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

 

Thank you! (note: for some reason, the font has changed for this post?)

I got a video of the engine (to capture the sound); I will try to attach it as a file here:

The symptoms have changed (presumably due to the car having cooled down, so it is no longer starting with the engine warmed up); the symptoms now (with all coils connected, and nothing removed from the engine) are:

- the engine sounds ok for a few seconds (not clunky at all!), then it shudders, and rattles as it slows down (at a normal rate - as if there is no power being produced). It does clatter a bit just before it stops (it seems to sort of try to start for one rotation right before it stops). I get a red warning triangle and check-engine-light. If I cycle the ignition on and off a few times, or if I disconnect the 12v battery briefly, I can try to start it again. The video shows that the last time I tried this, the engine sounded normal for maybe a second, then begins juddering for several seconds, and finally slows down (again, clattering one last time, speeding up before it stops rotating).

My interpretation :

- it sounds normal when the motors are turning the engine.

- the juddering / rattling is when it is trying to use fuel to run the engine (the second time after only 1 second from ignition on … probably because the hybrid battery is at ‘1 bar’)

- the weird ‘engine cuts out, then fires one cylinder one last time just before the engine stops rotating, which causes the engine to briefly speed up before finally cutting out’ is probably unburnt fuel being compressed, then igniting, just as the engine comes to a stop.

I am not sure what to do now. I certainly won’t take it to the dealer! I did find someone selling a mini VCI in this country, so bought that (it will arrive in approx 1.5 - 2 weeks). I will probably get the battery out, check the balance, and charge it up to 80%-ish … to allow for testing once I get the mini VCI.

Whatever has gone wrong - is essentially ‘my car engine tries to start but cuts out when it tries to use fuel to run the engine, and I have a red triangle and a check engine light’. The fact that the car ran earlier was just because it was already warmed up; it now won’t start at all

(Some other info : I am in the Southern Hemisphere, so it is just going into Autumn, so no snow, no extreme weather etc). Also : I did try running the engine with the air box cover removed and the air filter removed; that made no difference. I would have hoped that if there was an air leak after the MAF, this would have reduced the severity of it - but it made no difference!

 

To summarise everything, as the presentation changed a lot;

Current issues:
- My car won’t start. The engine sounds normal for a few seconds, but as soon as it tries to inject fuel, it shudders and immediately cuts out. When cutting out, just before stopping completely, it seems as if there is a small burst of power - the engine speeds up a little bit before stopping.
- I have a red warning triangle, and a check engine light, and will get a mini VCI in 2 weeks. I need to disconnect the 12v battery before it lets me try and start the engine again.
- if I remove the air filter (to reduce vacuum in case of a leak after the MAF), it makes no difference.

From the above : no idea what is wrong. I suspect there is at least some fuel getting into the engine, as that last ‘slight burst of power before the engine stops rotating completely’ reminds me of carburettor cars that would do something similar when you switch off the engine, and some fuel in th cylinder(s) ignites.

The current situation ‘my car won’t start’ might have been the only thing I noticed. So it is quite possible that ‘my car won’t start’ could have been the ‘presenting complaint’. I do have some extra clues:

The failure happened when the car was warmed up, and not during any harsh driving, and traffic was free flowing. The engine started juddering under load (with no warning lights and no symptoms leading up the failure). I only noticed the issue when accelerating, so essentially, something suddenly failed ‘silently’, and only showed itself when the engine was producing power.

When the engine was warmed up, the symptoms were:
- engine shuddering badly as if it was running or 2 or 3 cylinders, at idle and under load
- fuel consumption increased by ~20%
- despite shuddering, the engine would start every time during testing.
- flooring the accelerator in park caused the engine to slowly speed up (as normal), and the shuddering / misfire completely resolved. Slowly releasing the pedal over 30 seconds led to a slight unevenness in the engine when the revs were low, and a return of the very loud shuddering / knocking at idle.
- no coolant / oil mixing, no fuel contamination in the oil. Hot fans worked fine.
- removing one coil at a time resulted in the engine being much worse : even though it was a regular, loud, shuddering misfire, it also seems that it was firing on all four cylinders?. Disconnecting coils one at a time caused the engine to either cut out completely (after some shuddering) (two of the coils), or caused the engine to run smoothly for several seconds, then seem to lose power for a split second, then run smoothly for 7 seconds, repeating. I don’t know why there were two different behaviours, but smoothly running engine initially was due to the motor turning the engine.

