Coolant light on periodically, Prius 2012, 96k miles

One thing I did not know until quite recently was that there are two temperature sensors for the engine coolant (and of course several others in the separate loop for the electronics and motors). Both of those engine coolant readings can be retrieved over the diagnostic port, if your scan tool knows where to look.



It might be interesting to have a log of the various readings to see which ones where were high when the light was on. Some of them ought to set codes (either in the ECM, the combination meter, or the power management control ECU, depending on which reading was high). Any codes retrieved? And retrieved using what?

 

Coolant should never be lost in a good system. It is either leaking, burning (head gasket) or the engine is overheating enough (thermostat, pump, clogged radiator air) to relieve. All are serious problems and are putting your aluminum engine at risk.

Going up a steep hill will quickly raise coolant temperature from 195f to maybe 205f in my experience. Verify the temps when at the top of the hill are not much beyond that and verify the radiator fans are running.

 

There really is not a "coolant" light.
It is an engine overheat light.

Likely that something in the cooling system is not right.
Like a broken water pump, which is fairly common, or a dirty radiator or the radiator corroded internally or the radiator fans not running.

Your small loss of coolant is consistent with an occasional minor overheat.
BUT......this needs to be fixed soon because if it continues to over heat, the head gasket might blow and that might mean a new engine.

 

Excess pressure in the engine coolant reservoir has been mentioned here before. Can’t recall for sure, head gasket going?

 

Your symptoms sound consistent with the early stages of head gasket failure. Two tests that your mechanic can perform.
First is to get the engine up to full temperature then pressure test the cooling system. Remove the spark plugs. Leave it pressurized all night. Next day inspect each cylinder with a videoscope for any trace of coolant.
Second test is also with the plugs out and radiator cap off. Turn the crankshaft so that one of the pistons is perfectly "centered" at the top of its stroke with the valves closed (Top Dead Center of the compression stroke). Use a hose from a compression gauge (with the inlet check valve removed) and connect it to compressed air to pressurize the cylinder (to 120-150psi). If there are leaks it will push coolant and air out of the radiator. Rotate the crank to bring the next cylinder to TDC and repeat.

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SO.....like most owners, you have two choices:

1) Do a little simple reading in places like this and from "real" engine experts if you can find any, and then make an educated guess at what the problem might be. But it will still be a guess and it might take several guesses before you hit the right answer.
ANYTHING that restricts the flow of coolant in the engine can cause it to overheat under a heavy load.
I listed the most likely culprits in my previous post.
Sorry I forgot to mention the thermostat.
It also could be the pressure cap not allowing the system to pressurize. That is the quickest and cheapest thing to try......but that usually allows the loss of a LOT of coolant too.

2) Pay a professional mechanic to diagnose the problem and fix it.
This is often the least expensive option in the end.

 

I'm not 100% certain of that. It may be true, but I haven't confirmed it.

The light is controlled by the combination meter. We know (it's in the repair manual) of at least two reasons it can light: when the sensor that's wired directly to the combination meter (that's the one in the engine coolant hose) hits about 248℉, or when the ECM sends a CAN message to the combination meter to show the light (which can be based on the other engine temperature sensor, the one on the cylinder head, that's wired directly to the ECM). I don't know what temperature that happens at.

What I do not know for sure is whether or not any CAN message to show the light results from inverter or MG overtemperature readings. I think I have seen posts suggesting it may, but it's also possible they've been mistaken. I've never set out to test it myself.

 

This is often referred to as a leak-down test. There’s a specific set of gauges that can be used for this too.

 

bump

 

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