Coolant light on periodically, Prius 2012, 96k miles

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by peetucket, Apr 16, 2021.

  1. peetucket

    peetucket Junior Member

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    Ok, we have a coolant light that comes on briefly every few months, always when driving up a steep hill (which we do a lot since we live on one). It's already been to the mechanic twice: the first time they said everything looked fine, but I asked them to replace the engine and inverter coolant since we were at 94k miles and the interval for those was 100k. A couple months later, same thing happened, and I noted the engine coolant was a tad below the L level in the reservoir, so I topped off with fresh coolant all the way up to H. After a couple weeks, the light came on again and I noticed the level was back to L and there was some coolant that looked like it squirted out the top of the overflow reservoir. So I returned to the mechanic, they replaced the radiator cap, pressure tested the cooling system and said it all looked fine to them. No problems for the next few months, including a 400 mile road trip during which I monitored coolant temp with a bluetooth OBD scanner and iPhone app, and everything looked fine (coolant never passed 195F). Now three months later, the light comes on again and then goes off on a short drive coming up the hill. My wife said she heard gurgling sounds near the inverter coolant, but the engine coolant temp looked fine when measured via the iPhone app and I don't see any obvious signs of overheating or leaking fluids. The cabin heat works fine. We've never had the red triangle or check engine light when this happens and it drives normally - only the coolant light on and then off again after a few minutes. The coolant levels in both the engine reservoir and inverter reservoir are at or just above the L level. Engine coolant temp as measure via the OBD app is never above 195 even when working hard, inverter coolant temp reads 20-23 (not sure of the units??). The radiator fans seems to kick on and off just fine (when parked with AC on they cycle on and off periodically).

    I'm going to return to the mechanic again, but does anyone have any other ideas? Could it be persistent air in the either the coolant or inverter? A bad coolant level sensor?

    Thanks
     
  2. ChapmanF

    ChapmanF Senior Member

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    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.

    [​IMG][​IMG]

    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?
     
  3. peetucket

    peetucket Junior Member

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    Interesting - thanks. I am using the Car Scanner Pro app with a bluetooth OBD II from Veepeak. There are a lot of different sensors including a slew of temperatures, but it's not clear what they are all. I have just been monitoring the one called "Engine Coolant Temperature". I read the codes with the scanner tool and there weren't any engine related codes and the check engine light has never come on
     
  4. rjparker

    rjparker Senior Member

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    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.
     
  5. sam spade 2

    sam spade 2 Senior Member

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    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.
     
  6. peetucket

    peetucket Junior Member

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    Ahh ok, so the coolant warning light on means overheat detected in the engine (not the inverter). I was monitoring coolant temps for a while after the first couple occurrences and never saw it above 195, even at highways speeds going up hills, but then I stopped monitoring when the light stayed off for weeks. I've never caught it in the act - though yesterday when the light came on, we did get the scanner on there within 10 minutes and then drove it for another 10 minutes and the temps all looked fine and the light was off again. I read the engine codes via the scanner and there is nothing there.

    Someone elsewhere suggested a thermostat issue? I don't want to mess around and am happy to replace things that need replacing but don't want to go preemptively replacing something like the water pump if its the thermostat or something else.

    Thanks
     
  7. Mendel Leisk

    Mendel Leisk Sand Pounder

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

    peetucket Junior Member

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    Maybe they should pressure test the cooling system again - I presume that would be manifested there. They pressure tested last time I was there and said it all looked fine.
     
  9. mr_guy_mann

    mr_guy_mann Senior Member

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    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.

    Posted via the PriusChat mobile app.
     
  10. sam spade 2

    sam spade 2 Senior Member

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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.
     
  11. ChapmanF

    ChapmanF Senior Member

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    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.
     
  12. Mendel Leisk

    Mendel Leisk Sand Pounder

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