Even more non-intrusive diagnostic techniques!

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

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.

The secret behind Top Technician: Process
I guess it’s just human nature to want to take a peek behind the door that says ‘Private, Members Only’. What could be so special, and what are we missing out on? It’s with that in mind that I’m writing this article. In the next four minutes I’ll reveal what goes on behind the scenes in the Top Technician technical tests, and what it takes to diagnose a vehicle in 20 minutes. You’re going to love it!

I first became involved with Top Technician as a judge in 2008, I was hooked from the start as it epitomised everything I loved about the industry. Watching dedicated technicians work progressively through each technical test was a thing of beauty. Not knowing who would win, as so few points separate most entrants, would keep me on a knife edge throughout the day.

Here’s the deal though; While this is a competition, the skills used to win are exactly the same skills that need to be displayed in your workshop every day. These skills ensure your diagnosis happens in a timely manner, and you can bill all of your diagnostic time. Not only that; Anyone considering taking a Diagnostic Technician or Master Technician assessment needs to display the same skillset. There’s a blueprint for diagnostic success, and if your follow it then you’ll progress in leaps and bounds.

What’s in a technical task?
There are a few core skills that a technician should possess, so each technical task is designed so that a competitor can display the following:
Diesel diagnostics for the workshop
I’m mindful of several recent diagnostic topics that focused on cutting edge opportunities such as noise and vibration analysis. It also reminded me of the most important aspects of fault finding; to focus on the symptoms, ask relevant questions and conduct a methodical approach based on systems knowledge, accurate data and a proven process.

All of this really boils down to training, experience, and confidence. There are no short cuts, cheap fixes or internet gurus. There are however basic steps that are easily introduced into your workshop procedures.

This brings me to the topic in hand. Can we conduct relativity simple tests on common rail diesel systems? Not only can we, but we must! Remember, the foundation rule of fault finding is a simple methodical approach. Don’t expect a magical fix-all in less than 1,000 words. However, I can provide a pathway that will illustrate the area of responsibility and potential investment in time and money.

Vital information
The first vital step is to listen and ask questions. Owners often have vital information. Remember this is not a recipe for short cuts or silver bullets for your machine gun. Your approach will always depend on the extent of problems. Will it run? are there any mechanical noises? Is there a loss of power? if so when? Is the fault intermittent and how did it start? There is an endless list of questions that will help establish a hidden history.

I often find that a physical examination or health check helps understand the way the vehicle has been driven and serviced. This will often expose basic problems especially with charge pressure circuits.

Try to explore all non-intrusive tests first. They may not be entirely logical in order of priority, but do provide results in the minimum time period. With experience, you will hone these steps into a razor-sharp intuitive process.

Serial investigation
Serial investigation is without doubt the correct first step. Do not jump to premature conclusions as serial data often shows symptoms, not cause. For example, a faulty air mass meter will cause EGR calculation error values, incorrect load and boost calculation. This is a common problem with many causes.

The volumetric efficiency relies on the intake system, swirl flap control, turbo spooling, and a free-flowing exhaust system. Please note that I keep my thoughts non-specific yet focused on all possible causes. This is a very important reaction in any diagnostic process.

Assuming a non-run condition, excluding any serial clues as often there are none, I would always check for the correct rail pressure. This can be done with a DMM. Expect around 1-1.5v with a quick rise time of 0.5-1sec. If it is slow to rise or low, check the priming system including the filter. This should be done with a gauge. Remember pressure, flow and pump current. This will depend on system type so check the schematics carefully. Most systems now prime at 5-6bar.

Isolate components
A slow rise time may be due to an internal leak or worn components within the high-pressure system. This includes the HP pump, rail limit valves, and injectors, as well as volume and pressure regulation devices. Always isolate various components and conduct a blind or proof test before suspecting the pump. They rarely fail, unless run dry or have contaminated fuel.
The PCM requires camshaft position data to sync the injectors and crank position once running. If recent belt replacement or engine repairs have been carried out, add this to your list. To check the injector sync against cam and crank position is a bit technical. To perform you will require a scope and current clamp.

Quite often the serial data identifies the incorrect timing sensor for position error. This is due to the PCM looking at the camshaft first. Slow rotation speed may be due to a faulty or incorrect battery, so check charge and health status with a suitable conductance tester. Yuasa have a fantastic free online training academy.

Next check relative compression. This is a simple cylinder balance check but when compared with current and rotation calculation will accurately predict correct compression.

Identify
A blocked exhaust or failed open EGR will prevent the correct combustion properties. Exhaust back pressure can easily be proven from the map and DPF pressure sensors. Plotting them with a scope will quickly identify intake or exhaust restrictions. The maximum DPF sensor value cranking or at idle should be 0.5-1.25 volts, 100mbar-1.5psi.

Injector type, solenoid or piezo faults will normally be identified within serial data. A single faulty injector circuit will normally shut down all fuel delivery. It is also worth noting that if a minimum rail pressure is not reached, the injectors will not be activated.
So back to priming. Leaks, faulty rail sensors will all contribute to a non-start.

If you are looking for more information, visit www.ads-global.co.uk for courses and dates, and Autoinform events.
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