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I performed a block test recently to diagnose a problem I'm having. The liquid stayed blue, but I want to clarify I have done this right. The purpose of the test was to see if I could prove a head gasket failure.

enter image description here

After removing some of the coolant using a siphon tool, and placing it in the coolant storage area of the vehicle, I started the engine and tried to test, the coolant level rose and I had to remove more coolant again, but only once.

Then I used fresh test fluid, I placed the test unit over the radiator to create the air-tight seal, and then let the engine run awhile, then I used the bulb to suck air through the test fluid.

The instructions say to do it for 2 minutes. What happened with me was that I apparently sucked out air until there was a negative air pressure, and no more air could be sucked out using that bulb.

So, I did the test once sucking as much air as possible in a short time, and after that, I did it again and paced myself so I could steadily do it for 1½ minutes. I was careful not to let any 'fresh' air into the tube or the radiator. It refused to turn green or yellow.

I did test the fluid itself. The same blue fluid turned yellow when exposed to the exhaust.

My question

  1. It it normal for this negative air pressure to occur?
  2. Is it better to remove air quickly, or slow/steady over the course of 2 minutes?
  3. Supposing there was a blown head gasket, would this negative air pressure still occur?

And of course I welcome additional relevant information such as how this varies between various vehicles, a summary on how to ensure a good test and/or the limits of what this test could prove or disprove.

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I would say that if the overflow tank only has a small hose from the over pressure nipple on radiator right under cap then that is normal and you should put the tester right on the radiator cap mouth, as the overflow isn't a reservoir and won't be getting circulation from coolant system until the cap is over pressurized. As opposed to a reservoir or expansion tank that has a (usually at least 1/2"id) feed hose to radiator.

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