TexasCHLforum.com

Rorary engine?

Does the fan run?

We had a 2000 626 and had intermittent cooling problems with it that usually turned out to be just the thermostat. Once, though, the fan wouldn't come on and I think it was just a bad fuse.

Sorry. That's the extend of my automotive knowledge.

Russell:

1. I had some radiator work done on a car I had. But then overheating was random and
a few mechanics could not figure it out.

The winning mechanic took paper and checked the air flow on the dual fans near the
radiator. Come to find out, one of the fans had been mounted backwards. Instead of pulling
the heat FROM the radiator TO the rear, it was pushing the heat FORWARD to the radiator.

Once the reversed fan was oriented properly, the heating problem went away immediately!

2. For another item to check: Your car may have 2 smallish hoses as part of the cooling system that
both attach to the firewall. They are an intake and outtake hose to feed the heater core, which keeps
your interior warm. The heater core is a low part of the cooling system and sometimes collects sediment
over time.

Remove the two hoses from the firewall, point a garden hose into the one or both of the hose outlets on
the firewall, and flush your heater core until you see clear water consistently coming out. The sediment
may be preventing your coolant from making the full trip. I consider this a long shot since we are just finishing
summer, so you would not have been using your heater.

Good luck. Let us know what it was when ID'd.

MAZDA = Must Always Zoom Down Asphalt! SIA

Zoom Zoom Zoom

If you have a blown head gasket, or cracked the head, you would probably have white, sweet smelling smoke, coming out the exhaust. You would also be losing coolant at a noticeable rate for it to overheat because of a head problem. If there is no smoke, the most likely culprit is a bad thermostat (I know it was just replaced) or air in the cooling system. Before you throw a new thermostat at it, take out the current one and place it in a pot of water on the stove. Put a thermometer in the water and turn the burner on high. When the water hits about 190-195 the thermostat should open. If it doesn't open, or sticks open, its the problem. Whether it needs replaced or not, you should also bleed the cooling system.

If it isn't the thermostat, head gasket, cracked head, air in cooling system, a lack of coolant, or improper antifreeze mixture, I would suspect that the fan isn't functioning properly. I'm not familiar with your car, but if it has a belt driven fan and the belt is slipping it might not be providing enough airflow at idle to keep things cool.

If you don't have a service manual for the car GET ONE! It can save you lots and lots of money if you use it. I prefer Haynes manuals, but Chilton also makes an ok book.

Oh, and to answer your original question. Having the thermostat open all the time will also cause the engine to overheat. If it's open all the time coolant doesn't spend enough time in the radiator to transfer its heat.

What kind of water pump did you put on? Was it an OEM Mazda? Fella on the Two Wheeled Texans forum just went through something similar with his Toyota Echo. Had the radiator rodded and cleaned, new aftermarket water pump, new thermostat. Still overheated.

Replaced the BRAND NEW aftermarket water pump with an OEM Toyota water pump and fixed the problem.

I would suggest that you do not remove the thermostat. This will increase the water flow through the system but it may increase the flow to the point that there is insufficient time for the water to lose heat while in the radiator. The chance of both the original and replacement thermostats being faulty is remote. But no harm in testing as mentioned in a previous reply - although I wouldn't bother.

A common sign that the head gasket has failed is coolant bubbling because of cylinder pressure leaking into the cooling system. You mention that the system is "boiling". This is unlikely in a pressurised cooling tank (the type with the pressure cap on the tank rather than the radiator) as the system pressure is designed to prevent this by raising the boiling point. The cap should relieve the pressure before the boiling point is reached.

So if I had to guess I would think that you do have a head gasket problem. The best way to check this is with a cylinder pressure gauge (about $20 at Harbor Freight). Pull all the plugs, prop the throttle wide open and screw the gauge into each plug hole in turn. Record your readings and compare them. You should see no more than about 10-15% variance from the highest to the lowest. My guess is that you will see one or more that is much lower than the others.

Good luck.

neilp mentioned the pressurization of the system, which made me think of another
possibility.

Check the rubber seal on your radiator cap. If it's old and cracked it won't have
a tight seal with which to pressurize the system.

If it looks bad, buy the proper cap for your radiator. That might be the best $5 you will
have ever spent.

SIA

Ah, I missed the boiling thing. Yes, if it is leaking cylinder pressure into the cooling system it would continue to "boil" after the car was shut off.