dhpmike
+1y
first, if its getting hot the radiator is probably stopping up at least a little.
Mazda put only the bare minimum of cooling capacity on those trucks anyway.
The coolant will never ever stay up at the top close to the cap. the system will always push it out until its about 2 inches below the cap.
The system is a closed system which means that during the day driving it gets hot, the excess coolant expands, pushes out into the overflow tank. then at night when the coolant in the engine cools down and contracts, it creates a vacuum and sucks it back in through the cap.
IF you have a bad cap it can do a few different things.....
1. doesnt hold much pressure, even if the hose is stiff when the engine is hot. The original cap was designed to hold (probably) about 13 to 16 psi at sea level. this will allow the coolant to reach more than 260 degrees before boiling. some coolants raise that temp also.
2. doesnt seal properly for the coolant to return at night. Then during the day when hot coolant flows out into the overflow tank, it evaporates a little each day until you are down quite a bit. plus when the cap doesnt seal, it wont suck coolant back in. It only sucks in air and then pushes out more coolant later or the next day. this is also evident by seeing an overflowing overflow tank.
3. The radiator can have a small leak. look where the tubes in the core meet the header plate (right below the top tank) and above the bottom tank. If you see calcium stains, green stains, white stains or wetness you probably have a leak. most of the time the leak is so small that the hot radiator will evaporate the leaking coolant and there is no trace of wetness, only a stain. many times an old radiator will have many small, very small leaks.
4. if there are small leaks that allows for less pressure to build up, little or no return at night and less coolant to cool the engine.
5. you may have a combination of problems. many times a cap will not be real bad but only hold 7 or 8 psi and you will also have leaks and the core stopped up a little which all these together will cause coolant loss and also running hot.
6. many times when you have a clean radiator and no thermostat you will have an engine running hot because the coolant goes thru the radiator too fast. especially on a downflow radiator like a B series has.
7. thermostats can stick partially open or all the way open or closed. this can make your engine run hot a number of ways described above and make it tough to diagnose.
The best thing to do is have a radiator shop do a pressure test on the cap, radiator and system. this is usually free. if they wont do it free, go somewhere else.
make sure you watch them test the cap and radiator. you may see the guage for yourself.
also, just flushing a radiator will not clean it out. It can get the loose stuff out, but if the radiator is stopped up enough to cause it to run hot then its the rust and calcium and solder bloom (rust on lead solder in the tubes) that is down inside the thin tubes.
there is only one way to ficx this. the top tank has to be unsoldered from the core/header and they run a flat stainless steel rod down each tube. this is done after a bath in a caustic solution to loosn the rust and calcium and solder bloom. then its put back together, flushed out and repainted and almost as good as new.
as old as these trucks are, many times the radiator is too far gone to be able to do this. in that case a new core or new radiator is the only fix. I prefer the new core since its cheaper than a new rad and you can get a thicker one than stock.
most B series came with a 1 row core. a 2 row HE (high efficiency) replacement will be much better than even a new rad.
now, as far as the head gasket goes, this can be the case as well. most shops have a tester that adds blue die to a tube, then they suck some hot coolant into the tube on the tester. if it turns yellow you have a head gasket leak.
as stated before small bubbles are also a telltake sign. BUT when coolant gets weak or diluted too much it will also make small bubbles.
a bad fan clutch can also cause a system to run hot.
also do not put straight coolant into an engine. its boil point, freeze point will not be any better than at 50/50 mix. also many times straight coolan will gel in the bottom of the block. this can cause many problems as well.
also there are only 2 plants in the United States that make coolant. so brands are pretty much all the same.
just make sure you dont mix the newer styes in any newer car calling for a specific type.
sorry for the book. I worked in radiator and cooling system shops for more than 15 years. Ive seen it all as I reworked 15 to 20 radiators a day. pulling them apart, rodding them out etc.
plus the other symptoms were seen in our remove and replace bays. if you have an further questions you need to ask me just hit me up anytime at