So, the saga continues. Bear with me please folks.
Having received the new OBD2 adapter a week or so back, I was busy logging like crazy every drive. Whilst I had no repeat of the symptoms, and no codes, I have seen quite a lot of variations in LTFT and STFT, sometimes totalling over 30% adjustment, which to me is excessive, however it is very variable, sometimes it'll only be 5% or so total. Average MPG on this car is around 5% less than the (mechanically identical) red hatch I'd had previously, over the same routes, to have a comparison.
This week saw me heading up North again. As I got up around 8000 - 9000 fasl, I was a bit down on power but no real misfires, and I wasn't sure if it was just the altitude. I stopped to use nature's bathroom and stretch my legs, I also opened the fuel cap just to see, and whilst there was a slight pressure difference, it was very little, and as I jumped back in and fired her up, I got my high coolant temp alarm, dash gauges showed fine, but Torque showed 105 deg C (in Lima it's between 85 and 90 C normally). I got moving and it dropped to 101 deg C, and downstream O2 sensor was showing a lean condition, as was the AFR measurement (although only ever so slightly). I stopped again for a minute, had a look over everything, found nothing, and carried on, rather nervously, but wondering if it was all altitude related. Temps finally started dropping to around 95 deg C, with downstream O2 showing 0.4-0.5V (I can't access the upstream O2 data). During this period, sum of LTFT and STFT varied from -28% to +46% (!)
Things seemed to stabilise until an hour or 2 later, getting close to my destination, a good few thousand feet lower in altitude, I had to pull up for an hour waiting for the road to reopen (roadworks where they close the road for hours at a time...), as we set off I didn't notice anything, as it was only 20-30 mph in a queue. As it opened up, I went to pass a truck, and had to abort - over 3000rpm / 50% throttle it was like hitting a brick wall. As soon as I could I pulled over and popped the fuel cap again, but there was no noticeable pressure differential at all, so it doesn't appear to be a fuel vacuum issue. I turned it all off and on again, and set off. Power seemed normal (for the altitude) below 3000 rpm and less than 50% throttle, but above that it was struggling. I limped it the last few km, and parked it outside the accommodation for an hour or so. I again had a very cursory look but I had to get down to the facility ASAP, so headed off again (only 2 miles, mostly downhill). Power seemed a bit down but it would just about rev past 3000 rpm and take more than 50% throttle, however still with a slight miss. The next morning before heading to work, I popped plugs on 1 & 3 out, they looked ok, possibly slightly lean, gap looked a bit big. The plugs are 5 months/4000 miles old.
I went to get my feeler gauge and...realised it was in my other toolbox in Lima. So they went back in as is. I also checked the coil pack resistances, all within factory specs. I then did a hard reset of the electrickery, and went to work. On the way it all felt mostly fine, just a slight miss passing 3000rpm. I also found something in Torque I hadn't found before - the misfire counters. I found it had registered 3 misfires on cylinder 3. On the way back from work, 2 on cyl 1, 2 on cyl 2, and 1 on cyl 4. So whatever the misfire is, it's across all cylinders.
Reading up that evening, on MS3s, there is something called the spring-stretch mod. These are coil on plug, with a spring for the contact. Several MS3 owners had random misfires around 3000 rpm, and it was basically that the spring wasn't making good enough contact with the plug, especially with Denso plugs (which I have), so they stretch the spring slightly to improve the contact. So I gave it a try. The entire drive back I didn't have any noticeable issues, apart from at one moment - there is a stretch of the road which never got resurfaced after the landslides of 2016, it's a few hundred feet of gravel. I can never resist having a play, and after that I had a slight hesitation for 30 seconds or so, which went away again, which was rather peculiar.
The rest of the trip back was ok (well ignoring the terrible driving standards and awful traffic), but still logging 1 or 2 (or 4 in one case) misfires across various cylinders every drive cycle, but hardly noticeable, if at all, whilst driving. Today, back in Lima, doing the normal run to school and work, the sum of LTFT and STFT are slight less, but still far too big a range : -16% - +32%. Also logging a couple of misfire (cyl 1 and cyl 4 this time). There doesn't seem to be any consistent correlation between fuel trims, vacuum, load, rpm, etc. About the only generalisation would be that the LTFT is more often than not at the lowest negative at idle, and the highest positive at higher rpm, but not always - say 70% of the time.
The first thing I'm going to do is gap the plugs, possibly followed by a plug change - I'm running Denso ITV16, which are supposedly equivalent to the factory NGKs, however some MS3s have reported issues with Densos (different plug, but still). I've now found a place with the factory NGKs in stock, at a reasonable price, so I may swing by there. The lean condition at altitude had me worried though, although the entire drive back was normal. I was wondering if the fuel pump was getting warm after 8 hours on the road and starting to fail, but it would do so on the drive back too, and it didn't. With regard to general stuff, no coolant consumption (nor oil), no evidence of HG issues (I will try and do a comp test at some point though), and as it's intermittent I think it's electrical/fuel related anyway.
I'm all ears to any suggestions or similar experiences...
I'm gonna guess south of France.
