Ideal coolant level (in the reservoir) for LC100 and Lexus RX350

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Hello all!

I've been looking for reliable info on exactly how much coolant should be filled into the coolant reservoir but got different opinions from mechanics and vague info from the manuals. I understand that coolant should always be full in the radiator. But regarding the ideal level in the reservoir tank (when engine is cold), I've heard/seen recommendations on FULL, 30% above LOW, or anywhere in between... Some mechanic told me that for LC100, it has to be at the FULL line, but for my RX350, it can be anywhere from LOW to FULL 😂 . Some mechanics even mentioned that coolant should NOT be at the full level when cold because much room is needed for the radiator to overflow coolant to the reservoir. Pretty confusing 😂

Could someone who did similar research please share your suggestion?
Thanks!
 

js47

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I haven’t looked deeply into this but I was under the impression that it doesn’t matter so long as the level is between Full and Low. I keep mine at Full, thinking more thermal mass is better, plus it is an added safety margin over keeping it lower. But I have seen no significant coolant temperature differences on my scangauge anytime the level is between Full and Low.
 
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I haven’t looked deeply into this but I was under the impression that it doesn’t matter so long as the level is between Full and Low. I keep mine at Full, thinking more thermal mass is better, plus it is an added safety margin over keeping it lower. But I have seen no significant coolant temperature differences on my scangauge anytime the level is between Full and Low.
Thank you for the helpful info!

Btw, in what situations will the scangauge/OBDII measured true coolant temperature differ from the dashboard temperature gauge reading? (e.g. temp gauge shows right below the middle line but OBDII reads a high enough temp to cause engine damage.)
 

JunkCrzr89

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Thank you for the helpful info!

Btw, in what situations will the scangauge/OBDII measured true coolant temperature differ from the dashboard temperature gauge reading? (e.g. temp gauge shows right below the middle line but OBDII reads a high enough temp to cause engine damage.)
Dash gauge won’t redline until you hit above 220s, maybe higher. It’s notoriously unreliable on all Toyotas. OBD2 reader is the only way to get accurate reading.

To your original question: Level in the reservoir halfway between Full and Low *after* a complete overnight cool down. Checking it any other time generally will be unreliable because the level fluctuates with heating and cooling cycles.
 
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Dash gauge won’t redline until you hit above 220s, maybe higher. It’s notoriously unreliable on all Toyotas. OBD2 reader is the only way to get accurate reading.

To your original question: Level in the reservoir halfway between Full and Low *after* a complete overnight cool down. Checking it any other time generally will be unreliable because the level fluctuates with heating and cooling cycles.

Thanks for the helpful info!
What's the ideal coolant temperature measured by OBDII when driving in hot weather with AC on?
 

94SRUNNER

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Thanks for the helpful info!
What's the ideal coolant temperature measured by OBDII when driving in hot weather with AC on?

From what myself and others have observed it seems to vary slight depending on whether you have a non-VVTi or VVTi engine.

For myself, I’ve got a 06’ VVTi and in summer ambient temps (70-90*) I see 186-190 depending on engine load. Having the AC on doesn’t seem to affect my coolant temps much.
 
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Coolant reservoir level is easy, fill it to the very top.
There is a overflow tube. The excess will come out of the overflow and the level will settle when the engine cools.
And that is where the level will stay.

The reservoir has zero to do with engine temp. Back in the day cars did not have a reservoir and the radiator was never completely full, but radiators were huge and made of copper and brass, which is excellent at transferring heat.
With the smaller aluminum and plastic radiators we need every square inch of cooling capacity, so the radiator needs to be full at all time, so the reservoir was added.

Coolant get hot, it expands, the radiator cap opens up under pressure and lets the excess coolant out and into the reservoir. That’s why it has a spring.
As the engine cools and the coolant condenses back the system pulls coolant in from the reservoir, so the radiator is always full.

The excess amount in the reservoir is there for any coolant loss as a buffer, that extra half gallon of coolant could mean the difference between getting to safety or not.
 

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