Hello I've been dealing with a specific situation for a while now in which my fb temperature gauge rises and lowers .. I've done a full flush of the cooling system and filled it back to spec. I also have replaced the thermostat since the one in it been there since 85 and the gasket was worn anyway. after that the car ran fine for few days then starting doing the same issue again.. I let the car run and watch the temp. rise and i noticed the top radiator hose wasn't getting warm until the the temp gauge was max to overheat for a little then it would start to cool down and i would feel the hot coolant rush through the hose. now my question is what should I replace next , the sensor or the water pump , or even fan and how could I test these three to come up with the actual fault?

 

You may just need to burb the system. Could be air trapped in there causing it. Park the car slightly up hill cold, open the radiator cap and let it run up to temp. Periodically squeeze the hoses to get trapped air released. Also make sure there is coolant in the overflow and the hose is good.

 

 

Does all this happen at rest or when driving car? Do you have the correct type thermostat,is it a Mazda thermostat? What was the coolant like that came out,dirty,rusty?
Not giving a lot of other details. Check actual temp of engine by aiming an infra red thermometer at top hose,thermostat housing and get some actual numbers. Sometimes trying to diagnose or replace parts by going by what the temp gauge is not the best way to proceed. The gauges weren't terribly accurate when new and 35 years later maybe less so. The temperature gauge sender will not make the car do what it's doing,it indicates the temps of coolant flowing past it and sends that info to the gauge on dash.

This sounds like a thermostat problem,may have the wrong type,rotary engines take a specific thermostat that has a flange on the bottom that closes off the bypass port below in water pump housing when thermostat is fully open. A thermostat without this doesn't allow coolant to flow properly in and out of a warmed up engine. Thermostat doesn't HAVE to be a Mazda part but needs to be correct type and of good quality.
There are a lot of crappy parts out there and it's entirely possible to have a new defective part. I would suggest removing the thermostat and check if it's the type i'm describing(what did the one you took out look like?)if it is,put it in a pot of water with a thermometer and bring water to a boil and watch what thermostat does while watching thermometer. It should begin to open smoothly as water heats up and be fully open according to reading on thermometer compared to the rated temp stamped on bottom.@180 is when it should be open all the way. If it doesn't operate in this manner,get another(assuming you have correct type) from a different manufacturer and test it to be sure it's working properly. Doing this will help you to be certain tstat is working and not the cause of your problem. Was this complaint the original reason you changed the tstat and you still Have same problem?

Condition of the coolant drained could be a clue as to where the problem is.Dirty/rusty coolant can corrode the impeller blades in water pump making it unable to pump/flow coolant at the proper rate through cooling system. This is not really a problem with oe type water pump with cast iron impeller but aftermarket pumps with stamped steel impeller it can be. Once you've replaced/determined tstat is working properly,fill radiator slowly while engine is running only enough to cover the tubes at top of core in radiator. As the thermostat begins to open add just enough water to keep fins covered(1/4"-!/2"). Watch water level in radiator and feel upper hose at tstat housing, when tstat is fully open you will feel the temperature change and should be able to see water flowing across the core in radiator at a good clip. If you don't,drain cooling system and pull water pump and inspect for condition of impeller blades. Make sure impeller is not slipping on pump shaft by holding pulley hub and trying to turn impeller,if slipping you'll feel it.If they're ok,water pump is ok,no need to replace it,reinstall with new gasket.

Next thing to look at is the radiator,if you've eliminated the previous items, you need to remove radiator and have it flow tested and or cleaned. Tell the radiator guy you suspect radiator may be partially clogged. Or just put a new radiator in the car.

Regarding cooling fan(clutch),they do fail but not usually in this manner. In your case with your symptoms a fan clutch would not cause the radiator hose to go from cold to hot like you describe. If everything else is ok with cooling system the car would warm up gradually with no fluctuations of temp gauge which should be between 1/4 -1/3 travel. You would have good heat from heater and if the fan clutch was defective,after an extended period of idling the temperature will slowly steadily creep up. The cooling fan is not necessary above speeds of 15 mph as the car moving forward pushes sufficient air through the radiator and the fan freewheels.

Do your diagnosis in this order and you can find and fix the problem without throwing unneeded parts at it hoping for a cure.If you're not sure the thermostat you have is correct,take a pic and post here so we can see it,same for condition of water pump.

 

How old is your radiator cap?

A leaky cap (or hose to the reservoir) can let air re-enter the system as coolant cools and contracts.