Quote:
Originally Posted by SD1too
I have never been entirely convinced by the chemical testers either.
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It's always a question of degree or concentration. These so-called 'block testers' rely on detecting carbon dioxide from the exhaust gas dissolved in coolant. In an obvious/severe case of HGF, exhaust gas will bubble continuously through the header tank, so you know anyway. In less obvious situations, CO2 has to be desorbed from the coolant and into the test solution in sufficient quantity to reduce the pH. There has to be enough to flip the colour of the indicator (due to acidity). So, a very small long term exhaust leak may not increase the concentration enough. The gas level is very low because of ongoing thermal desorbtion and venting, coolant changes, top ups and the like. Basically, the test is very prone to false negative results (as in the current case). However, it will never, ever give a false positive. Other factors also affect the reliability, like reagent freshness, the operator's competence and using reliable hardware. As with many engine issues, it's just one diagnostic among several. For me, the long term coolant loss and black oil clinched it.
TC