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post #1 of (permalink) Old January 9th, 2017, 15:32 Thread Starter
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Unhappy Yet another fast idle problem

(91 SW21 2.2L N/A MT)

This is driving me nuts. I have the infamous fast idle with surging. All the standard fixes haven't addressed the problem. Here's what's been done so far:

*The coolant system has been repeatedly burped. There is no air in the system. (This was my first guess.)

* I removed the IACV and it was dirty, but it has been thoroughly cleaned and moves freely. The actuator coils check out with the ohmmeter.

* I unplugged the water temp sensor and used a fixed resistor to tell the ECU that the engine was fully warm. No change. (This also confirms that air bubbles in the cooling system are not at fault.)

* The TPS had a dirty contact on the idle switch, but it was cleaned and checks out just fine.

I can confirm that there are no vacuum leaks. A finger on the IACV port inside the TB reduces the idle to a steady normal value (850 RPM). Unplugging the IACV and apply voltage to the ISCC pins causes the valve to close and the idle drops to normal. So it seems the ECU is wanting to open the IACV for some reason. (????)

I'm stumped at this point.

FYI -- I know what makes the surge at idle in case you were wondering: What's going on is that the TPS idle switch is closed telling the ECU that the throttle plate is closed. However, the ECU sees that the RPM is too high, so what it's doing is abruptly cutting fuel to the injectors. The RPM falls and the fuel is turned on again. The RPM increases. Wash. Rinse. Repeat. This is part of the fuel cutoff system. You can duplicate the surging by unplugging the TPS and shorting the idle switch. Slowly raise the RPM to around 1700. The surging will start.
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