4.3MPI thermostats keep sticking open, cold running

[flyboycj84]

Hello all.....no longer have my 97 SR185 with the Merc 4.3GenLX, and have moved up to a 04 Chaparral 270Sig with twin Merc 4.3MPIs. Having a small issue that I never encountered on my old 4.3. After a weekend of boating a few weeks ago, both engines were running cold (130 or less) on the way back from the island. I figured I sucked some island up and I replaced both thermostats figuring it hadnt been done in awhile from previous owner. Worked great for a few outings, and then happened again Memorial Day weekend.....but this time I was more cognizant of not being in too shallow water......literally one engine went cold on Saturday, and one on Sunday. Took both thermostats out, yes, they were stuck open.....boiled em with a thermometer and they opened up and closed as they are supposed to. So I put em back in and the boat warmed up fine. About 2 or 3 trips later....same thing. Cruised out of canals at idle for :45, got to ICW and ran at cruise for awhile.....and sure as $hit a few minutes after idling through the locks.....both engines cold again. Took it out to ocean and ran it up.....they warmed up enough to open up and work properly. But on the cruise back through the canals....they seemed to cool off again. Not quite the 130 or less like before, but not the steady 160 that they're supposed to. WTF is this all about? My old 4.3GenLX I had for 7yrs never had a thermostat problem, even when I would beach it on the island? Is there a chance I got 2 bad ones from Merc when I replaced? Im getting tired of pulling tstat housings and buying gaskets!!!! Happen to anyone else?

 

[81 Checkmate]

Id rather have a cool engine instead of a hot one......Maybe your gauges are off a bit...can you check with a IR gun to be sure. Also could be the water temp is alot cooler than when you had your other boat.

 

[flyboycj84]

Yes, cool is better than overheating. I have not checked with an IR gun. But both gauges going from 160ish to less than 130 almost at the same time? dont think its a gauge issue.....when I pulled the tstats they were both open a few millimeters until i put em in a pot and boiled em.....then whatever was in there loosened up and let the tstats close. the most recent time i was out though, i do not believe I was in any shallow water at all....at least not shallow enough to stir up the bottom.

 

[81 Checkmate]

Yes, cool is better than overheating. I have not checked with an IR gun. But both gauges going from 160ish to less than 130 almost at the same time? dont think its a gauge issue.....when I pulled the tstats they were both open a few millimeters until i put em in a pot and boiled em.....then whatever was in there loosened up and let the tstats close. the most recent time i was out though, i do not believe I was in any shallow water at all....at least not shallow enough to stir up the bottom.

Well thats a good thing...both reading the same. You could have some rust or debris in the motor thats holding it open........but i wouldnt think both motors would act the same at the same time also.


You checked both stats and confrmed they both work......Im no expert..but i think you are ok...now if it did not reach say 100 deg i would be alittle worried. Get that IR gun to be sure what you have.

 

[achris]

I have the same problem. Had a 4.3LX for 14 years with no problems. I've just replaced the thermostat on my 2006 4.3MPI... If the thermostats were slightly open when cold, replace them...

 

[flyboycj84]

I have the same problem. Had a 4.3LX for 14 years with no problems. I've just replaced the thermostat on my 2006 4.3MPI... If the thermostats were slightly open when cold, replace them...

I just replaced em less than a month ago. I guess I will do it again, maybe I got bad ones?