Block tester experience, head gasket leak testing

I ran into something odd today using a radiator / coolant bottle opening head gasket leak tester known as a "Block tester". The parts stores rent them.

Who here has used these on a regular basis.

I have used this one about 4-5 times. But I got an odd result today. And I am not sure what to make of it.

I last used it (I bought mine) on my sons ford that bubbled like mad and changed the fluid color in about 1 second the leak was so bad (exhaust into the coolant mostly).

I tried it on a coolant leak on the Saturn today, and no bubbles at all at 1500 to 4000 RPM, but the third test it sucked the fluid out of my tester into the coolant bottle at 3000 rpm???? I think the check valve at the bottom is sticking (It did that on the ford on me once before). But it emptied the fluid out at fast idle like it was pulling a vacuum on the cooling system? Could that be the water pump, a differential pressure oddity, or a an intake head gasket leak sucking in coolant?

A few bubbles came out of the stone at idle, but not at fast throttle. But the coolant was cold, still warming up and expanding still, so not conclusive yet on the bubbles at idle.

It is an overhead cam 4 banger engine if that matters.

This discussion should apply to testing any jeep as well with a block tester. I have found it to be a great tool for confirming a head gasket leak, or proving it is not one. I had a water pump suction leak when cold on my diesel once that drove me nuts, thinking it was a head gasket. This tool sent me to replace the water pump (thinking the shaft seal was bad), where I found the bad gasket under the water pump. It was sucking air into the pump at high rpms, but never leaked coolant. Damnedest thing I have ever seen in decades.

I block check all the time at work, I'm a mechanic, generally I notice no pressure to slight suction until the t stat opens. I've never really over analyzed the bubbles part. I typically drain the radiator or suck some out before I block check. It tends to stabilized things, gives a place for carbon monoxide to build up and prevents sucking coolant into the tool. Also I replace the fluid twice a year if I don't use it all or if the blue starts to turn color a bit on its own. I've seen new fluid react a lot easier then old fluid.

Lastly, if your trying to narrow down an issue some times I'll do this...

Add leak detection dye too coolant
Run car until t stat opens and dye is mixed
Turn car off, disable fuel system and crank (get rid of any fuel in the injectors)
Sometimes I put a psi tester on the cooling system or leave the cap off, try both ways but let it sit over night
In the morning crank the engine and pull the plugs
Lastly see if any plug has traces of dye on it.

I've found many blown and cracked heads this way.

No head gasket leak after all on the Saturn. :sunshine:The Water pump shaft seal was on its last legs, may have been drawing in air at high RPM at the seal face, and then blowing air out the bottle at low rpm or something like that. Had that happen on my Diesel once, acted just like a head gasket leak in the radiator side, but it was the water pump gasket and shaft seal going bad slowly. It was slinging coolant at high RPM, found the slinger/spray pattern on the hood.