Hot at idle and normal temp at higher RPM

Sorrento Dude

Sorrento Dude

New Member
Apr 27, 2009
61
Traverse City Michigan
Boat Info
1990 280 Sundancer
Engines
Twin 5.0
Alpha Ones
My port engine this spring started running hot at idle and would cool down at higher RPM. I changed out the thermostat and it was all good. I could let it idle for 45 mins. and it would never go above 150deg. Just recently I noticed it starting to run hotter again at idle (200deg). I can put it in neutral and rev it up to 1500 rpm and it will cool back down in about 10 seconds. Same symptoms before a change the thermostat out. But then the next day it will run at normal temp. at idle. Is it possible that the new thermostat is sticking?
 
Your impeller may not be pushing as much water as it should be on that side. When did you have it changed last?
 
On your t-stat housing... Is there a "T" looking structure with two hoses attached (one to port, one to starboard)? If so, inside it there will be a ball/spring contraption. Pull it out and stretch out (elongate) the spring about an inch by hand.
 
Something else to look at is the water hoses that bring the water in from the outdrives. I have twin Alpha I's also and a few years back I had the same problem you are having. I did all the things mentioned above to no avail. Turned out my problem was caused by that hose I mentioned. It was a bit too long and over time, kinked just enough that at low rpm it was restricting flow. At higher rpm the pump had enough pressure to force the hose open and it cooled normally. I spent most of the season chasing that problem and spent a lot of money replacing t-stat housings that were not the problem.
Obviously the boat needs to be out of the water to check that hose so I'd wait til you've checked the things the other guys mentioned. While out of the water, a new set of waterpumps in the lower units would not be a bad idea either.
 
Scorpio. I have read similar situations with the inlet hoses too. Could you see from inside the engine compartment where the kink was in the hose? Of course having the boat pulled out is the last resort, but if I does come to that I will the hoses and water pumps changed out on both. I also forgot to mention that my other engine is only running at 120 Deg. So, new thermostats this weekend and more inspections.
 
Lazy Daze said:
On your t-stat housing... Is there a "T" looking structure with two hoses attached (one to port, one to starboard)? If so, inside it there will be a ball/spring contraption. Pull it out and stretch out (elongate) the spring about an inch by hand.
Click to expand...

Bingo...I had this exact issue on my 5.7L two years ago. The issue was the springs on the by-pass valve were fouled with zebra mussels that didn't allow the valve to operate. Since the OP is from Traverse City, it is possible he's looking at the same condition. In the illustration the parts are #3 thru 6.
 
Sorrento Dude said:
Thanks play dough
Click to expand...

My pleasure. Not saying that's your problem, but... The symptoms screamed an impeller problem but when I had the issue, I couldn't believe my 1 year old impellers would be failing. They're changed every 3 years as routine maintenance and have never shown excessive wear. Fortunately, the mechanics I used at the time had likely seen the issue before and had the problem diagnosed in a flash. I was glad I called them in because all the "dock mechanics" would have had me change everything that wasn't broken.
 
Well stretched the springs out and changed out the thermostat again and I'm back to normal temps. The new thermostat that I changed out last month was actually stuck closed. Thank you everyone for the great advise.
 
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