Thermostat Issue

T

TMK988

New Member
Sep 17, 2009
118
Lloyd Harbor, NY
Boat Info
290 Sundancer 2006
Engines
Mercruiser
I replaced both my thermostats (Bravo 3's) because the engines were running cold (109 degrees). After about an hour of running, the port side engine is running cold again! Anyone have this kind of an issue before. Could this just be a bad thermostat (how often does that happen) or is a failing thermostat indicitive of some other issue?
Thanks.
 
Check you gauge for accuracy and you sending units. Not sure if it is even possible run at 109 degrees. ?? What is it up to when actually cruising and RPMs are higher?
 
Have you been running in mucky water and stirring up silt? Sand can cause a t-stat to stick open.
 
Thanks, but no, I really drive this boat very conservatively and am paranoid about getting in shallow water. And, there is barely an hour on that thermostat. I'm going to tap the housing with a hammer tomorrow, see if that does anything. I guess changing it out again isn't that big a deal, I hope this is really nothing more than a bad unit. I appreciate the thought...
 
Oh, and to Silverspoons thought, the boat was just idling. I hadn't driven it for a week and as I am on a bouy and by myself (I haven't tried to hook up alone yet) I just cranked up both engines to let them run for a while. The starboard engine worked its way up to around 160, but the port stayed at 104 initially, then I shut it down and started it up and then it went up to 109. I did rev the motor up to around 3k, but that really didn't do anything. I'll try actually running the boat tomorrow, althought I am uncomfortable running that port engine cold. Thanks for your comment.
 
My friend changed his thermostat and re-assembled it incorrectly which caused the same problem. He was sure it was a bad thermostat and I saw he placed the thermostat on the intake manifold (5.7 litre) and not inside the housing with the plastic insert holding it in place. He did not realize there was a cork gasket on top of the thermostat also. You may want to double check the manual and verify the assembly is correct.
 
Thanks fellas. I am going to disassemble it today and see if there is an easy explination. Maybe I'll get a new thermostat to be safe, they really aren't very expensive.
 
Running it cold will not hurt it at all. You will be running a bit rich and it will not be as efficient, but it will not hurt to test. If the thermostat is stuck open, you should eventually run up to at least 160 degrees or so. If you run it at 3000-3500 RPMs, (under a load, not just revving it up), and the temp goes to 160 or so, then it will be a thermostat. If it stays very low, then it is a sender or a gauge. Either way, your not hurting it.
 
Right, not hurting to run it cold, just inefficient. A thermostat is normally closed. So your brand-new thermostat you installed would not be stuck open already. And if you installed it upside down, you wouldn't have cold engine problems.

Sounds to me, like others, the gauge and/or sending unit (if electrical).
 
Yikes... here is an update. I went ahead and put in a new thermostat. It worked just fine. Then, on my short run home, the other engine, after running at around 160 for close to an hour, suddenly wouldn't go above 112! And... I couldn't get the motors to rev over around 3200 rpm and I couldn't get going faster than 20 mph.

Question, is there one sensor for each engine? I am really stumped here.
 
Yes - separate sensor on each. You really should get an IR thermometer and start checking these readings.
 
You could be having some kind of electricl problems.
Get the IR thermometer on it to see what you have. Compare from the same spots on both motors.
 
MLauman said:
Yes - separate sensor on each. You really should get an IR thermometer and start checking these readings.
Click to expand...

Bingo!

When you changed to the second set of T-stats, did you notice any granules caught in the seat? I know you mentioned you are cautious about shallow running, but it doesn't take much to cause one to stick open.

Also did you clean any old gasket material off the mating surfaces? I didn't notice what engines you have, on 350 Mags, there are brass contacts in the gasket that must make contact between the T-stat housing and the block. There is also a black collar that holds the T stat in place that has a alignment groove that must line up before it will go together properly. I have seen an exploded view that shows a seal that rests on top of the T stat that contacts the housing...but neither of my engines had them, and mine run at 160....
 
Thanks so much fellas. And to Stray Cat: There really wasn't any gritty material in the thermo. case when I changed both of them. I was extra careful cleaning off the old gasket before putting on the new one. I also used a gasket sealer. I was also careful with that flange thing, which really doesn't even go in properly if that guide you referenced isn't lined up correctly. So... I guess using that external temp. guage is the next step. Owning a boat now is like owning a car in the 1940's... seems that we have to keep on top of so many things. Thank God for you guys!
 
Bingo Boatrboy! I had no idea that there was another gasket in there. Now I'm going to have to take apart the other one as I didn't replace that one either. Thank you all so much for your guidence.
Sincerely,
Tom
 
TMK988 said:
Bingo Boatrboy! I had no idea that there was another gasket in there. Now I'm going to have to take apart the other one as I didn't replace that one either. Thank you all so much for your guidence.
Sincerely,
Tom
Click to expand...
So was that the solution for the other motor as well?
 
Well, I thought so. All was fine for an hour or so of running, how that very same engine is running cool again (around 105 degrees)! I'm actually going out today and am just going to drive the damn boat (slowly) and deal with this later. I just never heard of guys having cold running issues rather than hot issues.
 
Did someone suggest an IR thermometer!? That will help pinpoint the issue, instead of just guessing...or ignoring the potential problem.
 
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