I posted recently about Cold Start and Fuel Economy Problems - LH2.2. Ultimately, disconnecting the AMM temporarily made cold starts easier, and allowed it to run smoother when very cold. Since I had covered off most things, but had to turn the idle adjust in fully to acheive a low idle, it was reasonably concluded that I may have an intake manifold air leak. Art also suggested I check my TPS function and suspect a sticking IAC valve.

In the last couple of weeks, the car started stuttering/hesitating a fair bit when accelerating from idle. In the past, grounding the test point for setting idle speed seemed to get the appropriate response, so I figured my IAC was fine. With this recent hesitation off idle though, a sticking IAC seemed plausible. I checked the TPS idle switch function at the ECU, and it seemed fine, so I proceeded to clean the IAC. I've tested the IAC in the past by feeding 12V to the centre pin, and alternately grounding each outside pin. This again seemed to rotate the valve to each endstop, but spinning the unit by hand did not cause it to rotate well, as other posts have suggested it should. I did the cleaning, submersing the valve end in paint thinner, while applying voltage to force the valve back and forth. I finished off with throttle body cleaner, and achieved the endstop clicking while turning it by hand, as suggested.

As an aside, I found that the male spade connector on the knock sensor had a cold solder on it. It was very loose, so I resoldered it. I wonder what effect on engine performance this intermittent would have had.

When I restarted the car, everything seemed the same - ie. normal idle with the adjustment knob turned in fully. Then I realized that I hadn't properly seated the IAC connector. As soon as I did this, the idle dropped significantly. This indicated that a) the IAC was being controlled, and b) that it hadn't been closing fully before being cleaned.

Now I proceeded to do the idle adjust test. I grounded the test point, and opened the adjust knob enough to achieve normal idle. Releasing the test point however didn't seem to change the idle at all, like it did in the past. I redid the test from before, feeding 12V to the centre pin, and alternately grounding the two outside pins. Blowing through the hose showed that this opened and closed the valve as expected, proving to me that the valve was working fine. When I back-probed the two outside pins of the IAC, they read roughly 11.9 and 5.5 volts at idle. (I believe this is just the average DC voltage determined by the duty cycle of the driving voltage). After grounding the test point, the voltages changed by about 0.1 volts each (I forget if they changed in opposite directions, as I would guess, or the same direction, but the centre pin voltage was constant). This change tells me that the test point is working, although the result was not as much as I was expecting.

I'm wondering if at a specific ambient temperature, the IAC valve at idle has little air contribution, whereas at different temperatures, it is significantly different. I know that in the past, I've gotten significant idle speed change when grounding/ungrounding the test point, but that was probably in much higher ambient temperature conditions. Or do you think I have an ECU issue?

Thanks for taking the time to read my essay! (and for any thoughts you may have).

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David Armstrong - '86 240(350k km?), '93 940T(270k km), '89 240(parts source for others) near Toronto