Surging idle, 89 S4 - closed vs. open loop at idle
08-05-2011, 06:36 PM
#1
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Surging idle, 89 S4 - closed vs. open loop at idle
So the car went into the shop as they were trying to fix the high NO. The car has also suffered from surging idle, but it was very occasional - every month or two for a short period.
They changed the O2 sensor, and the idle control valve, fixed a couple of vacuum leaks. The NO has improved a lot but not quite enough to pass.
But the wierd thing is I now have a CONSTANTLY surging idle. It's almost impossible to drive.
Now I had this once on my XJS - the idle switch was incorrectly adjusted so it was in closed loop mode at idle - should be open loop at idle.
Can anyone tell me if the S4 is the same? by that, I mean, should it be in open loop at idle and closed loop when the throttle is pressed?
What else can cause surging idle?
They changed the O2 sensor, and the idle control valve, fixed a couple of vacuum leaks. The NO has improved a lot but not quite enough to pass.
But the wierd thing is I now have a CONSTANTLY surging idle. It's almost impossible to drive.
Now I had this once on my XJS - the idle switch was incorrectly adjusted so it was in closed loop mode at idle - should be open loop at idle.
Can anyone tell me if the S4 is the same? by that, I mean, should it be in open loop at idle and closed loop when the throttle is pressed?
What else can cause surging idle?
08-05-2011, 07:11 PM
#2
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All LH equipped Porsche 928s like yours are closed loop at dile, and when the throttle is open at low loads. When you mash the throttle it goes open loop to give the mixture enrichment required.
What you do have on your car is a closed throttle switch, it is this that enables the idle control loop when the throttle is fully closed. You should hear a faint click at the engine if you manually operate the throttle quadrant on the side of the intake plenum a fraction off the closed position. It should always reliably click again as the throttle plate closes fully.
However your car sounds as though the closed throttle switch is working. The idle speed hunting up an down is a good sign that the idle loop trying to get control of the idle speed, but for some reason cannot. The hunting suggests your idle control valve is good, and that the LH ECU ISV drive is also probably good.
Over what range does the idle speed hunt ? Have you tried unplugging the O2 sensor ?
Try a battery ground disconnect for a few seconds... this can sometimes help by restting the stored idle and fuel adaptation parameters stored in the LH ECU..
If the mixture is too far out, the idle loop won't be able to get control.
What you do have on your car is a closed throttle switch, it is this that enables the idle control loop when the throttle is fully closed. You should hear a faint click at the engine if you manually operate the throttle quadrant on the side of the intake plenum a fraction off the closed position. It should always reliably click again as the throttle plate closes fully.
However your car sounds as though the closed throttle switch is working. The idle speed hunting up an down is a good sign that the idle loop trying to get control of the idle speed, but for some reason cannot. The hunting suggests your idle control valve is good, and that the LH ECU ISV drive is also probably good.
Over what range does the idle speed hunt ? Have you tried unplugging the O2 sensor ?
Try a battery ground disconnect for a few seconds... this can sometimes help by restting the stored idle and fuel adaptation parameters stored in the LH ECU..
If the mixture is too far out, the idle loop won't be able to get control.
08-05-2011, 10:09 PM
#3
Nordschleife Master
I'd be taking it back to the shop and having them re-check their O2 sensor work. I've had similar symptoms fitting a non-working O2 sensor. PITA to drop the exhaust and swap sensors again, but it worked for me.
08-06-2011, 04:35 AM
#4
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Who did the work? I'd be glad to run diagnostics on it and look it over.
If the throttle switch is misadjusted or the throttle cable is too tight, the idle will be erratic.
If the throttle switch is misadjusted or the throttle cable is too tight, the idle will be erratic.
