Some more observations and thoughts in the ongoing quest to pinpoint the intermittent engine performance issue...
Tonight I was installing Diagbox on a netbook from work and while testing it on the car I noticed a fault logged on the gearbox ECU that I have seen appear sporadically before:
The gearbox ECU receives the engine speed signal from the engine ECU (which in turn gets it from the crank sensor) and uses it as a critical piece of information for regulating the torque converter clutch slip etc. If I understand it correctly the above fault is indicating that the speed sensor signal from the engine ECU was temporarily missing or incoherent.
From memory this fault has only ever been logged in neutral, and I have a theory on why that might be - if we imagine that a problem with the speed sensor signal causes it to receive less (or no) pulses per second, how would the gearbox recognize that ? My guess is that it compares the engine speed signal to the torque converter output speed sensor in the gearbox and checks the plausibility of the two speeds.
If you're in neutral then the engine speed signal should always be equal to or faster than the torque converter output speed signal - if its less for any significant length of time the gearbox ECU can assume there is a fault with the signal and flag a fault code. If it can detect the output of the torque converter is turning at 640 rpm but the input according to the engine is only turning at "300" rpm, (due to missing pulses) its safe to say there is a fault.
However if you're driving in gear then its normal for the engine speed to be lower than the torque converter output if you're on the overrun with the throttle closed, so the gearbox can't check the coherency of the two speed signals in gear, which I believe is why the fault is only ever logged in neutral even though the fault itself is almost certainly occurring while driving in gear as well.
Anyway, I let the engine idle for several minutes while testing the Lexia, I noticed at first that there was a bit of an intermittent roughness in the idle, with what seems like maybe one miss/stumble every second or so, which gradually went away over 3-4 minutes. This symptom has been there for at least a few months now, and I haven't been able to find any cause for the rough idle when cold.
I then went for a drive and surprise surprise, the engine is going like a rocket again....
Tons of power, very eager to accelerate, not quite 100% at low rpm, but anything over about 2000 it seemed perfectly normal. If it ran this well all the time I'd be very happy indeed.
Only this morning when I drove to the train station I let the car idle for only 20 seconds before driving (hey I was running late!
) and when I tried to accelerate up a steep hill in 1st it was really struggling, it felt like it was missing badly, struggling to even reach 3000 rpm in 1st with 3/4 throttle, as soon as it hit 3500 rpm the "missing" stopped and it suddenly roared away like an invisible turbo boost button had been pressed...
This symptom of poor performance below 3500 rpm for the first minute or two after starting, particularly noticeable if trying to climb a steep hill has been going on intermittently for quite a while now, and has been really obvious since we moved to a house which is on the side of a hill.
So what the heck is going on ? Putting together the clues of the engine performance and the speed sensor fault logged by the gearbox (not for the first time) I'm now thinking about a poor quality crank sensor signal - something I've speculated on before in this thread.
This reminds me of the intermittent misfire the car in this video had, see around 12 minutes in:
In that case the problem was a crank sensor with an excessive gap. The crank sensor signal is a sine wave which increases in both amplitude and frequency with engine rotation speed which is then converted to a pulse waveform inside the ECU. If the gap is too big the amplitude is too low which means at LOW rpm there will be insufficient amplitude for the Schmidt trigger (or whatever the ECU uses) to produce a clean reliable pulse waveform.
At high rpm the pulse waveform will be fine because there is amplitude to spare, but at low RPM the pulse waveform will start to become intermittent causing missing injector and/or ignition pulses and intermittent errors in timing and dwell times. (See the video at 12 minutes)
What I'm thinking may also happen under these circumstances apart from power loss through lost or mistimed ignition pulses is that if the timing of ignition pulses is not steady and reliable, when the fault is symptomatic it may sometimes be causing pre-ignition / knocking due to the spark being intermittently too far advanced, thus triggering the knock sensor and retarding the timing map. So I really need to get my scope onto the crank sensor signal.
I know I've asked this before, but can anyone tell me where the crank sensor connector is, and how I can access the crank sensor itself to check/adjust the gap ? I would look up the wiring for it on Sedre unfortunately Sedre is on my dead laptop...