Weird Temp Reading

[NorCal Boater]

OK Guys, I have a situation that has me baffled. Hopefully one of you brainier guys has a suggestion.

The boat is an '89 340 DA with 340hp 7.4's and v-drives and 600 hours. The temp gauge on the starboard engine will be right at 165 degrees and then creep up to about 180 and then go back down. Sometimes it does it at idle speed, sometimes at cruising speed. (2800 rpm)

Today it went to 200 and as soon as it get up it starts to go down. That motor got a new pump impellor last fall and both engines got new 165 degree thermostats. The old ones were 125. Coolant was changed at the time the impellor was replaced. A couple of weeks ago I checked the resevoir and it was low so more coolant was added and I thought that might have been the problem.

Should I change the sending unit? The port engine is fine.

Thanks to anyone that has an idea.

 

[comsnark]

Closed loop system? Almost sounds like there is air getting sucked through the system. . .but others will have more intelligence than I.

Personally, I *doubt* the sender is the problem. If you need to verify, you can shoot the temperatures with an IR gun while running.

 

[fwebster]

This sounds like a bad thermostat, but since you already changed them, have you replaced the thermostat housings? There is a bypass spring and ball valve in the housing that can malfunction when it gets some crud in it.

 

[NorCal Boater]

Thanks guys, I will take a look.

 

[JAndrewG]

Shawn,

I would verify that the stbd side engine is indeed getting warm. You could use one of those point-and-shoot thermometers (or whatever they call them), checking both sides while your bride runs the boat.

I bought a 300WE new in '87. The stbd side always gave different readings. Just before my batteries were old enough to change out (they wouldn't hold a charge for long), I had the high temp alarm on the stbd side go off. I slowed to an idle and lifted the hatch to see how hot it was. It wasn't. Baffled, I returned to the marina.

After I changed the batteries, this didn't happen. After much head scratching, I found that it was an electrical thing. The stbd side is the house side, and carries the house load. Apparently, the stbd alternator was sending a ton of voltage to the weak batteries on this side, causing the temp to read high. One of the clues was that the oil pressure guage was giving a high reading as well.

A few months after I sold the boat, the new owner called me and asked if I'd ever had a heating problem. I said no, and then remembered those false alarms. He got kind of sheepish- seems he already had the heads off the stbd motor and the mechanic couldn't find anything wrong. I told him when I put new batts in, the problem went away. He asked that if his wife ever called me, please don't tell her this story...

Might be worth looking into...

Good luck!

 

[velooce]

I had the exact same situation and the engines read exactly the same with the Infer-red gun. My gauge was the problem.

 

[NorCal Boater]

Thanks guys. I will buy the gun. When the Admiral says why, I just tell her I don't have one.

 

[brimanst]

I have had a bad gauge once and this happened, but most recently, it was just an air pocket in the cooling system. That is the easiest and cheapest to check first.

briman