LS400 idle and cutting out issue

Razor61 · August 20, 2020

I've had idling issues and the cutting out issue on my 1996 LS400 Mk3 more or less since I bought the cat in Oct last year which I think I've now solved. I have posted regarding this on other posts but thought I would put it all together and it may help other owners with the same or similar issue.

Cutting out issue after a decent run and stopping:

When I first experienced this issue I did some research and the general feeling was that it is caused by the ECU capacitor issue so I duly ordered the capacitors and removed the ECU to replace them. After examining the ECU I couldn't find any traces of leaking capacitors or 'doming' on them, ECU looked pristine so put the ECU back in the car. I'm not saying the capacitors are 100% ok (see further information below) but decided not to fit them. I decided instead to purchase a second hand ECU, replace the capacitors and fit it to the car. That way I wouldn't end up with a knackered ECU and not being able to run the car if I screwed up. I couldn't obtain the exact ECU but did obtain an updated version that is ok for my car (I also posted about this regarding ECU compatibility). I examined this updated ECU and it was also pristine and as new so didn't replace the capacitors and didn't fit to the car at this point.

Erratic idling sometimes well over 1000 revs, engine revving by itself at low speeds and driving the car forward when it shouldn't:

I stumbled upon a post on a USA forum regarding the 'ECM/ECU Coolant Temp Sensor' causing excessive fuel consumption and idling issues (I did a post about this) so I have replaced the sensor. But at the time I replaced the sensor the aircon compressor had just packed in. After I replaced the sensor the idle was immediately spot on at 600rpm and wasn't erratic at all, also the car didn't cut out after a decent run and stopping. All good for a few weeks (aircon still not fixed). Great result and gave myself a pat on the back🙂 But after I fixed the aircon the erratic idling came back with the same symptoms and maybe a bit worse, I reset the ECU and it was fine for a few days but came back again. After putting up with it for a few weeks I decided to fit the updated ECU to the car - 1. to see if it actually worked and was ok with my car and 2. to see if it made any difference to the idle (being an updated newer ECU). I was pleasantly surprised to find that it did actually work on my car as Lexus said it would and the idle is much much better, idle isn't 100% rock solid as before because the aircon is kicking in and out etc but I think it is 'as it should be'.

Based on the above my conclusion is that the cutting out issue on my car was solved by replacing the 'ECM/ECU Coolant Temp Sensor'. I'm reasonably sure the erratic idling was solved by replacing the 'ECM/ECU Coolant Temp Sensor' as well or a least when not using the aircon. Replacing the ECU with the updated newer one I bought definitely solved the erratic idling etc after I fixed the aircon, either the capacitors were starting to fail in the original ECU or just fitting a newer updated ECU solved it or maybe the combination of a new 'ECM/ECU Coolant Temp Sensor' and a newer updated ECU solved it all.

I hope all that makes sense and will maybe help someone else with the same issues because the erratic idling and cutting out is very annoying ....................