How hard to replace thermostat?

[Wild Turkey]

Yes. The factory thermostat is over engineered. It uses a rubber seal around the center plunger that eventually mushrooms out, deforms and causes the plunger to be stuck open. Common failure on all NSXs. Go with the Stant all-metal version and you'll never have to replace it again. Make sure to also buy the rubber seal gasket. I used a Mahle, but you can also use the OEM. I like to add a very light coat of silicone grease (Shin-Etsu, Pool-Lube, DC-4, etc.) to the rubber to improve sealing and preserve the material life to keep it soft and pliable. Finally, you may want to replace the thermo cover bolts and clean out the thread holes- they can often be rather corroded, which means you won't get the right torque setting when you snug them back up.

Follow up: Changed my OEM to the Stant stainless one you recommended with a new OEM gasket over the winter. Also the bolts even though they looked clean - cheap insurance. Just took it for a spin and it warms up in a few minutes, even on a cold day. If your NSX is acting like mine was, take a close look at the thermostat.

Thanks [MENTION=18194]Honcho[/MENTION]!

 

[ZX4NSX]

I finally replaced my thermostat with a new OEM thermostat (it took me HOURS). I think it has been bad for years and progressively got worse (my 1995 has almost 111k miles). After inspecting the old thermostat, I noticed the rubber plunger seal was not mushroomed like most mentioned on here, but pieces of the seal were broken off (see photo) - which means they could be clogging the radiator or someplace else in the cooling system.

After replacing it, it is like night and day; the temp gauge needle starts moving a couple of minutes after I start driving and reaches op temp much, much quicker - and remains there. I used to think the long warm up time was normal since the radiator is in the front and the engine is in the back. I think it may not be very noticeable, because the thermostat seal likely degrades slowly over time - causing a gradual increase in warm-up time over a long period of time.

So, if you notice long times for the temp gauge to start moving and reach normal op temp, and if it cools down when driving at speed, you should check and replace your thermostat. It may have broken apart like mine.