Perkins T160 running cool

We have twin Perkins engines on our 44' Gulf Star trawler. I noticed that the port engine was running significantly cooler than the starboard last summer and followed up on suggestions obtained from this site. Underway at about 1800 rpms the starboard sets at about 180 degrees on the gauge and the port about 130. We have recently had the heat exchangers on both rebuilt and the port thermostat removed and "bench tested." It worked properly. Today in the slip I ran the engines about 45 minutes with the following results: gauges (both lower drive station and bridge): port about 130, starboard about 170, laser gun: port 127, starboard 168. Otherwise, both engines seem to run a sweet as cream. Would appreciate any suggestions.

Recheck the thermostat or just replace it. I can't think of anything other than a bad thermostat that would make the engine run that cool.


David

Yup!! It's either the gauge or the thermostat and you've already proven that the gauges are reasonably accurate. It sounds like it's opening ok, just not closing down enough to keep the temp at 180. Change out the thermostat.....

If you have a water heater attached on the port engine it may be allowing flow to bypass the thermostat.

Thermostat. Unless it's on a Cummins, they are cheap. Get a spare.

Also make sure it is not installed upside down !

As an update

Had rough time locating proper replacement thermostat. Got one at NAPA, didn't fit. Geneva Marine on Shelter Island finally got to the bottom of it. Perkins changed the size of the coolant inlet design in 1980 and stopped making the earlier thermostat opting instead to sell a smaller skirted thermostat and offer an O-ring so the newer one fit pre-180 engines. Installed same. Disappointed. Both lower and upper gauges still showing engine running 120 -130 after 30 minutes. BUT, check with infrared gun revealed engine actually running about 175. So, thermostat was bad and both gauges wacky. Took 4 hour cruise today checking engine 5 or 6 times. Tight at 175 - 176.
Thanks for all the suggestions and remember "its the thermostat, stupid."