Logic, process and intuitive thinking

Frank highlights the importance of a logical approach when dealing with a diagnostic conundrum

Published:  13 January, 2021

Every so often a challenge comes along that demands, knowledge, skill, and a high degree of logic for the approach. But first, a little reflection over the last few months, and a trend I have noticed, namely that I do not quite understand why we are undertaking such major repairs on relatively recent vehicles.
   
What could be causing this? One possible reason could be a combination of complacency, lack of affordable maintenance funds on the part of the owner, or a substandard maintenance history. At ADS we have probably replaced six or more power plants with costs reaching very high thousands.
    
Are they owners purchasing vehicles they cannot afford to maintain properly, thereby leading to catastrophic mechanical failures? Or are these unlucky drivers simply not receiving the right kind of professional advice from the independent sector? If the latter is true, then we all need to take on the responsibility before we are branded bandits and opportunists cashing in on vulnerable owners. I’m not suggesting that a garage should become a charitable institution, but surely there is a profitable middle ground?

Distinct priorities
Back to the point at hand. This month’s problem could have developed into a major diagnostic failure had it not been for ADS’ Dave Gore, our diagnostic lead technician, a.k.a Diagnostic David. I would also like to thank James Dillon for th week’s boot camp training. Peter, our workshop technician, returned enthused and confident in his new skills.
    
The vehicle under consideration is a VW Golf 2.0 diesel EDC 17 common-rail with SCR after-treatment, which includes dual EGR.
    
I am going to begin with an overview of the potential complexity and problematic SCR additive system. Manufacturers are wrestling with a greasy pig in their attempts to clean up diesel combustion. I accept there has been big improvement, but it falls well short of the ideal and has without doubt introduced more problems than improvements.
    
The dual EGR system has two distinct priorities from cold. The hot exhaust gas is diverted by the high pressure EGR valve directly into the inlet manifold. The purpose is to rapidly heat the catalyst and DPF.
    
The low pressure EGR acts in a traditional manner with its priority to reduce combustion temperatures therefore reducing NOx. So far two valves, and the third valve is an exhaust brake. This throttle is fitted after the DPF/catalyst in the exhaust downstream, and is partially closed to raise the exhaust gas pressure during SCR additive treatment causing the gases to make a second pass through a water cooled egr cooler and DPF/CAT. This ensures the urea is fully saturated within the substrate reducing NOx.
    
The intake module, as it called with VAG vehicles, also has a water-cooled intake air cooler.

Discreet and regular
Our Golf was subject to a discreet and regular loss of coolant. No external leaks were detectable, except what appeared to be a leaking pressure cap. The car had no obvious issues, was smooth running with exhaust emissions that appeared normal.
  
Rather than just dive in and confirm the problem, I think it’s much more important to explain the tool options and diagnostic process. I have often used this phrase on my training courses many times; “The process is more important than the repair.” In other words, knowing how is a greater priority.
    
Water loss possibilities? External leaks or internal leaks? 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.

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