Help ! I have an old MGF VVC, the engine almost stalls for a split second as if the fuel is cut off, then continues as normal. Its very intermittant but happens at least once or twice each journey.
Note the problem occurs when idle , coasting or accelerating. It all started after my garage replaced the clutch. They have spent weeks trying to find the problem but gave up ( they didnt charge me)
I have already swapped out and re-coded a different ECU myself but no different.
Here are the engine management events when it happens - in this example the car was parked and idling ( i have a video of the live Pscan parameters which i slowed down to get the events order below. )
What do you think is the cause ?
Thanks for any pointers to my next task! Is the oxygen sensor the next thing or coils etc ?
I have also asked Claude AI to diagnose this problem and I've pasted its suggestions at the end which may prompt ideas.
Here are the events in order
1. Engine speed is around 910RPM in idle
2. Idle Error ranges from -10 to +11 RPM normally
3. When the engine problem starts the first thing to change is Idle Error jumps to -115RPM ( minus 115RPM)
4. Once 3# happens then the following all happen
Cam position changes from 1 to 0 degrees
Coil 1 and Coil 2 charging time more than doubles from approx 1,500 to 3,855uS
5. After #4, then Injector Pulsewidth changes from 2.5ms to 3.5ms, and Oxygen sensor voltage bank 1 changes from Zero volts to 16.37v
6. After #5 the Idle Error goes back to normal and the Coil charging time starts to drop towards normal, but the Inlet valve open angle changes from 2 degrees to 3.2 degrees
7. After #6, then Oxygen sensor voltage bank 1 changes back from 16.37v to normal zero volts, and Inlet Value open angle starts changing back to normal around initially at 1.8 degrees then back to 2 degrees , and Cam position returns back to 1 degree.
8. After #7, all parameters settle back to normal.
The whole process from #3 to #8 takes approx 6 seconds.
It can happen in idle or when driving but not accelerating or when accelerating. When accelerating you can really get pushed forward and back in your seat as the car abrutly almost stops for a split second then continues like normal.
Claude AI diagnosis:
Based on the sequence of events you've described, this issue seems to be related to a momentary loss of engine performance, followed by the engine management system's attempts to compensate. Let's analyze the potential causes:
1. Oxygen Sensor Malfunction:
The sudden spike in Oxygen sensor voltage from 0V to 16.37V is highly unusual and likely indicates a problem with the O2 sensor or its wiring. A faulty O2 sensor can cause the engine to run rich or lean, leading to performance issues.
2. Variable Valve Timing (VVC) System Issue:
The changes in cam position and inlet valve open angle suggest that the VVC system might be experiencing problems. This could be due to a faulty VVC solenoid, oil pressure issues, or a problem with the VVC actuator.
3. Ignition System Problems:
The significant increase in coil charging time (from 1,500 to 3,855 μs) points to potential issues with the ignition system. This could be caused by worn spark plugs, faulty ignition coils, or problems with the ignition control module.
4. Fuel Delivery Issues:
The change in injector pulse width from 2.5ms to 3.5ms suggests that the engine management system is trying to compensate for a fuel delivery problem. This could be due to a clogged fuel filter, weak fuel pump, or faulty fuel injectors.
5. Mass Airflow Sensor (MAF) or Throttle Position Sensor (TPS) Malfunction:
The sudden drop in engine speed and the ECU's attempt to compensate could be triggered by incorrect readings from the MAF or TPS.
6. ECU Issues:
Given the complex sequence of events and the fact that the problem occurs in various driving conditions, there's a possibility that the ECU itself might be malfunctioning or have corrupted software.
Primary Suspects:
1. Oxygen Sensor: The extreme voltage swing is a strong indicator of a problem here.
2. VVC System: Given the MGF's known issues with the VVC system, this is a likely culprit.
3. Ignition System: The dramatic increase in coil charging time suggests problems here.
Recommended Actions:
1. Check and potentially replace the oxygen sensor.
2. Inspect the VVC system, including oil pressure and the VVC solenoid.
3. Check the ignition system, including spark plugs and coils.
4. Perform a thorough diagnostic scan to check for any stored error codes.
5. Inspect fuel system components, including the fuel pump and injectors.
6. Consider having the ECU checked for potential software issues or corruption.
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