A temporary ECU/CPH swap is definitely on the agenda, David Hallworth has kindly offered to let me try the ones out of his Activa V6 conversion donor car which is SORN'ed pending engine gutting, its just a matter of finding a common time when we can look at it, so it could be a while away.addo wrote:Well, ECU etc swap if you don't want to try the battery terminals first. But I would 100% change them myself.
Meanwhile I've been thinking more about it and was watching another one of ScannerDanners new videos (as you do ) and realised there's another possibility I've overlooked for intermittent misfiring that gets worse when it gets hot - intermittent shorted turns on an injector as demonstrated in this video:
The only basis I have for believing the injectors are not at fault is that I did an injector balance test once last year using an exciter and measured the fuel pressure drop - which they all passed.
However that was done on a cold engine and the failure mode demonstrated in the video above is that as the engine heats up a shorted turn develops in the winding in the injector. This causes the inductance to drop dramatically and upsets the current ramp drawing a lot more current and too quickly.
This would affect the mechanical on time of the injector and because the injectors are wired in parallel if the ECU has over current protection then one shorted injector would steal current from the other one in the pair possibly causing the one that's not faulty to fail to fire at all.
Being very intermittent and tending towards being worse when hot does fit the symptoms I'm seeing. The only way to diagnose or rule out this problem is to look at the current waveforms for the injectors, so I'm seriously thinking about stumping up for a current clamp probe that I can use with my scope and multimeter.
It would also allow me to identify (or rule out) any problem with the injector and coil pack primary side drive from the ECU, which is the thing most likely to be wrong with the ECU, if anything is. Whether I get the current clamp before or after the ECU swap remains to be seen (obviously I wouldn't if the ECU swap did fix it) but that's my line of thinking at the moment.
Any problems with power feeds etc will show up as problems in the coil and injector current waveforms as well, in short those two sets of waveforms tell you pretty much everything you need to know about the ECU outputs that could potentially be causing a misfire.
I would try this except symptoms usually set in well before the fans kick in, and I haven't been using air conditioning lately either, yet the symptoms are still there. There is a noticeable drop in engine revs when the fans do kick in but I think this is more a symptom of the engine lacking power and/or mis-misfiring under sudden load increases as it does a similar thing when I try to accelerate off the mark.
Another thing to try is pulling the fan relays (without a battery disconnection event) as this will reveal current supply inadequacies. Don't boil it though.