I have a 2002 Mazda 626 with 134,00 miles. About a month ago there was a large coolant leak in the middle part of the engine. My mechanic replaced the coolant by-pass hose from the thermostat housing under exhaust manifold to engine block. A few days ago there was another leak around where the coolant reservoir is. Gauge read hot and had to top off coolant on way to repair shop. Topped off reservoir and next morning most of the coolant had drained out. My mechanic has done a pressure check, drove the car, let it sit and idle for up to an hour and there was no leak or overheating. Any suggestions?
He did a pressure test so one would assume the head gasket is okay. I’m sure he checked for a frothy white substance in the oil, indicating a breach. No persistent white smoke out of the exhaust, I hope?
It could be that, when you had the first leak, the engine wasn’t burped correctly - meaning, maybe there’s an air bubble in the system somewhere, which could cause overheating.
You could park the car slightly uphill or on a ramp, front up.
Let it cool so you can take the radiator cap off. Start the car and watch what happens when you slightly rev the engine. The coolant may drop a little and you may notice ‘burp’, in the form of an air bubble coming out.
When that happens, add some coolant (properly mixed, of course). Repeat until you see no more bubble and it is all topped off.
On some cars (ie Subaru and older Hondas) it is often best to take the top radiator hose off and fill the engine from there first when coolant has leaked out. Then, after filling, you quickly put the hose back on the radiator. Not sure whether that is true of Mazdas, though.
Some cars have a bleed nipple towards the top of the engine that you can open to let the air out and more coolant in. Someone here may be able to tell you whether your car has that. It basically does the same thing.