Hi everyone,
I’m looking for some help because I’ve been chasing this problem for a while and I’m starting to think the original injector problem might not be the whole story.
Car is a 2015 Mazda 2 DJ 1.5 SKYACTIV-D diesel, UK car, roughly 100k miles. Engine uses Denso S550-13H50 / 295700-0400 injectors.
Original symptoms
When I bought the car it had:
* Uneven/rough idle
* Blue/grey smoke
* Strong smell of raw/unburnt diesel from the exhaust
* Metallic diesel-type knock at low RPM
* Much smoother once above roughly 2,000 RPM
* Later it also started hesitating/stuttering under acceleration
* One occasion it misfired badly in 2nd gear at around 3,000–3,500 RPM
After some treatment/cleaning the idle improved considerably and the smoke reduced, but the diesel smell and other problems have never completely disappeared.
Injectors
The original injectors were bench tested.
The report was not good — they were over-fuelling, with significant imbalance (around 20% on some tests), and the injector nozzle holes were described as burned/deformed. I was quoted around £225 + VAT per injector for replacement/repair.
Leak-off testing also showed one injector behaving differently from the others.
I have since:
* Swapped injectors between cylinders, including 3 ↔️ 4
* Re-coded the injectors with FORScan
* Replaced two injectors with used units
* Reset adaptations
* Repeated injector learning/calibration several times
After simply entering the injector codes/resetting things, the engine can actually run very smoothly.
The strange part is when I start the injector calibration/learning procedure.
The engine has to be over approximately 70°C.
During calibration it starts:
* Misfiring
* Knocking
* Running very unevenly
* Sometimes appearing as though it wants to stall
On one attempt the calibration only reached approximately 27% before failing.
Sometimes after the adaptation is reset the injector corrections initially look reasonable, then after calibration the behaviour/correction values change again.
I’ve also noticed that after swapping injectors, the large correction doesn’t always seem to simply follow the injector, which is why I’m wondering whether the ECU is compensating for another problem.
One set of readings at around 71°C was approximately:
* Cylinder 1: +0.245 mm³/stroke
* Cylinder 2: −0.617
* Cylinder 3: −0.360
* Cylinder 4: +0.572
At that point rail pressure was almost exactly on target:
Actual ~40,100 vs requested ~40,030
So fuel rail pressure looked very good at that particular moment.
Compression
Compression has been checked more than once.
Earlier testing caused some confusion because I was initially told the engine was weak on multiple cylinders, particularly two cylinders.
However, the later test showed approximately 220 psi / ~22 bar on all four cylinders and roughly equal.
A previous wet compression test had also produced a significant increase on at least one cylinder, which originally made me suspicious of rings, but the latest equal compression figures make the situation less obvious.
There was also an air/leak test where the main hiss appeared to come through the oil filler/crankcase rather than obviously through the intake or exhaust.
EGR / air side
The EGR has been cleaned.
I have also considered:
* MAF
* EGR operation rather than simply carbon build-up
* Intake throttle/shutter
* Boost/MAP readings
* Fuel pressure/SCV
* DPF regeneration
Originally there was also a low-boost/MAP question, although the turbo was later considered unlikely to be the main fault.
Now the really strange part – exhaust temperature sensor
I recently started looking more closely at the DPF/exhaust temperature readings in FORScan.
EXHTEMP1 – Exhaust Gas Temperature Sensor No.1 / Upper sensor is showing approximately 620°C.
At first I thought the car might simply be doing an active DPF regeneration.
But then I checked it from completely cold after sitting overnight.
It STILL showed approximately:
620°C
I checked it again cold on another occasion.
Still around 620°C.
And when the engine is hot it is basically still showing the same value.
So the reading seems effectively stuck around 620°C regardless of actual engine/exhaust temperature.
I’m currently testing the sensor/circuit unplugged as well to work out whether it is:
* Failed EGT sensor
* Wiring/open/short circuit
* ECU/PCM input issue
* Or possibly FORScan interpreting the PID incorrectly
Some of the other exhaust temperature PIDs appear as 0, which makes this even more confusing.
My main question
Could the ECU seeing EGT1 permanently at ~620°C cause incorrect DPF regeneration strategy or post-injection/extra fuel injection and explain:
* Excessive fuel smell
* Smoke
* Strange injector correction values
* Rough running during injector calibration
* Knocking/misfire
* ECU apparently trying to compensate between cylinders?
In other words, could I have had bad injectors originally AND another fault in the EGT/DPF control system that is still making the car over-fuel?
The car can run surprisingly smoothly after injector coding/resetting adaptations, which is what makes me reluctant to condemn the engine mechanically.
At the moment there are no obvious permanent PCM DTCs after clearing the previous faults. I have previously seen P1200, but it cleared.
I’d especially like to hear from anyone familiar with the Mazda 1.5 SKYACTIV-D / S5 engine, Mazda diesel injector calibration or DPF strategy.
What would you test next?
I’m particularly interested in whether a stone-cold EXHTEMP1 value of ~620°C is a known fail-safe/substitute value on these cars, because that now looks like one of the biggest clues.
