Yeah, Nah!
Yeow! Another long one. I hope it's interesting enough to make it worth the read.
My attempt last night didn't fix it. A second trip and... Seems like another factor is "Fully Warmed up".
Thinking back to the original No 1 problem with the car - very bad EGR issues, restarted back there thinking that "disabled" (vac line off and plugged) maybe wasn't enough.
Pulled the vacuum actuator off (easy, 2 x T20 screws) to see where that was at. There is a lot of spring tension in that dashpot, and I already knew that the arm was returning fully and easily - really snaps back if you pull the vacuum line off at idle - and could see when I took the pot off that the valve shaft was fully rotated to the closed position.
The design seems sound, there's a few degrees of spare (spring-arm) rotation remaining, which keeps the shaft tightly against some hidden stop when closed. However, also saw some soot around the shaft bearing and thought it was a tad loose. Seems like it's a simple butterfly valve, and maybe there is some internal wear of the butterfly or throat so maybe it was still leaking a bit. Lots of doubt...
So, made a blanking plate and fitted that in, but on my second trip out, it played up again... Slightly. And again today, this time another "Big Rev" 3N1 downchange. (It actually completes the change but usually I'm stopping so it's hard to confirm.) This time, the traffic moved ahead in time so that I could calmly confirm it. Also took careful note of the leadup to this change. It's not what one might expect, at all.
The accelerometer idea seems to be blown out of the water, as in this case and two other major rev cases, the braking was gentle but variable, and the changedown seemed to occur when the brake foot pressure was declining or released. It has always taken me by surprise, which makes it that much harder to remain the detached observer.
Anyway, after last night's failure, got thinking again... Apparent facts:
The
TCM seems to be doing something it has been programmed to do, albeit an exception to the general rule. It's not simply, random accident.
It is clearly (from the OBD Data) proceeding in a planned, timed and logical manner, and seems to complete its plan without fault. It changes from 3rd, through Neutral into 1st without graunching or breaking anything. For all I know, (as a long term former driver of crash boxes), it's actually performing a full Double-Declutch sequence, complete with appropriate engine rev-up, but I haven't yet found the PIDs which would confirm that.
Given that it must have been a whole lot of extra effort, time and money to implement this feature, it must have been regarded as worthwhile and important enough to do. I'm still thinking it's to solve a safety or driveability issue.
Suddenly hitting a very steep hill in too high a gear at low RPM might have been one, except that that scenario isn't playing out here. To the contrary, the decels in question are generally very gentle and drawn out. Not enough to reach the accelerometer threshold, I imagine. (That said, the observed behaviour suggests a fault somewhere, so the steep hill possibility is not necessarily ruled out.)
The problem, so far is that I haven't figured out what is triggering the activation of this exception.
For now, I've shelved the steep hill idea, but have found another that is looking even harder to pin down.
The first thing I noticed about the car (after sorting the EGR) in switching from the 2011 was that the brakes were noticeably weaker. Initially passed it off as someone fitting cheap aftermarket pads and, as you do, pretty quickly got used to it.
But now I'm wondering about the vacuum pump...
This is already too long so we'll get to that later as this is another big can of worms.