Sudden severe misfire, *much worse* if *any* coil packs are disconnected!?

Background : I have had no issues with this car, and have owned to since 2019. It has about 290,000 km / 200,000 miles. The previous owner had it for a few years and parked it indoors on a carpeted garage! I have also maintained it, including oil and filter changes with good quality parts. This car is my daily driver currently!

Today : I went to collect shopping. About 200 metres before arriving at the shop, the car felt very ‘rough’. Was turning (left if in the US / Canada), waiting at a light. I thought I must have a flat tyre (due to the camber when waiting to turn across the road). I drove the last 200 metres on the battery. And it wasn’t one of my brand new tyres, sadly.

- I checked the oil (clean, normal level, no water etc). I changed the oil many weeks ago (it isn’t worn out yet - I have been doing a low annual mileage, but also using the car once / twice a week, and the driving I do do, results in the car getting up to temperature, and doing plenty of miles after that each time.
- I checked the oil and coolant. The oil was normal, no emulsion, not ‘watery’, no smell of fuel etc (smelt like normal engine oil).
- the inverter coolant was clean, correct level, and flowing normally.
- the engine coolant is also uncontaminated, and normal level.

I drove the few miles back home after collecting my shopping. I did this in a ‘cautious hypermiling’ way; gentle acceleration occasionally, and lots of coasting. During the journey home:
- the car was completely fine on the battery, but ‘chuggy’ (felt like 3 cylinders) when the engine was running.
- I put the interior fans on to ‘maximum heat’ on the way home; instant hot air (thinking of head gasket)
- the car interior slowly began to smell more and more of exhaust, which was odd, as I had the circulation on ‘internal’.

Further testing at home:

1. I found some plastic debris (something from, the road, I guess) caught behind the air filter. Probably a red herring, and I can’t remember the last time drove over something. It would have caused, at most, 30% occlusion of airflow, but I tend to use medium acceleration, so probably not an issue. The air flow seems good still - definitely no issues with the intake now. I thought this was the issue, because of the ‘engine running only on battery for several seconds’ after restarting it.

2. I rechecked coolant and oil. Still no issues / no signs of fuel or coolant in the oil.

3. Initial testing of coil packs
- disconnecting one coil pack at a time results in clear worsening of the misfire. I can’t remember exactly what happened with each coil pack, and I can check again, but, of the coils, disconnecting one at a time,
- two of them result in the engine running smoothly for a few seconds, followed by the engine slowing down for a fraction of a second, then running smoothly. This cycle repeats, and the accelerator has no effect. I assume the engine is doing the ‘no fuel on startup’ thing here.
- for the other two coil packs, the engine runs smoothly for a few seconds, the gets incredibly rough, and the engine stalls.

4. With all 4 coil packs installed, the engine behaves like this:
- Firstly, a few seconds of being ‘smooth’ on startup up
- It then feels as if it is running on 3, maybe 2 cylinders. (once the fuel is injected, presumably)
- it stays very rough, feeling like a regular misfire.
- if I press the accelerator down in ‘park’, the engine speed slowly increases, and the misfire stops.
- slowly releasing the accelerator results in a gradual reduction in engine speed. It begins to feel slightly uneven as it approaches idle, then becomes very rough once it reaches idle. The engine never cuts out (except for the normal engine cutoff)
- the engine can output power still; the battery can be force-recharged still (which I did briefly, being mindful of the HV battery and many attempts starts with 3 coils).

Regarding fault codes:
- no engine light or any other warning appeared before the ‘misfire’ started, or appeared since
- no other triangles or hybrid errors occurred either.
- I did get a check-engine light when testing coils disconnected one at a time, though - but only then, not on the way home etc.
- My mini-VCI diagnostic cable is 100 mile away, and a new one will take weeks to arrive from overseas
- I have an OBD-2 port scanner; this never worked on this car, but did on an NWH20, and a Leaf. This car (NWH11) only shows codes with Techstream

Other issues:
- the car has NWH20 cells (done by me around 2020/2021 lockdowns). The battery is still strong, and isn’t showing warning lights. I could recharge it after all the start / stop testing above.
- the car doesn’t have a catalytic converter. I’m not sure why, presumably theft? - it is legal to have no cat here (for a car of this age, imported during a certain time window years ago!!). The exhaust seems to be flowing freely.

Overall;
- am not sure what tests to do next;
- it is weird that the car seems to have a severe, and regular misfire (feeling like it is down by 1 or 2 cylinders, and always the same 1 or 2 for each (completed 4 stroke cycle);
- it is much worse (the engine cuts out or doesn’t try to start) when removing coils one at a time (this suggests all cylinders are ‘working’, which is weird given the ‘severe misfire;
- no engine codes except when disconnecting one coil at a time
- the car runs fine on battery power, and the battery can still be recharged by the engine
- flooring the accelerator in ‘park’ results in the engine slowly increasing in speed, and results in the misfire resolving
- slowly releasing the accelerator results in a slight unevenness as it approaches idle speed, followed by extremely severe misfire resolving / engine shudders once back at idle.

Also when driving home : the engine was very down on power, but I didn’t ‘thrash’ it - used brief periods of low-medium power. The fuel consumption was about 20% heavier from the info display (weird, as the way I drove home should have improved the economy, or at least, not worsened it)

I would be grateful for any advice on what to look at / test next? (given the weird symptoms … and lack of mini VCI for at least a few weeks!)

Thank you for any advice / thoughts!

(VVTi? I won’t restart it until that is ruled out)…

 

Am doing some reading around and thinking what it might be;

The ‘misfire is much worse when disconnecting coils one-by-one’ thing is odd. It suggests that : “there is fuel getting to each cylinder, and there is a spark getting to each cylinder” (because otherwise, if the issue was in one cylinder, then I would expect the ‘misfire’ to ‘not get worse’ when disconnecting the coil for that cylinder.

Regarding the head gasket - zero symptoms so far. Also, the issue appeared suddenly, and the car was warmed up and driving well before the misfire appeared. The car had started fine and driven fine up to that point. Also, there have been no warning signs up until this point. It could still be the head gasket, but the very sudden nature of it does make me wonder if it is something else (it appeared suddenly out of the blue and wasn’t followed by overheating, mixing of coolant / oil, pressurisation of coolant, white smoke or coolant smell from the exhaust).

I do worry about the VVTi system, as the car is nearly 1/4 of a century old, and early VVTi had issues. I was worried about the VVTi locking pin issue (causing knocking on startup), but I hadn’t noticed knocking on startup, and it seems that the Prius specifically doesn’t even have a VVTi locking pin?. The fact that the engine runs perfectly smoothly with the accelerator pedal held down in park (?2500rpm) makes me wonder if the timing ‘suddenly went out’, such that the timing is ‘good’ for ‘medium RPM under no load’, but bad for ‘idle, or any RPM with the engine under load’.

Has anyone come across issues like mine that were due to the ‘camshaft timing oil control valve’ solenoid, or the actual VVTi sprocket on the camshaft? I don’t want to start the engine again, in case the VVT solenoid has died, and the VVT gear is bouncing around, and about to break and destroy the engine