Testing Coolant Life

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Jun 29, 2014
Melbourne, Australia
Hi all,
I have a Toyota MR2 which some may know is a PITA to change the coolant on. I bought the car quite recently, so I do not know when the coolant was last changed.

I have read on various forums, including this one, that by measuring voltage drop in between the battery and coolant in the filler neck, you can check the remaining life of the coolant.

Instructions for checking coolant life are :
Check your coolant conductivity with a multimeter.
1. Start with cold engine
2. remove radiator cap
3. start motor
4. Set voltmeter to 'DC' (20 volts or less)
5. after reaching operating temp, insert positive (+) probe into coolant.
6. Rev engine to approx 2k and place probe onto negative (-) side of battery.
7. If the meter reads less than .4 volts your coolant is good.

Theory behind this is that electrolysis causes corrosion, and that coolant, over time, will lose its ability to insulate electric current from metal parts of the engine (radiator, water pump etc.).

Of course, theory and reality are completely different things, so if anyone has a experience with using this method, please let me know.

Some thoughts that I have right now are that electrolysis isn't the only source for corrosion, and that in regards to the procedure itself, how is it possible to run the engine to operating temps without the cap, much less run the car at all, without making a massive mess?

Cheers, Kevin
I owned an 88 supercharged MR2 and I know exactly what you mean. I was warned about the problem so at every oil change I would drain out just under a quart and replace it with a quart of 50/50 mix. I'd do it with the heater on and it never caused the dreaded air pockets. The cooling system and the coolant stayed clear like brand new. Be wary of anyone that claims to know how to do it. Ask them where the "secret bleeder" is before you let them continue.
This is a tried and true test. I would recommend putting the voltage scale to DCmV and making sure that the meter has an accurate DC Voltage spec. Also make sure the positive lead does not touch any metal parts. The negative post of the battery is the most negative part of the vehicle. The negative DVOM lead should be on the center of the battery post and not on the cable as well.
Hey hows the MR2 going now?
I remember you posting a while back when you first got it haha Good to see you're using Engine Armour and MoS2 which is pretty much the same as what I put in the AU, either Engine Armour or Durablend 10w30 and half or a full bottle of Moly. I've used nearly everything and this seems to be one of the best combos

As for coolant I get the Caliber 5 year/250,000 stuff from SCA and I'll change it every 2 years to be on the safe side
Funny, as I was playing the same thing on Sunday, as I'd noticed that the coolant in the Caprice' overflow looked a little yellow...was just over 0.2V, checked on the old interwebs and found 0.3 was OK...started and it picked up another 0.03V just after the AC fan started blowing...castrol test strip still fine.

My Nissan which has had a 30% change every year since year 7 or 8 with 50:50 plain green premix (I mix it) was 0.15V cold or running.

Had an idea reading the thread that if you had a bad bleed spot, you could run a hose down the vertical tank, and tip coolant in the hose, pushing old out the filler, no need for air bubbles at all.
I would have thought coolant would be conductive, it would be easy to test, anyone bothered?

Would anyone care to run the theory past me because it sounds like hogwash. The test does not prove that the coolant is conducting electricity only that there is a positive voltage which would indicate a load between the coolant and the negative post somewhere. The load could be outside of the cooling system. So even if it were true the results could be skewed by existing electrical faults like a poor earth which can be common on many vehicles.

Another poster stated the engine does not need to be running so with the key off the coolant would have to be generating its own voltage, like a battery, to get a positive voltage.

Has anyone done the test in say a plastic container?
The coolant is conductive, and is sitting in a system f mixed metals, that are all electrically joined together...it IS just like a battery...which is why we use coolants, with all sorts of additives in them to stop corrosion.

It's not about the coolant being a back-up battery cable (but can be with poor earthing).

A month or so ago, I pulled apart a couple of carbon/zinc batteries, cleaned them up, and melted epsom salts (sodium sulfate), potassium silicate and a few other ingredients into the case with teh carbon rod in the middle. There's 1.2V between the carbon and the zinc, and if I poke the probe into the middle of the "electrolyte", there's still over a volt.

Jabbing the probe in the electrolyte is what's happening in this test.
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