Non-intrusive diagnostic techniques

Frank Massey examines various ways for you to get the answers you are looking for when working on a vehicle

By Frank Massey |

Published:  04 September, 2020

The last two topics in recent issues focused on combustion issues and the various tools, service and repair process available to us. Two reasons have directed me to develop this debate further, firstly an email from my much-respected friend Phil Ellison at ASNU, and a VW Golf edition 30 presented to our workshop with poor running at low and transient throttle position. I was also involved in a conversation with friends in Perth, Australia over valve timing issues.

I’m going to respond to Phil’s interesting input first and clarify something especially important to all diagnostic techs. All decisions we make must be evidence based and not opinion. This is an extremely broad statement, but simplifies the fact that if you do not have access to the required tools, software, or process skillsets your decisions will be opinion-based!

I can relate this to my time building military aircraft, where nothing ever happens as a result of opinion. You could quite literally switch off and simply follow the build schedule and submit your work to inspection. You were not paid to have an opinion. This is why I left!

I may have previously left an impression that it was not necessary to fully evaluate injectors in a test bench, if this was so, then I apologise as my thoughts are the exact opposite. My intention was to ensure that you fully explored all causes of incomplete combustion while the engine is running, as most engine work now carries a high labour content! Do not, however make the mistake of letting cost dictate your process. Phil did pick up on the common issues of injector removal damage where specialist tools are required. The use of fuel additives, which can be a common cause of internal injector damage especially to plastic filter baskets, where any debris is then deposited in the basket effecting fuel flow. Direct injection technology now demands the absolute best fuel quality, often reinforced by manufacturers placing fuelling advice inside the filler flap.

Phil also picked up on a common issue I did omit; Stop/Start. Hot engines with an increase in stop events, with fuel trapped in the injector often causes lacquering of the pintle. Heat in the combustion chamber dries any combustion residue and oil on the injector tip. I’m coming to the inlet valves very shortly…

Fuel trim or correction does not fix problems, it can exacerbate them, imbalance in injector delivery or as Phil pointed out deterioration of the spray pattern will cause bore wash, premature lubrication failure, and an increase in crankcase emissions, larger fuel droplets do not combust fully.

Interestingly, he pointed out that new injectors are produced with a +/- 5% tolerance.

Potentially misleading evidence
The Golf appeared in our workshop just a few days after I had finished my topic.  I was not involved in most of the diagnostic process or repair but was in discussion over potentially misleading evidence.

The vehicle had covered 106,000 miles, and was suffering from poor idle and incomplete combustion, with a mil light indication.

Step 1/ serial interrogation
0568/P0238 boost sensor, signal high, frequency 1
0768/p0300 random/ multiple cyl misfire, frequency2, counter re-set 255
0772/p0304 cyl #4 misfire intermittent frequency2 counter re-set 255

The next step taken was a cranking current differential test, showing no apparent mechanical imbalance? Back to this later.
Coil and plug failure is a common problem and is an obvious job for the Pico scope, no problems with burn times or primary current saturation here.

David Gore, our diagnostic tech, opted for the first look sensor in the exhaust next. I’m not sure if he opted for WPS in cylinder or not. This would have been my preferred choice, but as the saying goes too many chefs…
If you refer to Fig.1, The image is triggered from ignition, sequentially 1342 from left to right. I’m going to let you debate this image, as I intend to cover this in detail next month. I bow to Brendon Stickler’s wisdom on exhaust pressure evaluation. My debate is focused on the properties of pneumatic pulse delay from the cylinder head to tail pipe. I have since proven this and will discuss this in the October issue.

The next and obvious decision was to remove the manifold and check the intake tract and valves for carbon.
So, as you can see in Fig.2, there is excessive intake valve carbon. This is due to several factors, the most common of which is no self-cleaning from the fresh fuel air intake cycle. Other factors include, lengthy oil service intervals, not replacing oil separation filters, poor fuel quality, driving environment, poor or incomplete combustion cycles, incorrect atomisation and air swirl during the intake and combustion preparation cycles. Remember, direct injection can separate the fuelling into several events on both the intake and compression strokes.

Value
Back to a comment I left open earlier, I hope you are still interested? The value of compression is determined by the mechanical engine efficiency and volumetric efficiency, Pumping losses! So why didn’t a problem show up during the cranking balance check? As this test is based on compressional resistance. Accepting that when the engine was at idle it ran badly and would eventually disengage the injector cycle in cly #4? the answer is rotation speed increase reduces the available time to draw in fresh air. If you compare nominal compression values say 10-12bar against the value at idle they will only be around 3.5 bar!

The detrimental effects of intake fouling only tends to occur at closed and partially open throttle, where the pumping losses are the greatest. The dtc relating to boost pressure sensor value high, can be caused by ignition misfire or unstable intake pulses.
Finally, the injectors were subject to the Spanish Inquisition in the ASNU bench. The results (see Fig.3) confirm substantial fuelling imbalance causally relating to my previous comments.

My grateful thanks to Phil, David (and myself), for the technical input in this topic. I’m off to the workshop to check the delay characteristics with WPS in cylinder and FIRST LOOK sensor in the exhaust.

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