Diagnostický případ

Jen diagnostikované vozidlo

Libor Fleischhans

Jazyk případu:

After switching over to CNG, the engine idles on 3. cylinders

20.02.2025 | aktualizováno 20.02.2025 13:53:07 0 13x

Informace o vozidle

VIN:	TMBJM7NE3F0112564
Výrobce:	Skoda
Model:	Octavia III
Rok výroby:	2014
Kód motoru:	1,4 CNG
Typ motoru:	CPWA
Výkon (kW):	81 kW
Převodovka:	Manuální
Palivo:	LPG/CNG
Najeto:	194 026 km

Jak se závada projevuje

Nefunguje / funguje nesprávně. When using CNG, the engine runs on 3. cylinders at idle. It then automatically switches to petrol where the misfire isn't that noticeable.

Nouzový režim.

Jak často se závada projevuje

Sporadicky / za specifických podmínek.
- Za jiných podmínek. Only after longer idle periods while running on CNG. The engine runs much smoother on petrol.

V jakém systému se závada vyskytuje

Motoru.

R-CODE

během jízdy
[P336400] Plynový pohon válce 3 - Zjištěny výpadky spalování

trvalá závada
sporadická závada

Doposud provedeno bez úspěchu

Customer's statement: In another service centre, they tried replacing the control unit, the lambda sensor, and the piston rings in the cylinders (the entire engine was disassembled because of this). However, the issue still persists after all this work.

Závěr

VOZIDLO JSEM NEOPRAVOVAL, PROVÁDĚL JSEM DIAGNOSTICKÉ PRÁCE

Four independent clues (photographs of the valves, compression test, cylinder intake capacity, and exhaust pulsations with the ignition disconnected and the starter engaged) typically point to a mechanical problem in the cylinder head, including the valve timing system.

The cylinder head must be removed, and everything needs to be thoroughly inspected—from the camshafts and valve lifters to the valves themselves, valve seats, valve sealing, valve lift, and the combustion chamber.

Obrázky, fotografie a soubory

FCD Mini Testerem was used to verify the CNG valves (18872/1)

When the valves are switched with the same frequency and PWM load, all CNG injection valves produce the same sound.

The recording does not indicate any issues with mixture formation (18872/3)

The mixture adaptations are within the allowed range for both petrol and CNG. The permitted range is below 10%, but our maximum deviation is -7% and otherwise it fluctuates around zero. Our continued troubleshooting doesn't need to focus on air intake, intake system sealing, or the fuel system since they are 99% trouble free according to the log.

Cylinder #3 has lower compression (18872/4)

Relative compression is pointing to a mechanical issue on cylinder #3.

Intake pulsations = cylinder #3 is not sucking (18872/5)

The pressure pattern on the MAP sensor, with the ignition disconnected, the starter cranking, and the intake system manually blocked. One cylinder is not pulling in air – that’s cylinder #3.

Photograph of differently colored intake valves on cylinder #3. (18872/6)

The coloration of the valves indicates "different flushing" when the valves overlap compared to the other cylinders.

The valve heads of the remaining 3. cylinders are black (18872/7)

The coloration of the other cylinders (1, 2, and 4) is the same—"black."

Dynamic air pulsations in the exhaust (ignition disconnected) (18872/8)

Yellow = MAP
Blue = Pressure sensor instead of the lambda sensor

Test procedure: The engine is cranked with the starter, the throttle valve is open, and the engine does not start (blocked by disconnecting the ignition modules).

Evaluation: The blue curve shows a different pressure wave pattern compared to the others, indicating a clear mechanical fault in the valve timing system.

Soubory ke stažení

pdf

Diagnostics – record (18872/2).pdf Cylinder no. 3 showing misfires
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mwf

A recording where everything is visible (18872/9).mwf For self-study (channel names added).
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