94 B4000 Coolant System Trouble

[Thatonediabeticguy]

I have recently bought a Mazda B4000 and I'm having trouble with to coolant system. Once it gets hot it forces all the fluid to the reservoir tank and I don't think it's letting any fluid back through. If anyone has any ideas on what could be the problem, please let me know.

 

[RonD]

Welcome to the forum

Probably not what you want to hear but need to check this or you WILL WASTE MONEY on non-fixes

Because its a 1990-2000 4.0l engine this symptom is most likely a blown head gasket or cracked head
The seller probably knew this, so I assume you got a good price

When engine is cold remove rad cap and smell the air inside radiator, if it has a exhaust smell then that's the problem
Confirmation test is the Glove test
Rad cap is still off
Remove overflow hose and seal that port in rad with a vacuum cap, putty, gum, ???
Then put a latex glove over rad cap opening and seal it with a rubber band, or you can use a balloon or even a condom instead of the glove

Unplug the 4 wire connector on coil pack, we want a No Start

Crank engine over and watch the Glove
If gloves starts to Bounce then you have a bad head gasket or cracked head
If it doesn't inflate then we can move on to something else

If glove bounces you can remove 1 spark plug at a time to find the leaking cylinder
Not that it matters, both heads need to come off and be pressure tested and new head gaskets installed


Each cylinder in an engine has 150+PSI compression when cranking the engine over, each cylinder is surrounded by coolant passages, the point of the passages is to cool the head, where the Explosions occur
When that explosion occurs there is 1,000+PSI pressure
If a head gasket blows(or head cracks) that means a cylinder is leaking that pressure into a cooling passage, this displaces the coolant and increases the pressure in the cooling system above the 14psi rad cap rating
Causing coolant to be pushed out into the overflow tank
And it won't get sucked back in because of the higher pressure in the cooling system
The reverse flow of coolant in "normal" operation is from the coolant cooling down after engine is off and shrinking in volume so pressure in the cooling system drops below outside pressure, so the coolant in overflow tank is sucked back in to radiator, that can't happen if cylinder pressure is in the system

Hopefully is not a head gasket/cracked head issue

But your description is exactly what happens

And no, coolant rarely gets into the oil any more when head gasket blows, it can, just not as likely as it was years ago
And with a cracked head coolant can't get into the oil

4.0l OHV has a weak spot between valve seats, and if this engine is EVER overheated, a head will crack there