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One of the most common (and annoying:evil: ) problems we 900 owners face is abnormal idle. Hunting--where the idle speed cycles up and down repeatedly--is one of the most common of these.

The two major causes of idle hunting seem to be a bad Hall sensor and a sticking AIC valve.

On our 1990 900S automatic, the idle has never been right. I fixed an intermittent high-idle problem by adjusting the TPS (throttle position switch) so it closed when the throttle did, and have regularly flushed out the AIC valve as a preventive measure.

But the hunting, which was sporadic in Park, became more and more frequent--especially when loading the engine by putting the car in Drive. This often caused it to buck like a copulating (insert suitably comical mammal here) when waiting at a light with foot on brake. Annoying :(

On a hunch, I replaced the AIC valve with the one I had kept from my recently scrapped '88 5-speed--which had idled perfectly for its entire 270,400 miles.

Problem solved :D

My retrospective guess at the problem: I kept the AIC valve clean, and its rotor always moved freely--so no sticking there.

Most likely, the AIC's commutator and brushes are worn so there's poor contact between them. Thus, when the ECU signals the AIC to move, there's a delay. The ECU keeps feeding voltage for a greater AIC movement (more air--increase idle speed); the AIC now responds with too much air, and spikes the idle. The ECU signals the AIC to cut idle speed, which now plummets. And the cycle repeats about once per second.

Sometime soon, I'll take apart the AIC valve to see what's going on. I'll also try refurbishing it, as was detailed by a post in this Forum a couple of years ago (I'm afraid I can't locate this post, though...)
 

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I've been having trouble with my Lucas AIC as well. The idle has been a bit wobbly for some time, but a couple of days ago the car wouldn't idle at all and died as soon as I took my foot off the gas. I've flushed the AIC through with carb cleaner before but have not managed to take it apart successfully (Lucas ones come apart more easily apparently). I also get the classic 'leaking bypass valve' symptoms of when I brake and put the clutch down when coming up to a junction, idle drops to 600 rpm. But I don't have a bypass valve, I have a Forge BOV.

I checked for vacuum leaks, went right through the guide in the tech help section, cleaned the throttle body out, blasted the inside of the distributor with WD40 but it still wouldn't idle. I then took the plug off the back of the AIC and sprayed WD40 on the connectors - they were not corroded at all, but it has been very damp here. Put it back in, turned the key and the car idled perfectly. I'll see how it goes in the next few days, but I'm wondering if a build-up of moisture inside the AIC could cause a variable signal too, before the thing just dies and won't idle at all. Anyway, giving it a shot of WD40 on the connectors might help.
 
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