Cold start : cuts out after ~7 seconds. Did this once every few months, but now totally dead.

I posted a thread about an issue I had with my car recently. Got mini VCI today, also realised something important. The problem is quite different from what I thought, hence : I need to make this thread

Background : I bought this car (2003 Prius NWH11) 2019, with a battery fault. I built a battery with NWH20 modules (I had two spare NWH20 batteries, one of which was a bit dubious). The battery worked fine. I lost my Windows install with TechStream, and my OBD2 scanner worked with all cars except this one.

Since then, once every few months, something weird happened on cold start;

- I would start the car, but not immediately start moving. After 7-30 seconds, the engine would die, with a red triangle, yellow engine light,
- I needed to unplug the 12v battery before it would let me try to start again!
- On one occasion it stalled twice in a row like this! Because I didn’t start moving immediately after starting the second time.
- My OBD2 didn’t work, I didn’t have a Windows install, and I assumed it was due to battery being out of balance.

I hadn’t had any issues for months. Then, last week, I was going to the shops. The car had started fine, and drove fine. Halfway through the journey. The car seemed rough, like it was misfiring. I drove home carefully (home was about 5 miles away).

Once home, the car started fine, but was rough. I couldn’t seem to isolate the rough running to one cylinder (disconnecting coils one at a time made it much worse, despite the apparent ‘misfiring’.

Once the car cooled down, it wouldn’t start again. It runs fine for 4-7 seconds, but then starts ‘knocking’ very badly for several seconds, then cuts out. Just before stopping spinning completely, it often has one ‘ignition?’. It speeds up, enough for three more ‘knocks’, then stops spinning for good.

I got a new TechStream today, and pulled the codes, which are:

(Engine and ECT) ECU : Under ‘Codes’, it has the following string : “None.” Above where it would show the codes, it says ‘Triangle; DTC Monitors are N/A’ and also ‘MIL : N/A’
(HV Battery) ECU : None. (live monitoring shows no issues, and very well balanced!)
(Hybrid Control) ECU : P3101 Engine System (this is tagged for Current, History)

Of note : I disconnected the 12v battery last week - it would have been flat today if I hadn’t. This might be what the ‘DTC Monitors N/A’ is about? Maybe?

I realised over the weekend that the rare cold start issues seem related to what has happened now! But I ignored them as they were rare, I didn’t have Windows and one of the NWH20 batteries I used to rebuild this battery was a bit dodgy (some blocks with one cell shorted, for example!).

in retrospect, the rare cold start issue is probably key here, in retrospect, given the very good balance of the battery!

I would be grateful for any ideas where to go from here. I don’t have any pressure gauges, the hot fans worked well on the way home (I’ve checked for other signs of head gasket failure too); oil is normal and correct level. I did fill my fuel tank from 50% to 100% about 6 miles before the engine because ‘rough’. However that fill was also 3 days before the issue arose, so it did manage a cold start (the last ever cold start) using that particular fuel tank, and I can’t see any reports of bad batches of fuel so far.

One idea I had : the throttle control body? The issue is just like when I blew some fuses in my NWH20. The issue was the throttle body not working (blown fuse) and it seemed just like this!

Does the throttle body behave in a certain way when attempting a cold start? If so, I can watch (do a video recording) of the throttle opening during an attempted start, in case that gives any clues?

 

I had come across that TSB yesterday, but that thread gives a lot more info! Something interesting is : this car is a Japanese used import (not sure when it was imported exactly but will check once I find my documents) - however, I bought it from someone north of Auckland in New Zealand (a subtropical area), and drove it a few hundred kilometres south (where we only get a few weeks in winter with nights below freezing); in retrospect, the failed starts probably all happened on cold days (which again made me think a weak battery was the cause); we are going into late? Autumn currently also;

If I have the faulty ECU, I will just live with it (and use the 12v disconnect if it happens - an easy fix, but not something Toyota could suggest to most owners in the past ). It does seem like something new has happened on top of this, so today I plan to check the throttle plate (video camera during starting, and will shut car down after 8 seconds to try to save HV battery charge); will likely check MAF and Throttle Body first (and order MAF cleaner and will only use that!), remove any pooled oil just in case, and clean Throttle Body. Also, only some of my car tools are here . After that will check fuel pump (I really hope it isn’t that), and if it works, will check relay(s) etc;

In the longer term;
- the ‘fuel pump can only be replaced with an entire fuel tank assembly’ is a death knell for these cars, so if I end up replacing it, I will figure out how to use a different fuel pump. That will require a fair amount of research; I envisage the fuel level connectors will remain on the old pump, and the fuel lines will go to the new pump. I also envisage challenges if the existing fuel lines are ‘blocked’ by the old pump - as any fix I do will be done ‘properly’ for obvious reasons!
- once back on the road, I will figure out what was done to bypass the lack of catalytic converter - as there have never been any persistent codes, despite driving for years - I am wondering whether it is a patched ECU, or some kind of sensor simulator?