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2004 engine coolant temperature erratic

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by mark010101, Jan 27, 2014.

  1. mark010101

    mark010101 Mark 04, 05, 08 Prius Owner

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    I have a 2004 Prius and have a scan gauge. I constantly watch engine coolant temperature. I noticed it being very erratic in the summer (going up to around 180 deg F and then falling back to the 160s then back up again).

    I decided to replace the thermostat hoping that this would solve the problem. It did not. Now that it is winter and very cold, the coolant temperature never reaches the thermostat temperature (82 deg C or 180 deg F) and is struggling to get above 150 to 160 deg F for trips > 10 miles. I have blocked most of the radiator and still the same problem. MPGs can get in the low 40s for longer trips (> 10 miles), but for short trips they are in the mid 30s (1-3 miles). This is on the low side for what I have gotten in the past. When the engine temperature is erratic, the combustion efficiency suffers because of uneven temperature in the cylinders.

    I have noticed the cabin heater is erratic at times as well. Great heat some times and at other times moderate heat. I attribute this (obviously) to the coolant temperature swings.

    Also I have not heard the storage tank coolant pump run after I shut off the car during cold weather.

    Would the issue be the coolant control valve? I have had no error codes set at all (specifically P1121). If it is the coolant control valve is the a test procedure to make sure this is the issue? Also are these covered under recall like the inverter coolant pumps were?

    Thanks and I look forward to hearing your thoughts.

    Note: I have the shop manual for the 2004 and am comfortable with nearly all Prius maintenance.
     
  2. Patrick Wong

    Patrick Wong DIY Enthusiast

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    Is it possible that you have some air in the engine coolant loop? It is not easy to get all of the air out, and that would explain why the cabin heater is sometimes ineffective.

    My posts #22 and #42 in the following string describe my procedure to get air out of the engine coolant loop. Subsequently, I found that the Lisle funnel is highly effective in assisting this process.
    Changing engine coolant | Page 2 | PriusChat

    If you have any concern about whether the coolant heat recovery system pump is operational, my procedure describes how to apply a short to the CHRS relay socket to force the pump to run. This is also a method to help get air out of the CHRS portion of the engine coolant loop.

    Here's my "how to" regarding the engine thermostat and engine coolant pump:
    How to replace engine coolant pump and thermostat | PriusChat

    If you don't have the check engine light on and no logged DTC, then I would not worry about the engine coolant control valve. It is not covered under a recall.
     
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  3. mark010101

    mark010101 Mark 04, 05, 08 Prius Owner

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    Patrick: Thanks for the response. I am 99.9% sure there is no air the in coolant system. I used established procedures described here and in the service manual and put back exactly the amount of coolant the was drained out when I replaced the coolant several years ago and for the recently replaced thermostat.

    Do other Gen 2 owners have issues with erratic engine temps? It lowers MPG. You need constant cylinder wall temperatures for efficient combustion. I can't believe the Toyota engineers design the ICE cooling system to behave this way. Another other thoughts or suggestions? Thanks all.
     
  4. Patrick Wong

    Patrick Wong DIY Enthusiast

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    What are the winter ambient air temps in your area?

    My belief is that the Prius gasoline engine produces relatively little excess heat and in very cold ambient temps, it would not surprise me that the engine coolant does not get sufficiently hot to open the thermostat.
     
  5. mark010101

    mark010101 Mark 04, 05, 08 Prius Owner

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    Well I do live in Illinois and it is -1 deg F at the moment :) However as I mentioned in my original post, I was noticing the erratic coolant temperature in the summer (mid 160s to 180 deg F summer vs 140s to mid 160s deg F for winter). Previously (before last summer) the engine temp would reach 180 to 184 and hold constant (summer or winter). Perhaps the coolant control valve is broken and sending coolant to both the coolant storage tank and the heater and it is not setting a DTC for some reason?

    I know it might be a pain, but it might be worth it to remove the coolant control valve and inspect it. I will have to look through the shop manual to see if there is a test procedure or perhaps a visual inspection will confirm the problem.
     
  6. Patrick Wong

    Patrick Wong DIY Enthusiast

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    OK, this is a reasonable thought. The valve might be stuck in a halfway position (which actually is an acceptable operating position depending upon conditions.) However:

    DTC P0128 is supposed to be logged if the engine coolant temperature persists below 75 degrees C (167 degrees F).
    DTC P1120, P1121, P1122 or P1123 are supposed to be logged if the coolant valve is not properly functioning.

    It might be worth your while to use a handheld IR thermometer to verify the engine coolant readings you are getting, at least when the engine thermostat is open and coolant is flowing out of the thermostat housing and through the radiator hose to the radiator.

    Good luck with this project.
     
  7. mark010101

    mark010101 Mark 04, 05, 08 Prius Owner

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    Thanks Patrick --- good point. I am assuming the engine coolant temp is a reliable reading. It may not be. I will need to check the shop manual on the test procedure but I like your IR thermometer idea too. I should also move the scan gauge to my wife's 2005 Prius and check readings/behavior in the extremely cold weather.

    I guess the hardest part of the pulling off the coolant control valve is draining and refilling the coolant system which I have done a few times in the past and should be an expert by now :)
     
  8. mark010101

    mark010101 Mark 04, 05, 08 Prius Owner

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    Patrick (or anyone else) I assume that the Scan gauge temperature defaults to the Prius ICE block coolant sensor and not the coolant storage tank sensor. I watched the temperature again as I drove to work today and it never got above 159 deg F. Something is definitely not right with these erratic temperatures that is why I started to wonder which reading was being taken by the Scan gauge.
     
  9. Patrick Wong

    Patrick Wong DIY Enthusiast

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    I do not know.