I decided to make this thread to try to clear up some of the incorrect information I’ve read online about thermostats (this is mainly geared towards the common 180 deg thermostat swap). I have seen a ton of wrong information scattered across the internet about this and would like to try to clear things up:
First of all, some things I’ve read that are 100% false:
I've read people stating that modern automobiles are designed to run hotter because the oil needs to be at a hotter temperature to lubricate properly. This is not at all true; ANY oil, whether it be automotive oil, wd 40, vegetable oil, etc... will always break down faster and lubricate poorer the hotter it is. Yes modern oils are CAPABLE of operating effectively at higher temperatures, but that does not mean they MUST operate at higher temperatures. They will still break down faster and be less effective at a higher temperature.
I've also read people saying that swapping to a cooler thermostat will result in poor gas mileage because the engine is not designed to run at that temperature. This can be true in certain circumstances, but usually is not the case(with a 180 deg thermostat), I'll explain this below.
Some correct information:
Energy, work, and heat are all related in this equation: Energy = Work + Heat. This is over simplified and there are many other factors, but these are the main ones and it drives the point home. Energy is the input, work and heat are the outputs. You can see by the equation that if energy remains constant, if you decrease heat output, you increase work output.
After reading the above it should be clear to you that ANY engine will ALWAYS operate more efficiently (in terms of gas mileage AND power) when it runs cooler. This is assuming it is used in the same manner (same loads, same fuel, etc...). One other note about this is that there is a point where too cool is bad, as the engine needs to be "warmed up" and hot enough to allow combustion to occur easily and reliably. That said though, this "minimum temperature" is well below an oem 200 deg thermostat, or even a 180 deg thermostat, and even a 160 deg thermostat. It is probably closer to 140 deg or so(I know this because I have run 160 deg thermostats on carbureted vehicles reliably).
An oem thermostat in most vehicles will be 200 deg F give or take 10 degrees. The #1 reason they operate at this temperature (so far above the minimum temperature for reliable combustion which I guess as 140 deg F) is for emissions. Emissions are very strict anymore and running the engine hotter allows it to run cleaner, and the manufacturers use this to their advantage. From this simple fact, it stands to reason you can swap to a 180 thermostat and reap the benefits. This is usually correct.
Modern automobiles have fuel management systems that are pretty much 100% computer controlled, and they have what is called “open loop warm up” mode, which is where the computer ignores certain sensors and operates based on predetermined fuel and ignition timing maps. Also during this mode the computer has what is called “fuel enrichment” which is similar to a choke on a carburetor; it adds fuel during warm up to richen the air/fuel ratio until the engine is at a predefined “normal, warmed up” temperature. Once the engine temperature reaches this predetermined temperature (say about 170 deg F) the computer goes into “closed loop” mode. This means that it takes input from all the sensors and is “warmed up”. The computer will then make minor adjustments to the air/fuel ratio to keep the engine running as efficiently as possible (for good power and gas mileage). This is why you can’t simply swap to a 160 deg thermostat, because if you do then the computer will never leave “closed loop warm up” mode and will continue to add extra fuel because it thinks the car is still warming up. On a carbureted vehicle yes you can swap to a 160 deg thermostat and suffer no adverse effects other than a heater that does not perform well. With computer controlled fuel management systems, which all are pretty much the same from all the different manufacturers, you can usually run a 180 deg thermostat without issues.
How would you know if you have an issue with your 180 deg thermostat? Monitor your fuel mileage for several tanks before you change the thermostat, then monitor it again after you change the thermostat. If your mpg dropped then you may not be reaching the proper temperature, and should maybe switch back to the stock thermostat.
Some people will even remove their thermostat all together on a carbureted vehicle, but this is not proper as then it will allow coolant flow during warm up and REALLY slow the warm up process. You should always run a thermostat, even on a carbureted vehicle you should run at least a 140 deg thermostat.
The bottom line is that with a cooler running engine (within reason) you get more power, better fuel economy, better oil performance and longevity, and overall longer engine life/less wear. Everything will break down faster with more heat.
The only two things to contend with are that with a cooler engine the heater becomes less hot (duh!) and the fuel management system’s predetermined “normal operating temperature” which is different from manufacturer to manufacturer, but usually somewhere around 170 deg F.
I hope this helps at all, or at least clarifies some of the information!