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Cummins 270B Thermostat Failure

After 3,150 hours our Cummins engines have never let us down but we had a close call this week.  I've been working most weekends recently and so we haven't had the boat out for about 5 weeks.  Since we're leaving on our annual winter trip this Friday and we've had 45 MPH winds this week and several days of freezing temperatures, I thought I should check the boat Tuesday night.  

Everything looked great in my once over the boat.  I normally never run the engines at the dock believing that starting them and not getting them up to full operating temperature is harder on the engines than not starting them for weeks.  And, we typically have the boat out at least twice and often three times in a month year round so it's really not much of an issue.  But, I did start them this night and the port engine warmed up to 180F as it usually does but the starboard engine was only at 150F.  I figured it was just inaccurate gauges but, to be on the safe side, checked them both with an IR heat gun.  Port was 176F and starboard was 144F.  Clearly there was a problem.

I pulled the thermostat and found that it had failed in an interesting way and I think I understand why Cummins has changing the thermostat designs several times.  This design, delivered in late 1999 engines, has a rubber collar bonded to the outside.  The rubber collar appears to have become partly detached over the last 7 years, jammed in the Thermostat and it now is always partly open.  It can't open fully so would overheat had I taken it out.  And, it can't close fully so won't reach temperature unless under load.  I'm glad I caught it before the trip.

Changing a thermostat in a Cummins B-Series engine (6BTA5.9M) is reasonably straight forward.  Disconnect the battery, remove the alternator, remove the lower coolant line that loops over the alternator at its lower connection to drain the coolant.  Remove the alternator upper bracket where it attaches to the thermostat housing.  Remove the thermostat.  Clean the mounting surfaces with fine emery paper and reassemble.  Ensure that all air is out of the system and it comes to full operating temp at idle and doesn't climb much over 195F at full load.