December 2020

DECEMBER/JANUARY 2021 AFTERMARKET 27 Fig. 3 Engine bay inlet module removed Given the current SCR additive system, engine layout, and lack of accessibility the process and tools will determine success or failure. Cylinder assessment Given that the pressure cap was showing deposits on the header tank spill, although not consistently, suspicion lay with compression entering the coolant jacket. Applying the chemical combustion leak detector on the expansion tank showed no evidence of combustion gases within the coolant. So, a new cap was fitted with no effect. The next option was to conduct a live in-cylinder compression test. The problem with diesel vehicles is the omission of pumping losses (the resistance to engine Volumetric efficiency), so it is imperative to introduce an intake restriction this allows for a drop of in cylinder pressure during the intake stroke. Most of you by now will accept my assertion that vacuum does not exist where as a pressure differential is much more accurate for in cylinder assessment. By restricting the intake, a greater pressure differential is present during the pistons descent, therefore confirming good sealing properties of valves, piston rings and hopefully cylinder head gasket. Driving conditions With faults still not found thus far, David’s next move, was in my opinion, a textbook in logic process and intuitive thinking. The clue lay in the fact that coolant loss only seems to happen during driving conditions. David attached the Pico WPS to both the charge pressure circuit and coolant jacket. When driving the vehicle on load, both pressure sensors indicated an increase in pressure during turbo assistance. In simple terms, the rise in pressure was symmetrical. Convinced the head gasket was not at fault, David assessed the problem to be the inlet cooler. Removing the cooler and conducting a pressure test confirmed an internal leak. So, in conclusion based not on opinion but actual test data evidence, David assessed the problem as a positive pressure differential during turbo boost, which was pressurising the coolant jacket, and pushing coolant out of the filler cap. This is the reason why I always discuss pressure differential rather than suction, compression, or vacuum. Why? Pressure differential produces flow, from high to low. In conclusion, avoiding the catastrophic error of a wrongly diagnosed cylinder head gasket, a new intake cooler was fitted.