[mjp]

Hi all,
I took the cover off my thermostat to see If I could test it somehow, and I cant seem to get it out. I cant push the plate in or out either. There is alot of corosion all around it. Is there a way to test it? I have a Merc 115 Fourstroke EFI 02.
Thanks,
Mike

[will941s]

If you can remove the thermostat you can test it in hot water, about 120 degrees or whatever the temp on the thermostat is. If it opens it's good. With the corrosion I would replace it though. Make sure when you put a t-stat back in you clean the hole it came out of really well to make sure it seals.

[mjp]

Thanks for the reply, Other than over heating the engine, what else could a bad thermostat symptoms be?

[will941s]

If thermostat freezes closed it will overheat and do major damage, if it freezes open you lose water pressure and the engine runs cooler. Which seems like a good idea, but through my findings is'nt what it's cracked up to be. Frozen open....water pressure makes the cool water run through all the chambers in the engine and cool in a designated path. Low water pressure means the water changed paths and some parts are running too cool and some parts are running too hot. Either way, always run a thermostat and change it every year...or at the most two years. Along with the water pump impellar. With everything working properly the water outlet should have a strong stream, then go to a slow trickle...then a stream as it gets hot and cold maintaining that determined temp on the thermostat.

[will941s]

If there is no water coming out and the engine it getting really hot it's bad. If there is a steady stream and it does not stop, and the water is always cool it's bad. After running the engine, the powerhead should be warm. Not really cool. Thats the way I determine if my stat is failing.

[sdsaw]

It will come out.
You might have to grip it with pliers and wiggle it while pulling.
If you don't damage it, you can clean it up by soaking it in white vinegar to soften the salt build up.
To test it (after it's clean) suspend the thermostat by a piece of string in a pot of water.
Have a thermometer in the water and start to heat the water
Below 120 deg F- no valve lift
Above 140 deg. - min 0.17 valve lift

Since you have an EFI motor, the stat plays an important role by maintaining the proper temp in the powerhead so the ECU (computer) knows what the proper fuel to air mixture to inject.

If the stat is stuck open, the ECU will inject a richer mixture because a cold engine requires more fuel to run properly.
So, your warmed up motor with a stat stuck open will be getting over-fueled which may lead to fouled sparkplugs and gas saturated lubricating oil.

I know that you're having running issues with your motor.
There's a good possibility that you didn't clean your injectors thoroughly since it was a DIY job.

Have you checked your compression to make sure that there isn't something majorly wrong?

If you don't already know it, your Mercury has the Yamaha F115 fourstroke powerhead in it.
There's a lot of info in the Yamaha/Suzuki section if you do a search over there.

Scott