Engine instantly off when gas pedal released

Based on the collective knowledge and experience of many owners and experts here and elsewhere, I am quite confident in saying that any perceived relationship between the EV switch and ICE shutdown at 34+ MPH is just coincidental.

Maybe a summary of EV mode, ICE warmup stages, and their relationship might help you understand exactly what is happening, along with ScanGauge indicators of ICE status. Not all of this applies to you, but for the sake of completeness ...

• EV mode generally works pre-S1, the first few seconds after power-up.
• EV mode doesn't work in S1, the first minute or so after power-up.
• EV mode generally works in S2, S3, or S4 at <34MPH.
• Even in pre-S1 and S2+, EV will not work with:
  • SOC too low (<50%).
  • Too much go-pedal pressure. This seems to vary in unexplainable ways independent of other variables. Usually EV will activate with light pedal pressure, but there are times during S2 or S3 where I get repeated "EV deny" prompts until I release the pedal completely and wait a couple of seconds.
  • Cold (<35F or so) battery.
  • Hot battery, though this seems to vary with SOC. I've had EV work with battery temps of 120F+, but with SOC >80%. Seems the car's desire to reduce SOC is a greater priority in those conditions than avoiding EV.
  • Defroster on full blast.

[Others, fill in the gaps if I’ve missed any.]​

• ICE will not shut down spontaneously in S1 or S2.
• ICE generally will shut down spontaneously in S3 at speeds of 34+ MPH.
• ICE generally will shut down spontaneously in S4, but it will enter S4 only after reaching 157F and then only after it is allowed to spontaneously shut down with the car at a near-complete stop for 7 seconds.
• ScanGauge will indicate ICE-off as zero RPM at 40MPH or less. At >40 MPH, an RPM of 900-some indicates ICE-off. The ICE is turning to prevent over-revving of MG1, but with no fuel use.

I don't see this mentioned much at all in the forums, but there are brief instances (a minute or two) where the ICE refuses to shut down even in S4 as a secondary coolant loop opens somewhere in the 170F – 180F range. Hobbit describes this in this CleanMPG thread:

I experience this regularly and it is particularly annoying. In my afternoon commute during all but the hottest of summer days it actually reaches this condition at up to three different and predictable points along the drive. Because of ICE-off gliding segments and stoplights that follow these points, ICE temperature doesn’t always reach the threshold where the loop apparently stays open for good. EV mode will work, but these points are all on moderately to heavily traveled streets with speed limits of 35 or 45 MPH where it is usually impractical, discourteous, and perhaps even unsafe to slow to <34 MPH to activate EV until I need to slow for the light. I’m wondering if you’re experiencing this phenomenon when the ICE suddenly decides to shut off as you're (again, coincidentally) pushing the EV button at 34+ MPH.

 

If there indeed is a causal relationship then you're the first to describe it. If others can replicate it then we can confidently say we've added to the knowledge base of this car's nuances. I'm skeptical but not closed-minded, so I'll try it myself and keep you posted.

Please, when you're able do it again, try to make a mental note of exact speed, ICE RPM (before and after shutdown) and ICE temp, and report them here.