My car has been overheating for a while now with a thermostat and w/o a thermostat!!! I keep on bleeding the coolant system and making sure everything is okay. Coolant overflow tank fills up periodically and then starts to overheat. There are no signs of smoke out of the exhaust when riding/startup and motor is pretty much newly rebuilt. I also see no signs of leaks and everything is new: waterpump, thermostat (2), hoses.
What do you guys suggest or think i might possibly be experiencing. I figured if it overheats w/o a thermostat, then what?? It takes a while to actually reach temp but eventually after driving for a while it starts to rise. Also add coolant light comes on which reminds me that car is also rising in temperature.

Thanks

1987 gxl
newly rebuilt, streetported
91 internals
racing beat headers
racing beat pulleys
racing beat clutch
apex n1 exhaust
magncor 10 mm wires
hks intake

Ist: why are you running with no thermostat?

2nd: if you want to do something like that, you are better off using a stock thermostat and busting out the top so that there is at least a little restriction. water pump wants to see some pressure build up.

#3 the add coolent light coming on only happens if there is cavation of the coolent in the water pump or there is air in the system. That points to the water pump is being over-reved, or is broken/not working right, or your bleeding the system in-correctly.

If you coolant overflow is throwing out water, you may have a bad\crack\corroded housing (unlikely if the engine was newly rebuilded), you need a new radiator cap, or you are not bleeding the system right.

Yep, air in the system. Idle the car up to operating temp with the cap OFF (and the heater on) until no bubbles are appearing. You may have a pinhole leak letting air in somewhere. Take the car to a radiator repairer and get them to pressure test the system. These leaks can be difficult to track down.
Don’t run without a thermostat, and make sure it’s a Mazda one.

Maybe you have a clogged up radiator. Did you try to use some of that stop leak **** in the radiator? If the system was clogged, it would overheat right?

Have you had your radiator/system flow-tested recently?

Could be a blockage, too....

Cheers,
Dave

Of course your engine overheats with no thermostat... the thermostat is also a coolant director and helps to build up pressure in the engine itself (NOT radiator cap pressure - that's different).

I agree w/the others, first get a new Genuine Mazda thermostat, and go from there... check for radiator blockage, make sure there are no air pockets in the cooling system, etc.

Are you overheating while moving at a constant speed over 40 mph? Or are you overheating when you are idling or in traffic?

If it's in traffic, your fan relays are probably faulty or your fans are not working.

Do 2nd gens have AST? If so, do you still have yours?

It's probably air or something inside the coolant but I just thought I would offer some options.

Radiator core clogged?&nbsp Run the car till hot and move your hands over the radiator core - BE CAREFUL!&nbsp Make sure the core is evenly hot.

You might need to remove the fan shroud to access the radiator core, if you still got the stock viscous fan on.



-Ted

Thanks guys for the replies. Next thing i'll do is buy a original mazda thermostat and radiator cap. I know for sure that i am bleeding the system right, i hope? But anyways I make sure car doesn't reach or past the 3/4 mark of the temp gauge until i have to shut off.

This problem usually occurs especially when riding, then at idle it may stay steady until i rev or ride out again.

Of course OEM parts, my god, none of those pepboys crap. make sure you get the stock gasket

chech your fan clutch.............i had the same problem, replaced my stock fan with an ele. fan, after that no more over heating

Some of the replies given here are confusing me. Why would a leaky radiator cause overheating and why would a thermostat help built up pressure and for what purpose does pressure building serve.
Personally I have been running a leaking radiator for 1 month, temperature needle 1/4 way up like it suppose to and the only drawback is coolant refill after every trip( 5miles to work ea. way).
I thought the only purpose of a thermostat is to regulate coolant temperature so that the engine can run at its ideal temperature. Now I had my Legend run for 3 months with a stuck open thermostat with no apparent damage done to engine, only problem was that it kept running rich until it was replaced with a new unit.
So straighten me out cuz I'm confused with your arguments.

