Non-intrusive engine evaluation

Ask for and share advice on using the PicoScope kit to fix vehicles here.
Post Reply
Steve Smith
Pico Staff Member
Pico Staff Member
Posts: 1352
Joined: Sun Aug 25, 2013 7:22 am

Non-intrusive engine evaluation

Post by Steve Smith »

Model: 2003 Volvo XC90 2.9 T6 petrol (Gas converted)
Engine code: B6294T
Mileage: 178,458
Symptom: Engine misfire (permanent)

A recent visit to the above vehicle provided a great opportunity for non-intrusive engine condition evaluation. How we tackle such symptoms of misfire is predominately governed by experience, probability and accessibility.

Non- intrusive engine evaluation provides a means to cost effective “initial” diagnosis that delivers conclusive evidence on how to proceed with rectification (if rectification is a cost-effective solution)

A quick vehicle scan revealed a misfire present at cylinder 5 and no doubt further analysis of serial data may well reveal more about the fueling conditions of the cylinder

Given a misfire has been detected by the PCM we must be conscious of fuel injector “cut” shortly after start up (Catalyst protection) Here then we have a very small opportunity to validate the misfire using emission related data either via the scan tool or gas analyzer.

Taking a step back from serial data, a relative compression test with Pico Diagnostics (PD) will provide a brief evaluation of the fundamental condition of the engine prior to more intensive diagnosis

Why a relative compression test? The answer here has to be why not?

Based on the vehicle mileage and the installed gas conversion, valve seating is always a concern and one we could not afford to overlook.

If the relative compression test results are OK, then we are justified in more intensive diagnosis. Rather this route than carry out intensive diagnosis to find we have a compression issue!

Prior to a relative compression test, the PD Battery Test is carried out to ensure our vehicle battery can perform as required:
Image 1
Image 1
Unfortunately, as can be seen above, the 12 V battery has failed and would require “support” to continue testing. Whilst this result is not relevant to our customer complaint, a new battery is required at over £100 and needs to be kept in mind based on the conclusion of our diagnosis

Relative compression test

Using no more than a x1 test lead across the 12 V vehicle battery, the results below speak for themselves and we are onto something immediately
The next question, where is our offending cylinder? Our scan tool suggested “cylinder 5 misfire” but how can we be sure?

Here we switch to PicoScope and repeat the relative compression test but incorporate a synchronization signal from cylinder 1. Below I have used the PCM trigger signal to the COP at cylinder 1, however a COP probe may also be used for less intrusion.

Based on the firing order 153624, cylinder 5 looks to be our offender
Image 3
Image 3
At this stage of the diagnosis, we can conclude cylinder 5 is misfiring due to a compression issue, backed up with evidence from PicoScope, Pico Diagnostics and Scan Tool data.
Note: We can also inform our customer a 12 V battery is required.

Using the techniques above, the maximum intrusion into the vehicle proved to be obtaining the PCM trigger signal to COP #1. However, all in all, minimal intrusion has delivered an initial diagnosis where the customer can now make an informed decision on how to proceed

Permission was granted for further diagnosis with a view to assessing the cause of our low compression.

Below we apply a First Look sensor into the exhaust tailpipe and a COP probe resting on cylinder 1 COP unit. The engine is then cranked with fuel injection prohibited which aids the analysis of exhaust gas pulsations thanks to the removal of combustion dynamics within the exhaust system.
Image 4
Image 4
Above we make use of the Cylinder ID chart provided within the Microsoft Pressure Waveform Overlays software which qualifies a sequential irregularity within the exhaust gas pulsations from cylinder 5.

You could argue this is expected from a misfiring cylinder and not a worthy test! However, it does reveal a considerable negative pressure exists on top of the piston when the exhaust valve opens for cylinder 5 (Denoted by the enlarged negative pulse [below 0 V] highlighted in the blue circle) This information can be held for later use during our summary

Below we add intake pulsations to the above test using a WPS 500 pressure transducer attached to the inlet manifold. Here the engine is running to evaluate the effects of our misfire on manifold pressure due to combustion at idle speed
Image 5
Image 5
What becomes immediately apparent is the fluctuations and overall sinusoidal appearance of the manifold pressure (channel D). Note how the intake “pull” for cylinder 5 is reduced by comparison to cylinder 1 (Highlighted in the blue circle placed around channel D)

Note also the complexity of the exhaust gas pulsations now combustion is taking place! With that said, we can still detect a missing pulsation (again highlighted by the blue circle placed around channel A) where uniformity is disrupted during the exhaust event for cylinder 5.

