IAC - idle speed logic?

daveo2002 · Aug 21st, 2023, 16:34

Hi. I have a 1997 940, b230k. I've recently changed the head gasket and fitted a bunch of new parts. It has the 3" maf, Classic Swedes stg2 ECU and EZK chips, cam and turbo etc.

The car is now back together and starts and runs enthusiastically. But the idle speed is 2,500rpm. If I disconnect the IAC it idles at 600rpm. Pinching the hoses onto the IAC reduces the idle similarly. So I think the problem is the IAC running fully open.

I've taken the IAC off. It's a fairly new part. Clean, ohms measure fine, the valve moves freely. It's a two pin connector, voltage opens the valve and a spring holds it's closed. So I believe the ECU is sending volts to the IAC when it shouldn't.

What might cause that?

Thanks

Forrest · Aug 21st, 2023, 20:12

The IAC receives a frequency pulse or duty cycle rather than a static voltage. They are notoriously hard to test properly. It only comes into play when the throttle is closed though. Is your TPS adjusted properly per the FAQ?

I think the o2 sensor may influence the signal to the IAC at idle. Is yours standard and fairly new? If you’ve been fixing HG failure, coolant in the exhaust might have fried the original.

daveo2002 · Aug 21st, 2023, 20:52

Thanks Forrest. I set the TPS as per the FAQ. I thought the TPS was good because it clicked nicely but I never thought to check the resistance, so that's a good place to start. I will try that tomorrow.

The o2 is new and a separate wideband sensor confirms the mixture is bob on. It also has a new crank sensor and the speedo works fine.

daveo2002 · Aug 22nd, 2023, 19:10

I believe the TPS was the issue.

Measuring resistance whilst fitted, it would click as the the throttle closed, but the resistance across the pins didn't alter. Awkward to get the crocs onto the pins for sure but I tried five times and consistently saw the same resistance whether throttle was open, closed or wide open.

With the TPS free in my hand, I could turn the spindle and resistance dropped to nil as you'd expect at one end of travel. Worn contacts maybe?

I fitted a spare TPS and she idles beautifully. Thanks for the pointer!

Aug 21st, 2023, 16:34	#1
daveo2002 Junior Member Last Online: Sep 4th, 2023 19:01 Join Date: Dec 2010 Location: Birmingham	IAC - idle speed logic? Hi. I have a 1997 940, b230k. I've recently changed the head gasket and fitted a bunch of new parts. It has the 3" maf, Classic Swedes stg2 ECU and EZK chips, cam and turbo etc. The car is now back together and starts and runs enthusiastically. But the idle speed is 2,500rpm. If I disconnect the IAC it idles at 600rpm. Pinching the hoses onto the IAC reduces the idle similarly. So I think the problem is the IAC running fully open. I've taken the IAC off. It's a fairly new part. Clean, ohms measure fine, the valve moves freely. It's a two pin connector, voltage opens the valve and a spring holds it's closed. So I believe the ECU is sending volts to the IAC when it shouldn't. What might cause that? Thanks

Aug 21st, 2023, 20:12	#2
Forrest VOC Member Last Online: Today 11:52 Join Date: Feb 2008 Location: Gloucester	The IAC receives a frequency pulse or duty cycle rather than a static voltage. They are notoriously hard to test properly. It only comes into play when the throttle is closed though. Is your TPS adjusted properly per the FAQ? I think the o2 sensor may influence the signal to the IAC at idle. Is yours standard and fairly new? If you’ve been fixing HG failure, coolant in the exhaust might have fried the original.

Aug 21st, 2023, 20:52	#3
daveo2002 Junior Member Last Online: Sep 4th, 2023 19:01 Join Date: Dec 2010 Location: Birmingham	Thanks Forrest. I set the TPS as per the FAQ. I thought the TPS was good because it clicked nicely but I never thought to check the resistance, so that's a good place to start. I will try that tomorrow. The o2 is new and a separate wideband sensor confirms the mixture is bob on. It also has a new crank sensor and the speedo works fine.

Aug 22nd, 2023, 19:10	#4
daveo2002 Junior Member Last Online: Sep 4th, 2023 19:01 Join Date: Dec 2010 Location: Birmingham	I believe the TPS was the issue. Measuring resistance whilst fitted, it would click as the the throttle closed, but the resistance across the pins didn't alter. Awkward to get the crocs onto the pins for sure but I tried five times and consistently saw the same resistance whether throttle was open, closed or wide open. With the TPS free in my hand, I could turn the spindle and resistance dropped to nil as you'd expect at one end of travel. Worn contacts maybe? I fitted a spare TPS and she idles beautifully. Thanks for the pointer!

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