Two weeks ago I noticed my port side engine was running on the cool side,  around 120-130.
I picked up a new thermostat and replaced the old one. On my test run I noticed on idle it seemed to be climbing up just like the starboard engine, but as soon as I start to bring up the RPMs and get on plain the port side temp drops back down to in the 120s. I did notice a piece of rubber in the housing around the thermostat so I think the impellers are ready for a changing but that doesn't explain why the temp drops. I did pick up some new impellers so I'll do that on the weekend.
 Any ideas why its running cool? And would running cool effect gas mileage?

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2001 298 Vista
Lagoon city lake Simcoe

Pull out the thermostat again. There is probably part of your impeller stuck in it again, keeping it open.

You and I have the exact same issue, I've replaced the thermostat and the impeller, and it happens to be the port engine as well.
Before replacing the thermostat, it would never get up to temperature, and hung at 120. After replacing it, while at idle it reaches normal temperature of 165-170. The instant the throttle goes down, the temperature drops back to 130ish and stays there. It drops so fast I can't believe the engine can physically drop in temperature that fast.

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1969 Chris Craft Commander 47
1999 Four Winns 328 Vista - SOLD

It may not matter, but with both of you. As this a closed system or open.

Good point Paul. If it is FWC, the impeller is on the raw water side and wouldn't interfere with the thermostat. If not, it's a possibility.

Sticky thermostats are much more common with raw water cooled systems which after all is an 'open' system, prone to corrosion and sand going through the system. If you had a closed cooled engine I bet you would never or hardly ever, have this problem. I have not had to replace a thermostat in any of my cars, in over 35 years, in fact I don't remember the last time I had a sticky 'stat in a vehicle. That's the difference between raw (open) and closed cooling. Honestly with inboard and I/O powerplants, they all should be closed cooled. The engines were designed to be in an auto application with antifreeze/water running through them and run at 195*, not salt/brackish water and run at 150*. To me that is one of the few advantages an inboard/I/O has over an outboard, the abiity to have closed cooling. Sticky 'stats and corrosion in the cooling passages is a big issue with outboards too....

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88 Four Winns 200 Horizon
4.3 OMC Cobra-4bbl
2002 Walker Bay 10/2012 Suzuki 2.5
2008 Walker Bay 8

1998 Jeep Grand Cherokee 4.0/Selectrac
2007 Jeep Grand Cherokee 5.7 Hemi/Quadradrive II

Yes mine is an open system, but I don't run in salt water. Once I change the impeller and clean out any bits I will report back with findings.

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2001 298 Vista
Lagoon city lake Simcoe

My water heater is tied in to my port side engine. I wonder if that could cause any of this.

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2001 298 Vista
Lagoon city lake Simcoe

Can't see how it would drop temp with higher RPM's. Plus once the water is hot (mine always is since it's plugged in at the dock), it wouldn't sap much of the heat away.

I'm going out tonight, if I get a chance, I'll stick a temp probe on the thermostat while accelerating to see if the engine really cools that fast to rule out electronics issues.

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1969 Chris Craft Commander 47
1999 Four Winns 328 Vista - SOLD

That's great weather, I'm going to mess with the sensors aswell see if I get any changes. Maybe I'll try swapping them or something.
Are you noticing any difference in your gas consumption since yours has been running cool?

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2001 298 Vista
Lagoon city lake Simcoe

Depending on the sending unit, if there is an air bubble, or a blockage in the system it will caluse the sender to get an incorrect reading (air instead of liquid). A simple automotive base test would be to buy some temperature strips (1-2bucks each) and attach them to the heads/intakes on both motors and compare the external temperatures, what may appear like a failure could be a miss reading.

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2000 H190 5.0GL

So I was going to change the impeller and found the the old one was still like new, so that is not an issue. I took a look at the thermostat again and it also looked fine. But this time on this run I brought a inferred thermometer and was able to find out that the port side engine is physically running cooler.
So now what?

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2001 298 Vista
Lagoon city lake Simcoe

If you have ruled out electrical/gauge issue by confirming the temp via IR thermometer it is almost 100% still a thermostat problem. Whether stuck open due to a piece of something in there, defective T-stat, defective housing (unlikely), or installation issue (thermostat shifted sideways maybe) it is the ONLY thing can cause the engine to run cool. By design it will start to open at temp, be fully open at 20deg above, and close 10 deg below the temperature rating (all rating approximate of course). There is nothing else in the system that could cause you to not hold temp.

There are lots of things that could cause an engine to overheat, but this is not the case with running too cool. I would check the t-stat again, and perhaps even test the new one in a pot of boiling water for the temps above (you tube has plenty of examples). Examine the housing to make sure there is no way water can pass at pressure. Is the rubber o-ring that holds it in good shape?

If you have not checked since finding debris it is possible that more debris have clogged up the new one too. If you are at least getting to 120 it is probably not stuck completely open, but definitely allowing something through.

If it were I, this is what I would focus on even if it meant having to pull the housing again and installing another new t-stat to confirm no issue is present.

You gas mileage will suffer with an engine that does not come up to operating temp. To what extent I am unsure.

If you have enough slack in the hoses this link may provide a possible diagnosis solution http://www.coolviewthermostat.com/info/ , it is not uncommon to have out of the box thermostats that are not functioning properly, they can be place in water and brought to temperature to verify when they open with a thermometer, your infrared gauge may work also just check to see the temperature the thermostast opens it should be the specified tempurature of your thermostat.

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2000 H190 5.0GL