put new thermostate in and temps fluctuating rapidly
 

I have a 2002 6.2 Mercrusier that I switched from a 160 stat to a 140 stat. Now my temperature is fluctuating rapidly from 100 to 150 or vice versa about every thirty seconds. The thermostate was not put in backwards which has the same symptoms if that's what your thinking. That was my first thought so I checked for that and there is no way to put it in backwards. This is running off the hose in my driveway as I was testing it before going to the lake. I'm stumped...whats your thoughts??

Did you replace the t-stat due to a cooling issue or just because ?

Check it in the water if you think everything was installed correctly. Speaking of which, Did you see if the t-stat spacer was installed correctly ?

EFI's are programmed to run with the spec'd thermostat...which is typically 160. So, if you run at 140 you will run richer.

Oil pressure was getting too low at idle (almost nothing) so I changed to a K&N 3002 filter and put Brad Penn oil 20W50 oil in along with changing the thermostate to a cooler one because the 160 that was in it was running 180-185 on the gauge

Wrong move to lower T stat - ECM is cal for 160 regardless - you will run rich all the time.

On the rivet button t stat gasket did you install a new one and correctly plus used the correct gasket ???

AND the big AND is did you use sealant on the gasket - if so wrong move there - the rivet buttons 2 on each side of the gasket are the ground contact points so the temp gauge can read correctly.

Also in the T stat housing did you clean the surface for the new cork gasket to lay down flat so the T stat lies flat in its position - if not water can by pass the t stat

These are common mistakes I see by whomever Improper installs even for T stat replacement.

other things that can cause your issues - Also water circ pump going out ??? When they are going bad and or are bad they flux the temp readings.

Also coolant temp sensor with very high corrosion and or mineral build up on it - can flux temp readings. Or its bad or your temp gauge is going out.

BUPS comments on the T-stat gasket and rivits is critical. I believe that is where your temp fluctuations are coming from. The fact that your engine was running 20 degrees hotter than the thermostat indicates a more serious cooling issue.

While the engine will run slightly richer, and i mean SLIGHTLY at 140 vice 160 this is NOT an issue. The coolant temp vs BPW table compensation between 140 and 160 degrees is insignificant. You are likely no more than 1/10th of an AFR difference between those two temps.

Well I didn't do any of the bad moves except replace the thermostat with a cooler one. Sea water pump is new this week and I will checktemp sensor for mineral build up I guess. Now I'm wondering why my last boat ran so well for 10 years with a 140 stat in it being MPI. That's what the boat had when I purchased it.

I'm gonna check the gasket too....this may be the problem!

BOOM...the gasket was the problem...Fixed!! Thanks Trash and thanks for you help too BUP!!

Thanks everyone...I didn't realized SB replied too. This forum is great and helps us DIY guys get the job done!!

told you so had to say that back at you maybe in a defense kind of way. and messing with you. . I give out indepth info of what can be related to the issues. sometimes it can pin point to one thing or have many things that can cause the issue(s).