Radiator fluid boiling? Both 95 and 96 camry

Well, Your sure that there is no air in the system and that the radiator is full of coolant ( not just the overflow tank, the radiator itself ). And you have replaced the thermostat and rad cap (aftermarket), But did you check the sealing surface on the radiator side of the cap ?? There are 2 seals here, one outer ( the cap itself) and the Inner ( this is for the pressure relief valve on the cap). If the inner sealing surface is deformed from the defective old cap then the coolant will not get pressurized ( decreasing the boiling point) and will just push coolant out into the overflow tank.
The way it should work is like this, radiator when full and cold should have the overflow tank coolant lvl to the cold line, when the engine is warming up the system will start to pressurize, once it reaches above about 13 psi the cap will open and let the excess pressure out and flow some coolant into the overflow tank and bring the lvl up to around the hot mark and stabilize.
Then once the engine is shut off and the system starts to cool off decreasing the pressure until it starts to go into a vacuum, At this point it starts pulling coolant back in from the overflow tank past the cap and into the radiator thus bringing the lvl in the overflow back to the cold mark.
Now if the radiator cap is bad (not holding pressure) or the Inner radiator sealing surface is bad the system will never gain pressure and just push coolant into the overflow tank (no pressure = lower boiling point causing more coolant to be pushed out). Now when shut off and cooling down the radiator cannot pull coolant back in from the overflow tank (no pressure to begin with ends with no vacuum to pull it back in) = low coolant on your next restart and it keeps getting worse.

And don't install a lower temp thermostat, you will just create driveability problems and possibly inhibit system monitors from running.

Also, the ez way to bleed the cooling system on a 5sfe is to remove the coolant temp sensor, The one on the upper radiator hose housing at the rear of the cyl head, it will have a green connector and takes a 17 mm wrench or socket, dint loose the copper washer. Fill the radiator until coolant starts to come out of this hole then reinstall the sensor and top off coolant.

So a quick little update...

I have driven the car about 130km or so today (70-80 miles) and I am noticing the fluid levels are fluctuating still a lot. Few things I noticed this morning was after driving it for about 20 minutes (with one stop along the way) I came home and with the car off, the fluid was actually pouring out slowly from the reservoir bottle top.

Anyhow I went to canadian tire to pick up a few things and when I came out, there was I'd say about 300-500ml of rad fluid on the road. I checked my reservoir bottle and it was pretty much empty at this point. I drove the car home, check the levels, now the level was back to about half way point (between full and low). I shut the car off and kept an eye on the reservoir bottle...within a few minutes it was back to the full line.

So my question...my mechanic says all of this is normal, so long as the temperature gauge never rises. He thinks we simply over filled the coolant levels when we replaced the thermostat and cap and it's simply dumping excess fluid out. He also said that rad fluid is "supposed" to boil and therefore I shouldn't be worried if the rad fluid is boiling inside my rad hoses and reservoir bottle, these are all normal occurrences...

Am I just being too overly cautious or should this be a cause for concern?

3 things:
A leak anywhere in the cooling system can compromise the whole thing as it may not be able to pull a vacuum and refill the engine block when the system is turned off and cooling down.

As for hot engines purging;
Water expands when heated. Thus a system full to the brim will likely force some coolant out during engine warmup.

When the engine is running the burning gas is creating BTUs that must be shed. The exhaust dumps the most BTUs, the radiator, and the block do the rest. BUT, right after you chop the key, there is still a huge amount of thermal BTUs to be shed but no exhaust flow of cooling system to do it so the engines temperature continues to rise for about 5 - 10 minutes and spikes. Often times this is when the reservoir gets coolant dumped into it as the pressure exceeded the caps rating and coolant is forced out of the block. Then the system begins to cool down.

As the engine cools, the density of the water increases, it takes less space, and if all things are correct it pulls a vacuum against the valve in the radiator cap pulling fluid back into the system from the reservoir.

A final note: ever notice how the fuel gauge and temp gauge remain pretty steady no matter what. I they have a means of averaging so they don't scare the hell out you! I know for certain the MZ's temp gauge can mask a 30 degree variance (+/-15F). Watch the OBDII data!

73sport said:

Its normal for fluids to go back and fourth between the res and the block. Its expected. Just because it dumps into the res does not mean its boiling over. If it routinely boils over you'll see the reservoir overfill and dump inside the fender. You would likely see the temp climb as the engine begins to over heat.

If this concerns you and you need an answer for your sanity, grab one of those laser temperature gauges and shot it at the water outlet before and after the thermostat. Depending on your elevation, glycol mix, and radiator cap, you might expect to see a reading of 224f as it boils over. Purging into the tank at lower temperature is more than likely just the water expanding as the block heats.

You could also by some OBDII scanning software/hardware too and glean some more data!

Click to expand...

Ok so let's add a couple things to the equation. As you had mentioned the car still seems to be pushing fluid from the reservoir bottle back to the rad. In some instances when the fluid I guess gets super hot it begins boiling over and slowly pouring out the cap of the res bottle back into the fender engine bay area. As of four days ago my fans were not functioning and therefore the car was overheating and the Gauge would go closer to the red (3/4 the way). So I'll assume since I'm no mechanic that the car is NOT overheating since replacing the fans and the fuse, it engages the fan operation normally.

Is it possible that the system simply is pelt pushing air out of the system being it's over filled and eventually this will all go back to normal operation? Seems after doing a rad coolant change in my 96 it did this very same thing and eventually over time the problem vanished..

Am I nuts or is this definitely an issue? As of now if the needle stays at the half way mark as it should be I'm not worried driving it..but can any of this do any damage with hot rad fluid leaking to the engine bay area??