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420SEL (1987)
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10 Posts
Discussion Starter · #1 ·
After a thorough soaking of the Idle Air Control valve, my idle went from 1200rpm to about 700. And there was much rejoicing. :)

But recently, the idle has gone to 1600. :(

I put 12v on the IAC valve leads and got an authoritative click; this makes me think the valve is fine. I slid the IAC valve connector back just far enough to get voltmeter leads onto it and got readings between 0.8v and 1.5v; this makes me suspect an electrical fault upstream of the IAC valve connector.

Anything wrong with my theories above?

I am pretty sure the idle speed control module is fine, as pulling it caused idle to rise -- although only by 200rpm or less. Could this still be the problem?

Last possibility is the over voltage protection relay. Does failure of the OVP relay cause that much of an idle jump? Is there a test for function of the OVP relay, or do I just replace it as a matter of course?

Am I even close on this? I am not averse to handing it over to a trustworthy mechanic if this is over my head, but if it's relatively simple I should probably learn to address it myself. (And with an idle that high, I wonder if it would have to be towed...)
 

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Depends on the day!
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Well, even if the valve clicks that doesn't mean it has it's full range of motion anymore. The .8-1.5v is kind of odd..correct voltage is around 3-4 volts if everything is working properly. When the valves are bad I'll see a full 12v @ the leads as the ICU is trying it's hardest to bring the idle down... So with the valve appearing not to be receiving it's full voltage my suspicion would lie w/ the ICU.

Best of luck!

Jonathan
 

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1991 420sel
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Often the ICV responds to cleaning but then after a few days is worse than ever. On my 2 420's, each needed a new ICV after about 110Kmi. Each cleaned up and worked for a while, but then the idle went up again. New ICVs (and the 2 small associated hoses) fixed the problem. So dont assume ICV is fixed. Check the current running through the ICV at idle. Hook the ammeter into the circuit in series !!! and read current when car is idling. If 1000ma, then the electronics are OK and the ICV is bad (or there is a vacuum leak somewhere), as 1000ma is maximum control and the idle module wont do more than that. I have found that the voltage throught the valve is about 5.5 volts when working properly. Your low voltage reading could still be the ICV. Check resistance of the ICV when disconnected and compare with spec. Easy way to check module is to borrow a known good one and put it in to see what happens. Charlie
 

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420SEL (1987)
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Discussion Starter · #4 ·
Well, I couldn't get the ammeter secured underhood to my satisfaction. That, and quite frankly the thunderous noise of a 1600rpm idle makes me uncomfortable. But somewhere beneath the engine roar I think I heard air rushing, so I will poke around in hopes that there's a vacuum leak that's obvious enough for a dolt like me to spot.

If not, I am on the fence between handing it over to a $hop (fingers crossed for an easily-rectified vacuum leak) or buying a replacement idle control part or two.
 

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What secure?? Just hold one probe to each terminal while the car is running and take your reading for a few seconds..boom done.

If you are worried about the car dropping into gear while it's at 1600 RPM? If so engage the emergency brake firmly, chock one of the wheels and put someone in the drivers seat w/ their foot on the brake.

The air you hear is the BIG throttle plate sucking in air as you are running 1600RPM.

Jonathan
 

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benzwag said:
Check resistance of the ICV when disconnected and compare with spec. Easy way to check module is to borrow a known good one and put it in to see what happens. Charlie
Hi
What is the resistance value of the ICV?
 

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1991 420sel
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john--I hope you have solved this. You hook the ammeter in the ICV circuit in series and measure the current with the car idling warm. You can hook it up while it is off and then start the car. If you see 1000ma, then the ICV is probably sticking.
 
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