Let’s summarize before intruding further:

• The exhaust gas pulsations (during cranking) confirm the pressure above the piston for cylinder 5 is lower than the remaining cylinders
• Exhaust gas pulsations remain disrupted during idle speed
• The intake “pull” for cylinder 5 is reduced by comparison to the remaining cylinders

At this stage we have remained non-intrusive but now require the removal of the spark plugs.

Note: Other than engine covers and hoses, this is the first-time components have been removed to assist with our diagnosis!

Below we compare cylinder 2 (Blue) to Cylinder 5 (Magenta) at idle speed
Image 6
Image 6
No surprise that cylinder 5 has a lower compression than cylinder 2 but can we see other clues in the above waveform as to why.

Note the deeper expansion pocket of cylinder 5 compared to cylinder 2 given our peak compression is low, yet the swept volume between pistons remains the same!

I.e., If we start with a low compression at TDC and our pistons “sweep” identical volumes, then our expansion pocket will inevitably be deeper (lower negative pressure)

Remember, when the exhaust valve for cylinder 5 opens there will be a greater negative pressure above the piston than the remaining cylinders (See images 4 & 5 above) captured by our First Look sensor given the deeper expansion pocket

Moving onto the 360° point in our 4-stroke cycle, note the delayed “defined” closure of the exhaust valve. Confirmation of EVC is defined by the point at which we achieve peak negative pressure after 360°. Could this be a retarded exhaust valve event or a poorly seated exhaust valve?

Below we reveal another “giveaway” in the form of leaning compression towers suggesting a leaking cylinder (More on compression tower symmetry can be found here viewtopic.php?p=100606#p100606)
Image 7
Image 7
All things being equal (and they rarely are) the rise and fall in cylinder pressure during the compression and expansion stroke should be near equal resulting in a symmetrical compression tower

Below we now look at the valve duration where defining the EVC event is always challenging at 360° however, they differ considerably between cylinders 2 and 5 by 27° of crankshaft rotation!
Image 8
Image 8
Note the effective intake stroke of 202 .6 ° measured from peak negative pressure to our inlet valve close event (IVC). Now compare this value to the effective intake stroke of cylinder 5 below at 190.1°
Image 9
Image 9
The effective intake stroke of course has a proportional effect on the effective compression stroke!

Below we have an effective compression stroke of 114° for cylinder 5
Image 10
Image 10
Below we have an effective compression stroke of 100.2° for cylinder 5
Image 11
Image 11
To finally summarize

• Low cylinder 5 compression (peak)
• Cylinder 5 leakage (leaning compression tower)
• Poorly defined EVO
• Poorly defined EVC
• Delayed/poor sealing of EV during intake stroke
• Reduced effective intake stroke length (Cylinder 5 reduced by approx. 12° of crankshaft rotation)
• Reduced effective compression stroke length (Cylinder 5 reduced by approx. 14° of crankshaft rotation)

To conclude

We have a number of issues to address with cylinder 5 and of course this engine in general. The above summary provides detailed information for our customer and the technician who will be tasked with the repair. Such detailed information helps to focus our attention to specific areas of the valve train during rectification which may have been missed if not documented.

I am thinking here particularly about the delayed EVO/EVC event which may reveal issues around hydraulic lifters/followers and the integrity of the engine oil pressure/passages.

Reading this back I should have added crankcase pressure as another non-intrusive pressure test to be carried out but as ever, hindsight is a wonderful thing. The vehicle has now moved onto pastures new where feedback is highly unlikely unless by some freak of nature it is returned to the same workshop by a different owner for diagnosis!

Once again thanks to Kevin Ives at Ives Garage for his support throughout

I hope this helps, take care……Steve

Post Reply