Whatever happened was instaneous, caused no check engine lights, and prevents the car starting now that the engine is cold. I am worried that it could be the VVT? (but Ithink the Prius doesn’t have the locking pin issue?). I wonder if a timing issue could cause the engine to run smoothly at ?2000 rpm when the accelerator is floored in ‘park’, and cause it to judder at idle and under load, and cause it to not start at all once it has cooled down?

 

Hi! I haven’t checked the fuel pressure yet - I am a fair way away from parts stores, and don’t have a gauge at home unfortunately; however - my new mini VCI arrived, so I managed to get some diagnostics done!

I did some testing, as follows:

(initial VCI test) - to ensure I can get it to work / or need to do more fixes on Windows etc;
Using Techstream,
- it lists various ECUs, including ‘Engine and ECT’
- each ECU is on a yellow background;
- I can select one ECU at a time and press the arrow. The mVCI does some comms for this;
- ECUs go white with no asterisk after querying them.
- Battery cells well balanced, 15.x volts (maybe 15.5?) all were close to that, except block 10 was zero, as the connector was not yet in.

(first actual test) :
- I put the HV battery orange connector in with the car off
- Attempted start:
- - engine sounds completely normal for about 4 seconds, and then starts vibrating and knocking very badly;
- - it does this knocking for a few seconds, then spins down. Just before it entirely spins down, it has a small burst, enough for three more ‘knocks’, before stopping completely. This is like previously, with the last burst of power probably being unburnt fuel igniting.

- with the car still on, I did diagnostics;
- all ECUs are yellow to start, before queries.

- Hybrid Control ECU shows
- - > P3101 Engine System (and highlighted for Current, History. but not for Pending or Permanent)
- - > this ECU doesn’t do live data.

- Battery ECU shows
- - > Codes : None.
- - > Live data : all blocks now very close to 14.9 volts. Fans off, and no issues whatsoever on that screen.

- ‘Engine and ECT’ shows
- - > Codes : None. (specifically, it has the word ‘None <full stop>’ for ‘Codes’.

Above, and not part of the codes window, it does also say (for the ‘Engine and ECT’ ECU)
- - > DTC Monitors are N/A
- - > MIL : N/A

After querying each of the three ECUs above, they were now written with a white background, and no asterisk. This includes the ‘Engine and ECT’ ECU.

My interpretation:
- the final burst of ‘power’ when the engine spins down does seem to suggest there is unburnt fuel which ignites, while compressed, right before the engine finally comes to a complete stop
- it is weird how the Hybrid Control ECU says there is a P3101 Engine System code, yet the Engine and ECT ECU says codes ‘None.’
- and weirder that both those ECUs go white (from yellow) after the query, which TechStream says means communication was successful;

If it is true that for the ECUs, going from yellow background going white background (without asterisk) means communications was successful, then

I can’t figure it out. All I can think is that the ‘Engine and ECT’ is broken, however the engine can actually run (roughly!) without an engine ECU, as long as it is warmed up, but it needs input from the ECU in order to run during the warmup cycle?

I am wondering whether the VVT sprocket failed without warning, and failed, leaving the valve ports in a position such that air can still flow through the engine, and fuel somehow gets in as well. Then, the engine ECU, by chance, also completely failed at the same exact moment, but the engine was still able to run (presumably without spark), maybe ‘dieseling’ because of no spark.

That sounds ridiculous, but it is worrying that it explains the whole ‘the engine sounded exactly like it was running on 3 cylinders, in a completely regular misfire kind-of-way, but removing (spark!) one at a time made it much worse for each removed spark, meaning that ‘the engine was running on all 4 cylinders, and also on only 3 cylinders, at the same time’.

I would be grateful for any thoughts on what to check next; I am trying to avoid too many restarts, as I am getting close to needing to pull the battery to manually recharge it!

Something I am thinking about : the throttle body.

Does anyone know : for the throttle body, does it normally move in a certain way during a cold start? If so I could look at it during a start, in case that provides any clues?

 

Also a possible clue;

I have a wifi OBD2 reader that worked on my newer 2008 Prius, and also a Nissan Lead, but it never worked on this 2003 Prius.

Only TechStream worked on the 2003 Prius. I didn’t have a Windows install in recent years, so I couldn’t check codes……

On about 5 occasions, over about 3 years? I had a rare, but recurring problem. The problem was:

- need to use car
- start car (sounds normal) : go and grab something to put in the trunk. Come back 15 seconds later : engine not running, red triangle and engine light, car won’t even attempt to start!
- disconnect 12v, reconnect 12v. car starts fine.
- no further issues for months each time.

In retrospect, the issue above seems related to what has happened now. I assumed it was my batttery going out of balance, it



[I need to delete this thread, because things have changed so much, that I actually have a different issue now]