addo wrote:As I suggested, the timing belt change makes nil difference in a correctly timed motor.
I didn't think it would make any difference either, so I'm not surprised. Not that it's a bad thing having the timing belt done! I'm certainly running a risk on mine...
The idle controller is a very simple device; all you are doing with a reset is setting the stops. If the ECU says more air, it will open further. I think you'd find it can also work like an anti-stall dashpot on a late carbed car, "kicking" the fuel/air feed slightly if the idle is dropping too low.
Well the ECU controls the idle through the ICV via a feedback loop - if the rpm is below idle speed it simply increases the opening percentage until its back at the correct speed. If the ICV is working smoothly the change will be subtle and smooth. If it sticks and can only move in lurches then when a load comes on (A/C, radiator fans etc) the extra delay will cause the rpm to droop a lot lower than it should, then when the ICV finally does move it will over compensate and the revs will overshoot the desired target.
This is exactly what mine does - when a load change occurs the revs dip down to near stalling then surge above the correct revs and then drop back to normal again. It does this when the A/C kicks in, radiator fans kick in in particular, but it also does it when I pull to a stop and the gearbox changes from 2nd to 1st just as you're stopping.
If you take it off and toggle it with brief pulses of battery voltage, you can also see how readily it slips back and forth (ie; if it's sticky).
That's exactly what I was thinking of trying when I take the valve off, if you read one of the linked threads you'll see pictures of the inside of a faulty valve that have been cut open - some get sticky due to carbon/muck build up and can be cleaned, however when they get really bad it seems to be that the bearing at one end develops a lot of play, the play introduces enough slop that the rotating section of the valve intermittently drags on the housing. There is a picture of one where the rotating section is obviously scuffed, and that was out of a 70k mile car.
So just because it turns to the limit stops with a full 12 volts applied doesn't necessarily mean it is turning smoothly for small corrections, and I think the key to the problem - both the dipping idle and poor idle regulation, and the poor mixture regulation while driving is that the valve is not able to make small precise adjustments and stay where the ECU wants it.
Sadly at around £200 new a new one is out of the question! So cleaning it or obtaining a second hand one would be the only way of dealing with it if it is the cause.
It would also be surprising if it doesn't self-calibrate in service.
I can't quite see myself what the "calibration" would be doing either... especially if that calibration process occurs with the engine not running. But I'll try it anyway.
It's really at a point where you might compare all injector signals at the plug with test lights, and also test up the coils/leads with a flying earth as I did on the job recently - this way you can see the pairs of plugs and A:B their spark intensity. If all is good on a hot motor, then you're at the point of ECU substitution.
I can't agree really, I thought I was at the point of ECU substitution a week ago but now that I've discovered the effects of blocking off the purge solenoid and read this guys thread I'm following a different line of thinking entirely.
Read his "invisible caravan" thread (don't worry, its only a few pages, not 60+ pages
) and its almost like reading a version of my thread from an alternative universe. He's had the exact same symptoms as me and larppaxyz, has tried many of the same things as me and developed some identical theories, he's thought 100% that he's fixed the problem on several occasions only to have the problem come back days or weeks later, the difference is he seems to have finally nailed it once and for all...
I believe that I've had additional unrelated problems thrown into the mix too - fouled spark plugs at one point (and possibly again now) failed coil pack and leads etc, but even with those issues sorted out the underlying intermittent performance problem remained, particularly at low rpm. A mixture control problem is most likely what is causing my spark plugs to foul as well, so I think if I fix this problem I'll have no more spark plug fouling.
And finally, thinking back last year, the performance did start to go down hill at around the same time that the idle speed started surging - sometimes it was bad enough that after a hot start the engine would nearly stall, surge above idle, nearly stall etc as much as three times in a row before finally settling down. There was no idle speed surging problem back when the engine performance was 100% in the first 6 months that I had the car...
I've been suspicious of the ICV for a long time now, but only in relation to the idle control/surging issues, what's new is the realisation that it may be screwing with the mixture while driving and causing performance issues as well.
The two Allen bolts holding the valve can be reached but not the hose clamps attaching the underneath of the ICV to the throttle body, which on mine are still the original awkward to remove crimp types as well. (Which I will change to jubilee types)
