Engine temp fluctuates - RWC 350 MAG MPI

[RonF]

I just had some engine service which included replacing thermostats and all water hoses on both raw water cooled V-drive engines. The thermostats are rated to open at 160 deg. Now the port engine won't hold a constant temp at low RPMs. The gauge varies back and forth between 170 and about 185 at approx 1 minute intervals. When shooting an IR thermometer at the top of the water manifold right over the thermostat, the reading varies between 95 and 105. So, it's not just the gauge, even though the gauge may be reading high. The mechanic put in a second thermostat, and the same thing still happens. I heated the first one in a pot of water, and it started opening at 160.

Any Ideas?

 

[Alex F]

How're your water intake stuff (scoop strainer, seastreiner, impeller)?

 

[ardeprint]

When does the described fluctuation happen? While idle or under power?

 

[RonF]

>> How're your water intake stuff (scoop strainer, seastreiner, impeller)? <<

The boat was just launched after service. Scoop and sea treainer clear. Impeller is a year old, with about 40 hrs on it. Exhaust water discharge is normal looking, as good as the other engine.

>> When does the described fluctuation happen? While idle or under power? <<

I'm going to say it's at or around idle. We've been at cruise RPM a few times since the service, and I think the temp stabilizes around 175 deg. Will try to get another day on the water before Emily gets here.

 

[ardeprint]

If at idle, I would look at the impeller. A year old impeller could have you running on an a weak impeller that pumps less pressure at slow speeds. Second, air in your circuit entering at a joint, cracked or broken hose.

A simple test: put a bucket to receive the expelled water from your exhaust. If you are getting less water from the problem engine than your good engine, its the amount of water your are pumping (impeller, blockage, etc...). If the amounts are equal, then you have to look at thermostat or a more severe issue like head gasket (doubtful!)

 

[RonF]

thanks

 

[Alex F]

I'm not sure what service was done, but something to keep in mind is when you change the impeller is the presence of all vanes. If you have some that came off they'll be blocking the water flow wherever they'll make their way and get stuck there. You can have new impeller, but until the lost vanes are recovered it won't make any difference. This is why it's recommended to change the impllers annually or at least every other year.