Autozone block tester kit question

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Dec 29, 2008
Missoula, MT
I rented out one of these to test my head gasket recently but have yet to use it. Sounds like they work well, but I was just curious on a part of the instructions. It states "With engine warm and idling" as you use the kit on the radiator hole. My question is how do you open up the radiator cap if you are on a warm engine, taking that as 'full operating temperature'? Though limited, my knowledge has always been you don't want to open the radiator cap if the engine is hot. Do you just do it very very slowly?
When i had to open hot radiator cap i used rag few layers and heavy duty rubber gloves just in case. I turned the cap extremely slowly letting the pressure to escape before removing the cap. If you never done it before you can practice on cold radiator and after on warm (not hot) radiator.

My guess if the manual is saying that it should be warm that it should be warm and not at full operating temperature. But it is just my 2cents. Try to search you may find more information about at what temperature you should do the test.

Good luck and be safe.
Maybe open the radiator then warm it up?:doh:
I waited about 10 minutes after arriving at home since it'd be too hot for comfort, then removed the cap, removed the amount of antifreeze necessary (more than you first might think), then cranked up and warmed it up.

FYI, be REALLY aware of vibrations kicking up even a little bit of antifreeze near the test kit, it'll give a false positive reading on you. Axe me how I know.
I removed the necessary amount of coolant. Started my truck cold. Cranked the heat and just let the truck idle until warm. It does not take that long to warm up.
I have used this tool many times. I usually do it about 10 minutes after driving the vehicle. After letting it sit for 10 min, I take the cap off, then start it and I do the test a few times. I do it when I first start it and then wait a bit with it running and do it again. It's really that easy and fast, you can do it several times. It's the best way to know if you have a bad HG. I've used this thing on all types of vehicles and I love it.
Is running it with the radiator cap off going to foul up the test at all? Wouldn't you be purging exhaust gas if there were any present?
Is running it with the radiator cap off going to foul up the test at all? Wouldn't you be purging exhaust gas if there were any present?

The exhaust gases, if present, are in the coolant itself not just as a bubble in the radiator. You are actually testing to see if there are exhaust gases mixed in the coolant. As the coolant passes by the HG, if it's bad, it will pick up traces of exhaust gases. The sniffer pulls the fumes coming out of the radiator opening for exhaust gases being emmitted from the coolant. Pretty simple and effective test.

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