2012 with 279k miles overheating

B1503 code is pretty specific. Maybe look ➡here⬅.

 

B1503 Exhaust Heat Recirclation System overheating after sealant:

This is pointing to a) no flow through the heater (there is no valve in the heater loop), b) heater core restrictions caused by the sealer, c) clogged engine thermostat d) reduced flow from the engine water pump (typically a cracked or swollen pump impeller) or e) insufficient coolant volume resulting in the heater core being air locked (a low coolant volume side effect).

Radiator clogging is not very common but aftermarket radiators or water pumps are not recommended.

Its not clear how you verified the water pump flow but it can be intermittent or it can have partial flow without setting its own code.

However your b1503 code directly points to the Exhaust Heat Recirculation System (EHRS) which has a second coolant temperature sensor downstream of that system on a small diameter hose.



That sensor sets your code at 248f, which is over 50f hotter than normal operating temperatures. Not good.

Monitoring both temperature sensors helps by using Toyota Techstream or a good bluetooth based app like "Car Scanner". Car Scanner is easier to read and graph. Using Techstream, you can read one of them in the Data List for "Engine and ECT", and the other one in the Data List for "Combination Meter".



Normally exhaust flow through the EHRS system is diverted through its heat exchanger when cold and mechanically opens when hot to allow straight through flow.



We have seen cases where the EHRS remains closed in its initial diverted state causing high downstream temperatures which then trigger the b1503 sensor.

A few stuck EHRS cases without other factors like sealer, head gasket or water pump issues have not caused overheating on the engine block as reported by the other temperature sensor.

Further isolation is needed. The fact your reservoir cap relieves indicates high temperature and pressure there although the reservoir loop bypasses radiator cooling.

 

Any block sealant can and will block any narrow passages; including and not limited to the thermostat, radiator, EGHE, heater core, and coolant switch valve - if so equipped. I'd pull the bottom radiator hose to see if there's any sludge there. I believe that stuff is heat activated, so that stuff is more than likely solidified in the EGHE, as well as other areas of your coolant loops....

 

Stop-leak should be illegal.

Of interest, asking Google AI about how to dissolve/flush stop-leak products:

https://share.google/aimode/KLM19femvijjfffPa

And specifics about what cleaner to use:

https://share.google/aimode/O1p19gwBdiy8NOEBd

 

Well...at least the OP was warned about it.
Good info on the 'fix to the fix.'

 

New parts fail especially if they were aftermarket. Many have reported short lives of aftermarket water pumps.

No heat means no flow through the heater when you also have an overheating situation.

You are risking the engine if you have 248f at the first sensor.

Monitor the two engine temperatures, ideally with graphical trends and you can isolate.

Verifying the flow and debris at the bottom of the radiator as suggested above is useful. Especially if you can't monitor data.

Sealer can clog systems - its not theoretical

 

225f is hot for the first temp sensor but you are not monitoring the second temp sensor described above. Its the one that coded.

Are you seeing flow into the reservoir?

Don't forget no flow into the heater.

 

A bad reservoir cap needs replacement. However it won't cause 248f warning light overheating by itself. It won't cause no flow into the reservoir or through the heater.

I have mentioned twice above the other temperature is in the Combination Meter part of Techstream.

Zero flow movement is not a big concern? Assumptions are killing your engine.

A half empty reservoir in one day is not likely from cap relief. Its being vaporized by a bad hg leak (sometimes from loose head bolts or a warped head / block) or by a leak through the (wait for it) the exhaust heat recirculation system. You could be half empty in the entire cooling system.

With a diy hg repair and sealer, normal gen3 hg symptoms may not apply.

With no reservoir flow you are a) not viewing it properly when warmed up or (b) something is seriously blocked or (c) the water pump impeller is toast.

I would suggest towing it to a dealer and stop assuming. My guess is you are not totally conversant with the cooling system flows or how the exhaust heat recirculation fits in.

It might help to reread this thread, take notes and research things you need more insight into. When we can't even find out if the water pump is aftermarket because of assumptions (?) we are less than effective.

 

Did you flush out the cooling system before or after the head gasket was done?

You've lost a few points when you tell us it was running flawlessly for a month, yet your operating temp. is 225F????? It should be around 185F; so you probability popped the head gasket again by running it hot and/or warped something. Clogged cooling channels will cause hot spots everywhere and may block super hot coolant from touching the sensor. So you think everything is running great; until the dummy lamp lights up.....
Did you check for warp while doing the head gasket; since the previous owner obviously drove it like that for a good long while???? As I've stated before; the only time I'd take a gen3 is if it's FREE and looks good.

 

Stop and spit ball this issue:

No flow through reservoir or heater.

As you are a mechanic you know the reservoir is not an overflow tank; its part of the circulation when the thermostat opens assuming the pump works.

As an experienced guy you know stop leak clogs up flow and counterfeit water pumps are sold with genuine packaging.

No flow in reservoir or heater. Oh well...

Possible excessive heat in exhaust heat recirculation system

Excessive coolant loss overnight

 

You can reverse flush the heater core & EGHE, tie a panty hose on the exhaust end to see if you blow out garbage. Just because the coolant pump is turning; doesn't mean there's flow. Did you forget about the stop leak???
Did you run a straight edge on the aluminum block? It's really soft material when compared to those old iron blocks - so it doesn't take much effort to blow a dip into the block; when someone is using stop leak to bridge an issue. You know they've been abusing and running that engine HOT.
As a former mechanic, you figure things out and TEST - NOT guess and assume. Otherwise you'd be losing time and money; if you guess wrong; especially if it's a customer's car......