Even more non-intrusive diagnostic techniques!

Frank Massey continues his look at the benefits of non-intrusive diagnostic techniques. Cue the Audi RS3

By Frank Massey |

Published:  21 October, 2020

Last month, I was debating the opportunities with non-intrusive diagnostic techniques, and more to the point the reliability of results. I think it is important to accept, as with all skill-based process,  the accuracy and results depends very much on experience. A second opportunity presented itself for this topic in the form of an Audi RS3 with a very sick engine.

I’m going to make my thoughts truly clear at this point; I see no point in applying a potentially complex series of tests where simplicity prevails. The Audi RS3 is a prime example of this, with a totally dead cylinder. We must however understand all techniques where cost, accessibility and risk factors demand an evidence-based decision.

With that cleared up let us review and discuss the series of tests carried out. The owner was somewhat vague as to the history of the problem. He explained the problem had been present for some time and hoped he could drive through it. As this topic will later confirm he has driven right into it.

Mechanical resistance
Due to the severity of the misfire, a decision to conduct a relative compression test was sufficient to confirm a serious internal engine defect. David and I were curious to challenge other options to determine the full extent of failure without component removal.
Attaching a current clamp around the ground lead, we were able to compare the mechanical resistance to battery current consumption, this can also be performed with voltage drop or both. The logic here is that all cylinders should balance. This test will not confirm valve timing errors or low compression across all cylinders! However, if you apply the x3.5 rule to the amp/hr battery rating, you should be able to predict the correct work rate and rotation speed, assuming of course you have confirmed correct battery application and health status.

We have no current consumption from cylinder 1 possibilities, problems with valve operation or piston to bore seal. The next test was to attach the first look sensor to the dip stick tube (see Fig.1), with the obvious aim of predicting potential cost and action plan.
So, it about as bad as it gets, the drop in current draw is synchronous with a rise in crankcase pressure rise. Oh dear. Annette did a cost exercise with a new engine replacement and turbo, inclusive of labour with no change from £40,000.

Ultimate techniques
For the purpose of comparing in cylinder compression using WPS and first look in the exhaust we now move on to the ultimate engine internal analysis techniques. My interest here was to compare actual in-cylinder events and exhaust exit pressures in real time to ascertain any delay and if cylinder overlay could be used to confirm which cylinder event was responsible for the result.

I will re-state my opinion here, having spent the first 20 years of my career as a professional engine builder I do not care which cylinder is faulty or what the internal fault is! Why? If I’m going to rebuild the engine, then it’s all coming apart for examination. Professional pride and reputation is priceless, so unfortunately nobody wants to pay for it!

Having fallen of my soap box, I do accept as diagnostic technicians we must provide the customer with a factual and accurate estimate with the quickest low-cost process. apart from the fact I find in cylinder and vibration analysis fascinating.

Important variables
Before discussing the complexity of Fig.2, there are some important variables that affect results, remembering that we are dealing with pressure differential or absolute values otherwise none of this will make sense.

Assuming a good in-cylinder seal, the slower the piston speed, the greater the pressure differential. For example, cranking compression is approximately three times greater than when running at idle. This is because of pumping losses with a closed throttle, the descending piston creates an expanding volume that has more time to draw in a fresh air charge, therefore higher compression.
A weak cylinder will accelerate up the bore quicker due to a drop in resistance, lower compression, and accelerate down the bore as the pumping losses are reduced, lower drag.

The first look sensor in the exhaust will record an increase in pressure drop with a weak cylinder due to the lower initial compression followed by the expansion in volume and corresponding increase in pressure differential when the exhaust valve opens. This causes the classic intake pulse at the tail pipe.

With the scope, green channel, and overlay triggered from cylinder 1 PCM ignition pulse, you can clearly see an extremely poor compression, erratic pressure rises and poor tower symmetry. The exhaust cycle is erratic with poor definition when the inlet valve opens. The expansion and intake voids are poor, also confirming a faulty
cylinder seal.

The first look image, red channel, shows multiple increase in exhaust pressure voids which I find unhelpful.  It does not in my opinion add any useful diagnostic value. I’m happy to accept any alternative opinion.


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