What is to hot?


What water temperature is to hot and the threshold where the car should be pulled off the track to prevent likely engine damage?


Lako said his gauge got to 370* during the red flag yesterday and his car was fine.

I think the gauge may have been a bit off.

In all honesty im not sure w/ an AL head on an M20 but we cracked our head, across 3 cylinders, when we lost coolant at nationals 2 years ago.


Theoretically 258deg.

Our coolant pressure varies from about 2-12psi, depending on where you measure it. Boiling will start at the top of the radiator because that’s where pressure is pretty low, yet the water’s not been cooled any yet. Top of the radiator is 3-5psi. Boiling point at 3psi is ~220deg.

So at 220deg some boiling is likely to start occurring. The steam will further pressurize the system, which will suppress more boiling until the water gets hotter yet. This self-limiting cycle of “boiling makes steam which make pressure which reduces boiling” should keep going until the expansion tank’s cap can’t take any more pressure (20psi?)

20psi water boils at 258deg.


  1. Impurities in the water usually lower boiling point.
  2. Water wetter probably changes boiling point. Later: I looked it up. It doesn’t change the boiling point.


Good info, thanks. Are most cars running coolant temp and pressure gauges? I’ll search the threads for the proper setup unless you have a quick recommendation on what works.


Coolant temp gauge and the low oil pressure light/sender moved to the thermostat housing for a low coolant pressure light is Ranger’s hot ticket.


I agree with Chris, but I’d put the switch in the block not the tstat housing. The oil temp switch is 14mm, tstat ports are 12mm.

Find the hose barb on the block near the starter. The barb is for a hose that takes hot water to the throttle body. That hose barb is 14mm. It will be rusted in so is kind of hard to get out. Pull the hose barb and thread in the oil pressure switch.

The connector for the switch can be grabbed from the engine harness. It’s a standard Bosch 2 port connector.