05/09/2026
🔍 DIAGNOSTIC CASE STUDY – Nissan Qashqai 1.5 dCi
Another good example of why proper diagnostics is much more than simply plugging in a scan tool and reading fault codes.
🚗 Customer complaint:
• Severe lack of power
• Heavy black smoke under acceleration
• Smoking even at idle
• Struggling to rev beyond approximately 2,800–3,500 RPM
• No fault codes stored to point us in any particular direction
With no DTCs to help, we needed to let the live data and oscilloscope tell us what was happening.
📊 First clue – injector correction values
Looking at the injector correction factors, cylinder 4 immediately stood out.
The ECU was making a massive correction to cylinder 4 because it could see that this cylinder wasn't contributing to the engine in the same way as the others.
The ECU constantly monitors crankshaft acceleration between individual combustion events. If one cylinder produces less torque than the others, the ECU can alter the fuelling on that cylinder to try to smooth the engine out.
This doesn't automatically mean the injector itself is faulty.
It simply tells us:
⚠️ Cylinder 4 isn't doing the same amount of work as the others.
So why?
🔬 Next test – relative compression
Using our PicoScope, we carried out a relative compression test.
This is a brilliant, quick and completely non-intrusive test.
No glow plugs removed.
No injectors removed.
No dismantling of the engine.
Simply an amp clamp around the battery cable connected to the oscilloscope while cranking the engine.
As each cylinder comes onto compression, starter motor current increases. Comparing those current peaks allows us to quickly identify whether one cylinder has significantly lower cranking compression.
✅ In this case all four cylinders were very even.
So we didn't have an obvious conventional compression problem.
💉 Could it still be an injector?
Before going deeper, injector 4 was swapped with injector 1.
If the injector itself was responsible, we would expect the correction problem to follow the injector to cylinder 1.
It didn't.
The excessive correction stayed firmly on:
➡️ Cylinder 4
That allowed us to move away from the injector and start looking at the mechanical operation of that cylinder.
📈 Now the PicoScope WPS500 comes out…
Using a WPS500 pressure transducer, we were able to look directly at the pressure changes inside cylinder 4 and analyse the valve events.
This gives us much more information than a simple compression figure.
We can actually see what is happening inside the cylinder as the engine rotates and look at when the intake and exhaust valves are opening and closing.
And there was our problem.
⚠️ Cylinder 4 had an abnormal valve event and valve opening duration.
This pointed towards camshaft lobe wear.
A camshaft isn't simply responsible for opening a valve.
The shape and height of each camshaft lobe determines:
🔧 How far the valve opens
🔧 When the valve starts opening
🔧 How long it remains open
🔧 When it closes
As the camshaft lobe wears, the valve may no longer open far enough or remain open for the correct amount of time.
If the intake valve isn't opening correctly, the cylinder can't breathe properly and doesn't receive the air it needs for efficient combustion.
If an exhaust valve isn't opening correctly, burnt gases can't escape properly either.
On a diesel engine, you can then end up injecting fuel into a cylinder that doesn't have the correct amount of fresh air available to burn it.
The result?
💨 Black smoke
📉 Poor cylinder contribution
📉 Severe lack of power
⚠️ Abnormal injector correction values
And sometimes…
❌ Absolutely no fault codes whatsoever.
Camshaft wear like this can often be associated with poor servicing, extended oil-change intervals, incorrect oil or general lubrication issues over the life of the engine.
This is exactly why we diagnose before replacing parts.
Anyone could look at the injector correction and fit an injector.
The injector wasn't the problem.
The ECU was simply trying to compensate for a mechanical problem inside the engine.
🔧 Ultimate Diagnostics
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Don't guess. Test.
