940 idle adjustment

[Oneeyeblind]

Hey everyone. 940 3 I've found plenty of pretty detailed idle adjustment on different sites. All basically the same.
Just one thing that's not completely clear to me. When I'm adjusting the idle down everyone says go down to around 500, that seems low so I'm kind of assuming you wind down to 500 whilst you have the IAC hose clamped shut? Thus stopping it interfering with the idle?
https://www.volvoclub.org.uk/faq/Eng...itchAdjustment
That's the link that I'm going from if anybody wanted to know. Any help is appreciated!

[Rversteeg]

You are correct:
The basic setting is 500 rpm by the air passing the throttle valve plate. The plate is never fully shut but always slightly open when the engine is idling.
The additional 300 - 400 rpm is caused by air that is bypassing the throttle valve through the IAC.
So without IAC working the engine will not stall but continue running at 500 rpm. A dirty throttle housing will reduce the basic idle speed. Although it will be compensated by the IAC, its response time will cause the engine to sometimes (almost) stall.
Under normal circumstances it should not be necessary to adjust the basic idling speed, just cleaning the the throttle body housing should be enough.

[Oneeyeblind]

Thanks mate. I'm gonna clean the body as soon as a new gasket arrives, prior to re adjusting anything. I can't actually hear a click from the tps either, is it a noticeable click?

[Rversteeg]

I don't bother to take the throttle housing out, but just take of the rubber air inlet hose, fully open it and then clean the housing with a cloth wetted with brake cleaner.
I am not sure if you can actually hear the switch clicking. Best is to disconnect the connector and use a multimeter to verify the setting / functionality.

[SalvadorP]

Originally Posted by Rversteeg

You are correct:
The basic setting is 500 rpm by the air passing the throttle valve plate. The plate is never fully shut but always slightly open when the engine is idling.
The additional 300 - 400 rpm is caused by air that is bypassing the throttle valve through the IAC.
So without IAC working the engine will not stall but continue running at 500 rpm. A dirty throttle housing will reduce the basic idle speed. Although it will be compensated by the IAC, its response time will cause the engine to sometimes (almost) stall.
Under normal circumstances it should not be necessary to adjust the basic idling speed, just cleaning the the throttle body housing should be enough.

Hey Rob. I'm having some issues understanding this system and this quote actually clarifies some of my confusion, but one question still persists.
Theoretically, unscrewing the stop screw of the throttle lever should close the plate and therefor make the idle drop all the way to the 300-400rpm provided by the IAC, right?
If so, what does it mean when this doesn't happen? When without linkage rod and stop screw the rpm don't drop at all? Does it mean the plate is never closing all the way?

Or is it that the default end position of the throttle lever is always to let a certain amount of air through? But then why is the stop screw needed?

[Rversteeg]

Never in my over 500K km of driving my 940LPT have I had to adjust the idle setting, so I cannot tell you how it is done by heart. All I ever did was cleaning the throttle house body.
The basic idle speed of 500 rpm is set by letting just enough air past the throttle plate. Additional air to reach 900 rpm is byapssing the throttle, through the IAC valve. The IAC is the only means of the engine ECU to control the idle speed. In other words, minimum idle speed is always 500 rpm, the speed above this is controlled by letting more or less air bypassing via the IAC valve.

The ECU uses information from the TPS to ascertain it has to adjust the engine speed and that a higher than idle engine speed is not caused by you pushing the throttle pedal. A speed signal is used simultaneously to determine if a higher than idle engine speed is not caused by the vehicle braking on the engine. It is an on/off switch and does nothing to the idle speed itself. That is entirely done by the ECU and cannot be adjusted externally.
You can only adjust the minimum idle speed via the throttle plate leakage ratio. If you set this for example to 1200 rpm, the ECU will have no means of controlling the engine speed any lower than 1200 rpm. With the correct setting of 500 rpm, the ECU has enough bandwidth to control the idle speed via the IAC valve. Minimum idle speed at 500 rpm with IAC valve closed and maximum idle speed of about 1500 rpm (?) with IAC valve maximum open.

[SalvadorP]

Originally Posted by Rversteeg

Never in my over 500K km of driving my 940LPT have I had to adjust the idle setting, so I cannot tell you how it is done by heart. All I ever did was cleaning the throttle house body.
The basic idle speed of 500 rpm is set by letting just enough air past the throttle plate. Additional air to reach 900 rpm is byapssing the throttle, through the IAC valve. The IAC is the only means of the engine ECU to control the idle speed. In other words, minimum idle speed is always 500 rpm, the speed above this is controlled by letting more or less air bypassing via the IAC valve.

The ECU uses information from the TPS to ascertain it has to adjust the engine speed and that a higher than idle engine speed is not caused by you pushing the throttle pedal. A speed signal is used simultaneously to determine if a higher than idle engine speed is not caused by the vehicle braking on the engine. It is an on/off switch and does nothing to the idle speed itself. That is entirely done by the ECU and cannot be adjusted externally.
You can only adjust the minimum idle speed via the throttle plate leakage ratio. If you set this for example to 1200 rpm, the ECU will have no means of controlling the engine speed any lower than 1200 rpm. With the correct setting of 500 rpm, the ECU has enough bandwidth to control the idle speed via the IAC valve. Minimum idle speed at 500 rpm with IAC valve closed and maximum idle speed of about 1500 rpm (?) with IAC valve maximum open.

Got it. Thanks for the explanation. When you don't know a certain thing, sometimes a bit of info, that is massively useful to someone who does understand it fully, can send you on a goose chase or just scratching your head.

I don't fully understand this system. I don't get what is the pupose of the adjustment screw or why it needs to be between 0.15 to 0.45 if the throttle lever is fully close anyways when the car is idling at the proper rpm. Not asking for an explanation. Just saying.

What matters is that the car is idling better than it ever was. It does bother me a lot not understanding things and how they work. But not being a naturally mechanically inclined personal, i have to start accepting that sweedish engineers just work in mysterious ways.
Thanks to the good folk in this forum i've learned so much. I couldn't change a spark plug before.