Why the OEM Thermostat
 

I know everyone always says you should use nothing but the OEM thermostat but I was wondering if anyone ever sat down and compared something like what you would buy at napa to the stock thermostat. I currently have one from napa in my car and it's running a little warm but not to the point where it comes up on overheating. There are a few things I want to check before I run out and dump 20 bucks on a stock thermostat too though. I'm just curious if anyone has sat down and compared the stock unit to something else to see if there is some notable or measurable difference between the two that would make an aftermarket one not function correctly. For that matter, is anyone actually running an aftermarket thermostat without any trouble?

most aftermarket thermostats do not take the bypass port in the water pump into account, they are generic in design because they only care about selling their product not about protecting your engine. the stock thermostat has a plunger that blocks off this passage when the engine is warm, if this passage if not blocked (aftermarket thermostats) this passage bypasses coolant from the water pump back into the engine bypassing the radiator completely so the coolant does not actually get cooled.

i hope this answers what you were looking for. ;)


you can bypass this bypass (no pun intended) by placing a 18mm freeze plug into the housing but the engine will take longer to warm up. our cars had too many cold start assist features anyways it seems.

That's about what I was looking for. I can't see the water bypassing the radiator entirely just because it doesn't block off the bypass but it would make sense that if the water goes some fraction to the radiator and some fraction to the bypass, the temps will creep up a few degrees like I'm seeing right now. Thnx a bunch.

speaking of OEM thermostats....i bought a thermostat from the stealership and then another one from mazdatrix....i compared the two and they were different....one had the air bypass on the very outside flat brim while the other one had the bypass near the center spring........

that is just an air bleed to relieve air pockets, doesn't matter where they are located so long as they don't interfere with the operation of the t-stat.

I'm pretty sure I've got a Napa or some $hit in my car. Temps are 180-210 depending on how much I'm abusing it.

but i mean if its the OEM mazda unit wouldnt it look exactly the same?.......

I wouldn't do this on anything but a race car with a gutted thermostat.

By blocking off this bypass port, you are realying solely on convection to circulate coolant through the engine. The coolant around the spark plugs will see localized boiling and possibly overheat the coolant seals before the thermostat ever opens up and starts cooling the coolant.

This bypass port was to keep coolant circulating through the block when the thermostat is closed to prevent localized boiling.