MGF VVC Engine almost stalls for a split second. I have Pscan live meaurements.

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.

 

This may not be the root of your problem but it might be something to consider.
My guess is that your coil pack is mounted on the back of the engine, under the inlet manifold. This isn’t an idea place, the plug leads are inordinately long & it doesn’t follow that the longer the better. Buying a replacement is problematic, buy new & be skint or used & find yourself in an unknown territory.
If your car is a long term “investment”(!!), as mine has become, it may be worth considering changing over to the head mounted coil packs.
Obviously you need the two packs, you would also need adapter wires to feed from the original “low tension” side & also the cam cover for a later car. It’s all quiet straight forward providing you observe all the P’s.
Proper Preparation Prevents Piss Poor Performance.
More info if required.
M

Your throttle angle is incorrect at idle, 5% is too high and the stepper/IACV step should be around 20-24  so that is also high...
I'm not saying that is your issue but it is An issue.
Get the engine up to temp and try doing the reset i.e. Switch engine off, switch ignition to position 2, press accelerator to floor 5 times, wait 15 seconds, switch off. Start car and check the throttle angle is 4%.

Next disconnect the stepper motor and start the car, check idle speed. It should be 800. If different adjust the idle screw. 1/4 turn at a time wait for idle to settle before adjusting again if needed. Once you have 800, switch off and repeat the stepper motor reset. Start car and reconnect the stepper motor, allow the idle to stabilise, it should eventually be 875. Check the step reading.. If the step is not low 20s then you may need to clean/replace the stepper motor/IACV.

Next the voltage of 16 volts at the O2 sensor is just weird. Where is it getting 16 volts from? Check your alternator output, should be a max of 14v-ish!

Oh, and check the electrical plugs are correct on the VVC!

Other than that, I'm out of advice,🤔 

Disconnect two hydraulic control solenoids and check if values reported as incorrect are back to normal. If they are you might need to replace both hydraulic control solenoids.

thanks .  Do you mean all values or is there one very specific value I should focus on for the control solenoids? 

cheers. 

Thanks Airportable,

We are seeing the car as more of a liability now LOL, as we are in Sydney, Australia. So more often than not a used replacement spare part has to be shipped out from the UK which is eye wateringly expensive. We are on our 3rd alternator after a short developed. But too expensive to ship the old ones back for refurbishment. We swapped out the old air suspension, and got a new soft top a couple of years ago. Wish we lived in UK again, it would all be much easier to maintain but we have no rust !

Palmplex wrote:

thanks .  Do you mean all values or is there one very specific value I should focus on for the control solenoids? 

cheers. 
 

All above values which are reported as incorrect.