i guess my water pump was cavitating and it erroded the water pump housing pretty good. Also there is a hole between the main cavity and the upper cavity that goes back out to the radiator. No over heating problems or the slightest signs after changing the thermostat, pump and coolant, but the car will not vacuum back up any coolant, though it will pressurize and spit out the over flow. So the freaking gay buzzer goes off every other time I drive it. Also the car has been idling like it is under cold start and rarely idles correctly after warming up, its always at like 1400 rpm even when warmed up. Could this hole and bad flow in the water pump housing be resposible some how for making the ecu think it is under cold start conditions all the time? does it explain why the coolant is not getting vacuumed back up?

 

Pull the water pump & housing & take a good look at the small coolant passages at the top of the front rotor housing, if you've been running with some rust or other debris in the system, they might have become clogged, and this would explain your "cavitation", and although if you're not running hot, this theory might not "hold water", pun intended...

If your radiator is old & failing, this might cause your problems also. Is your overflow tank filling up, but not being "sucked" back into the rad on cool-down?

Was the "old" coolant full of crap when you drained it? And, did you pull the block drain plug and flush the engine passages out while you replaced the coolant?

 

That hole in the housing partition between the inlet & outlet chambers started out as a 1/8" air bleed hole.

Another erosion place is the bypass port that's covered by the spring loaded disc on the bottom of the thermostat.
The lip of the port gets chamfered.
I discovered this when the back wall of an '87 housing sprung a leak.
I repaired the housing with "Bondo Marine Epoxy" putty stick.
It was still holding a year later when I sold the car.

 

The radiator seems to be good, there are no leaks, no excessive amounts of bent fins, just from bugs and stuff. But...there's no sucking coolant back up out of the reservoir to replenish what gets pushed out of the system. The old coolant was good, It was about two years old from the last time I replaced the water pump and everything, when I first bought the car, no signs of rust or anything. And I had a 195 thermostat in it, replaced it with a 180 and the car is running much much cooler than it has before, but it just won't suck any coolant back up from the resevoir, which sets of the buzzer because it doesn't make a ground to the sensor. so for the mean time I've just grounded out the low coolant sensor so I don't go crazy on the drive to work in the morning before the t-stat opens up. But its very odd that the system is pressurized enough to work to push fluid into the resvoir and can't vacuum it back up when it cools down. I was wondering where my coolant was going and then after a longer drive and stopping at a gas station there was a small coolant pond under the very nose of the car, so I am pleased it isn't getting eaten up in the engine at least.

 

Ahh, now what you're describing is a bad radiator cap, pure & simple...

 

The water pump doesn't usually cavitate while the engine is under normal operation unless you have changed the pulley to a smaller size. I have seen a pump come off of an engine with broken impeller blades. But when you are troubleshooting, start with the easiest to remove and/or cheapest parts first. If you replace the pressure cap, don't use a Stant or generic one, get one from Mazda.