Mazda 03 Protege overheating, shifting
 

Newbee to this site, working on son-in-law"s 03 Protege 2.0 with two puzzling problems, and I need help. I've been searching this site for several days, heeding, and learning Mazda stuff.
First: the overheating problem that others have, cooling steaming back out the overflow jug. started with replacing thermostat, then water pump. replaced radiator cap, both radiator hoses, hoses were old and weak, suspected collapsing hoses. No luck .Idling and reving seems to purge system of air, gets good heat thru heating core. While idling in garage, everything seems fine, drive half mile down road, it loses heat from heater, and boils out thru overflow.canister, and using laser thermometer things get strange. Thermostat does not open, housing only gets up to 110, bypass tube on front of engine (from thermostat area to return radiator hose ) gets extremely hot at 250-300? Temperature gauge stays in mid range, even when boiling back out thru that radiator hose. . Suspect possible clogging in that front bypass tube? Lord knows he doesn't use Mazda coolant, beer costs too much. When removing thermostat housing, water pump, and water outlet housing, everything looks very clean inside parts, and engine, no sign of clogging there. Have checked compression, looked inside cylinders, no signs of cracked , warped heads, no mixing of fluids, oil or coolant, no moister in exhaust.
Any thoughts? Is that front tube that I call a bypass tube hard to remove?

Second issue: shifts from overdrive to 3 and back about every 10 seconds at 45-50 mph. turn off OD and it's fine. when it does shift, the OD light doesn't flash, no other lights, no codes. Is there a solenoid for overdrive? Trans temperature switch? Trans is cold at test driving. Tried changing speed sensor, no diff.
I'm a decent GM mechanic, but new to Mazda engine, I would appreciate any help with this from you guys.....

Remove & inspect the bypass pipe.

Has the radiator been removed and professionally cleaned out?

Have you carried out a TK test? ( combustion gasses in coolant )

Have you tried running it with no thermostat?

Also, pressure test the cooling system, remove all the spark plugs, let it sit under pressure for 1/2 hour or so, then crank the engine over off the starter & look for any coolant exiting the engine via the spark plug tubes.

Thank you Grim-reaper, will run through that list today, and report back.....

Any thoughts on the overdrive issue?

Thank you!

Still working on these problems. Found the radiator raised lip for sealing the cap had deteriorated and was not sealing. How ever that seems to be a result of the boiling back, not the cause. New radiator anyway. I was wrong about the bypass tube heating up so high, bad reading with laser. removed bypass tube(s), clean and not blocked. TK test neg, tried twice. Compression ok, tested twice. No sign of fluid in cylinders, or oil, or inter mixing of the fluids, check twice. In case you're wondering, I'm recapping everything looking to see if I missed anything, or if something might so up later. Still haven't gotten a good radiator pressure reading, the pressure drops on the gauge, but it's a borrowed public tester from the auto parts store, and has been beaten up pretty bad, no glass in the gauge. Looking for a better gauge tester. Can't find any signs of fluid leaks, hose, radiator, cylinders, or heater core. Problem has eased up quite a bit, it would spit air and fluid out the overflow just going to the corner, half mile. now I can drive ti locally, and it doesn't overflow, but still does on highway. Could be air trapped in engine? is there a bleeder some where? Going to jack it up, to drain, flush system and change it to the Mazda FL22 later today.

Have not tried running it with out thermostat yet, but did change thermostat. The problem starts before the thermostat housing even gets hot enough to open thermostat.
Still have the shifting problem where it jumps in and out of overdrive. Does the tranny have a temp switch which could do that?

Are the cooling fans coming on when it gets hot?

Definitely try running without thermostat. I've seen issues with aftermarket thermostats.

Did you replace the radiator cap?

Thank you Grim Reaper for your help with this.
Yes I have replaced thermostat (twice because I didn't trust the after market ones,) it appears the thermostat housing only reaches 110 degrees.
Replaced both radiator and cap,although cap is also after market.
Both cooling fans work and come on, the smaller one only when AC is on.
AC is low on freon, and compressor cycles on and off. I will address that issue today, someone mentioned that could cause the shifting out of overdrive. (said it kicks out of OD when AC comes on?) Have you heard of that?
Also mentioned changing Trans filter. I know this kid has never maintained anything on this car.
I jacked up the front and have brought it up to temperature and let it sit over night trying to bleed any air out of the system.
I'll be back at it today,
I appreciate any other brain storming ideas.....