I’ve been a mechanic for over 10 years now and I need some advice on this fluid I recently purchased. The shop I work at buys the block tester from a different company (Lisle 2nd picture) and I have noticed it doesn’t always change color. I mainly use it on vehicles that come in the shop that I suspect may have leaking combustion into the cooling system/blown headgasket whatnot. But one day we ran out and they were on back order so I went to HF and got some fluid. After a quite a few uses I started to get suspicious because every single time it changed color. This got me thinking if it was truly accurate so I started testing it on regular vehicles that never had any issues and the HF tester changed color even on them. Then to confirm I used the shop supplied fluid  (Lisle) and it never changed. Now before you go saying I did it wrong, I know how to conduct the test, I always lower the coolant level and I make sure it doesn’t suck any into the test tube, and wait until the Thermostat opens. 

So what are your thoughts on the HF block tester fluid. Does anyone have any experience with it? Could it be more sensitive than the Lisle fluid the shop has been using? I want to know what the best product is so I don’t go selling head gaskets/engines on vehicles that don’t need it.


How accurate is Harbor Freight block tester?