If you lose too much water, your engine will overheat. Obvious really. If you forget to top up your system one day...
Not quite true, it acts as a restiction which slows the flow of the coolant. If it moves too fast, not enough heat is transferred into it, which could cause overheating.
That's right, but it's not it's only purpose (see above).
A stuck open thermostat will just cause the engine to take ages to warm up, during which it'll run rich. This will accelarate engine wear.

The cooling system needs to be pressurized.&nbsp Other than the obvious reason you'll lose your coolant, pressure keeps the coolant from boiling.&nbsp If the coolant starts to boil, it overheats very rapidly.




-Ted

I think the temperature in my leaking cooling system should run lower as compared to one that is under good pressure. I compare this to a pressure cooker where food gets done quicker because the water inside is allow to go above the 100 deg celcius due to the raised boiling temperature. But since our fan is running that should keep the cooling system around the same temperature be it leaking or not. Or is there something wrong with this line of thinking.

Yep, your logic is amiss.
Pressure keeps the coolant from boiling.&nbsp Once you get bubbles in the liquid, the temps skyrocket cause this produces air pockets and steam.&nbsp It's called localized boiling, but the whole system gets hotter because of it.&nbsp Think about it, if the system actually ran cooler without pressure, why do we bother to put pressure caps on all cooling systems?

Your analogy with the pressure cooker is a whole different ballgame.&nbsp The pressure cooker allows the food to cook HOTTER without the whole thing frying itself.&nbsp The pressure keeps the heat inside the pot, rather than have the heat escape for the pot with an open lid.


-Ted

Well, the only reason I'm questioning this importance of having it pressurized (other than the mere fact that it will raise the boiling temperature, which in the end is to prevent evaporation or slow loss of coolant which I can always TOP off since my trips are short) is that I have driven my car almost everyday to work 5 miles each way without seeing any dangerous rise in the temperature needle, just 1/4 way up like my other NA. And till this day, still runs strong, no sign of engine meltdown.
But your comment about localized heatspot is scarring me so I get off my lazy butt & having it replaced soon.

Remeber, the temp gauge is a sensor at one spot of the cooling system.&nbsp In fact, the sensor is located under the oil filter right under the oil pressure sensor.&nbsp There could be localized boiling, but the sensor is not seeing it.&nbsp Whatever the case, a sealed cooling system is important to an efficient working system!



-Ted

And to put the thread back on topic, and help out the thread starter with the overheating problem:

although I didnt overheat, I would get the Low Coolant buzzer every other time I started up the car and drove for more than 20km. This was persistent for more than a month. Pressurizing the cooling system didnt reveal any losses whatsoever. My coolant would just go over to the overflow and stay there...

Turns out I needed a new thermostat, a new cap, new heater hose (which just split right open ), a new thermostat housing (hair-line crack ), and two new small hoses running around behind the engine (which also split open).

Second week and counting the problem seems to be solved. I did see some steam from the engine (coolant burning because of overflow) when I went to Patra for the carnival, but the radiator and the overflow were filled to the top anyway, so....I got the Low Coolant buzzer again, but then it stopped (it took back some of the coolant). Had to be some air pocket, or the water-pump cavitated.

Also, every pound of pressure in ANY pressurized system decreases the temperature by 3 degrees. Most RX7s have a 15 pound cap, so this means if there are no leaks, your boiling point is a lot higher than if there was a leak, like 45 degrees higher!!!! Yeah, I can multiply

Man - you guys are all over the place. About the stock thermostat: There is a spring loaded disc on the bottom of the thermostat that allows excess flow from hi revs to bypass. My 87 was showing overheating symptoms. What finally fixed it? Would you believe the water pump housing? The aluminum casting behind the pump was eroded by the turbulence of the high revving pump. The bypass valve seat under the thermostat had about a 4mm chamfer eroded away, and there was a 6mm hole eroded through the wall between the inlet and outlet side.
All this bypassing gives symptoms like a partially blocked radiator. I fabricated a repair with Bondo Marine Epoxy Putty Stick. It has held for a year now.
The other thing that sneaks up on you is the fan clutch. On the series 4 version the fan pulls hard up to about 3500, and then maintains ~3500 as the engine revs higher. Mine was slipping at 2500. You can get the fan clutch to bite better by bending the bimetal strip outward. I ended up buying a new fan clutch.