Last summer, before I tore my car apart, I was having an overheating problem that I couldn't figure out. I had the radiator cleaned, flushed the coolant several times, replaced the tstat numerous times, replaced the water pump. Eventually I discovered that it ran very cool without the tstat which narrows it down to a tstat or tstat/pump housing problem. I know there is a bypass hole and that if its not plugged, the car is supposed to run hot without the tstat. However, mine didn't. Maybe the previous owner blocked it. I also checked the tstat in a pot of hot water - worked fine. So something is causing the water around the tstat to not get hot enough for it to open. Anybody had this problem before? Logic tells me this has to be a problem with the tstat/pump housing. I am going to be building a new engine for the car, but I want to figure out the cause of this problem because I am going to be reuseing some of my old parts - the side engine plates, radiator, and I have the pump housing but not so sure if I should use it. Any ideas? What would be the effect of running a blocked bypass with a tstat? Maybe the bypass simply got clogged.

 

The bypass hole is there to allow the coolant to recirculate back into the waterpump inlet after its pushed out of the block.The hole is uncovered and flows when the t-stat is closed(cold).Since the outlet to the radiator is blocked,the majority of the coolant travels in a closed loop.This aids in engine warm-up and even heating of the housings.
If you remove the T-stat completely and dont block the bypass hole,some of the hot coolant will inevitibly skip the trip to the radiator and flow back into the waterpump inlet.This reduces radiator efficiency and basically heats up the cooled coolant, before it can draw any heat out of the engine block.If you block the bypass hole and remove the T-stat,youll lose the re-circ feature during warm up,but 100% of the hot coolant will flow to the radiator when the engine is hot,and thats the important thing.
BTW,if you elect to keep things stock,BE SURE TO USE ONLY MAZDA THERMOSTATS!Many of the aftermarket, junk T-stats dont have the bypass block-off plate on the bottom,and they will not work right,even if the book at Pepboys says its the right part.

 

I have to disagree with the above. I been running gutted t-stat with the by-pass unplugged for over 10 years - no problem.

 

[QUOTE=The_7]

Last summer, before I tore my car apart, I was having an overheating problem that I couldn't figure out. I had the radiator cleaned, flushed the coolant several times, replaced the tstat numerous times, replaced the water pump.

Either a clogged radiator or fan clutch problem.

Eventually I discovered that it ran very cool without the tstat which narrows it down to a tstat or tstat/pump housing problem.

if it was overheating, then why is running cool? please elaborate more.

 

I agree with Steve, always use a MAZDA tstat. It makes a HUGE difference. Here's how I know:
88 vert now runs MUCH cooler with a Mazda tstat and a bottle of water wetter
92 probe overheating problems until I used a Mazda stat
95 Millenia overheated until I used a Mazda stat

 

always use a mazda tstat!

 

If I ran with the tstat, it would overheat - it would just keep getting hotter until I shut it down. If I took the tstat out, it ran cool - about 1/4 up the guage or a little cooler, even if I drove hard. Then if I put the tstat back in, it would overheat again. It was very consistant. This leads me to believe that it has nothing to do with the radiator or clutch fan (although I had the rad cleaned by a shop and im pretty sure the fan clutch was working fine). It could have been the wrong tstat. I did use a few from autozone, but I think I had one from mazdatrix and it still didnt work. Not sure tho.

 

Does it overheat on a stop and go traffic?
Hows the temp at freeway speed?

 

My rebuilt R5 13B is overheating. It just gets hotter and hotter 'til you shut it off. The tstat I used was a Stant with the little cover piece at the bottom like a stocker. I'm going to remove the tstat and see if there is an improvement.

By the way, the waterpump is a stock FB cast iron unit; same as yours. I have access to an FC waterpump, and will throw it on with a Mazda tstat if the cooling issues are solved by removing the Stant. It'll be my supercharger engine.

Edit: since the FC pump housing is aluminum, it would be pretty easy to cut threads and install an NPT plug in the bypass hole or something... That may be the best thing for an engine that's expected to run hotter than stock all the time.

 

I just had an interesting discussion on another forum about the tstat, recycle port(bypass hole) and related systems.

The entire thread is here:
http://ausrotary.dntinternet.com/for...898&highlight=

This is an image of the top of my water pump where the thermostat sits - there is a welsh plug in the recycle port as you can see:


HTH