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PostPosted: Sat Nov 30, 2019 1:46 pm 
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Joined: Mon Feb 19, 2007 4:12 pm
Posts: 8792
Location: Portsmouth
Car Model: 96 V4 & 9-3 Turbo-X
I don't think its air escaping.

At idle, or just after you lift off the throttle, the flap in the throttle body is closed, the manifold is in vacuum. Air won't be getting out, it will be being sucked in.

The purpose of the IAC valve is to bleed a little bit of air in to stop the engine stalling. If it's not bleeding the air in, the engine will stall. If you have a leak, too much air will be getting in and it would run fast.

Or, if it's a fuel problem it will either have not enough or too much fuel for the air being bled in, so that could cause it to stall either.


My bet would be that the IAC valve is failing when hot and not bleeding the air in like it should.

The picture of the valve isn't the best, but it looks ok.

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PostPosted: Sat Nov 30, 2019 5:29 pm 
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Joined: Fri May 17, 2019 7:08 am
Posts: 27
Car Model: Saab 900 16v Turbo Convertible
I'm pretty sure I put it in correctly, The long tube goes into the larger rubber elbow that leads to the turbo and the bypass hose that leads to the throttle body onto the shorter outlet of the valve.... If I have cocked that up then I probably should have gone to a mechanic in the first place.....


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PostPosted: Sat Nov 30, 2019 8:51 pm 
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Joined: Fri Jun 16, 2006 9:26 pm
Posts: 604
As previously suggested did you clean out the throttle housing? I mean really clean it out, ideally taking it right off and scrubbing it clean with carb cleaner and an old toothrush.

You may have to reset the throttle stop if it has been previously fiddled with, possibly been done to bring down the idle with your leaking by pass valve. Clamp the idle valve hose off completly and set the idle speed on the stop to 600 rpm. You may then need to start and stop the engine several times for it to learn the new setting.

After adjusting the stop you should really check the throttle pot voltage in the closed throttle position. From memory I think it should be less than 400mv but that needs confirming. If above then the throttle pot screw mounting holes should be slotted to achieve this figure.


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PostPosted: Mon Dec 02, 2019 1:06 pm 
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Joined: Fri May 17, 2019 7:08 am
Posts: 27
Car Model: Saab 900 16v Turbo Convertible
I cleaned it from one side I have to confess and didn't take the whole thing off will do that the next chance I get.
2 more things 99 Pete, alas my rev counter only works rarely for about 1 minute at a time or on some longer journeys for 1/2 an hour so it's difficult to know exactly where my revs are.

'After adjusting the stop you should really check the throttle pot voltage in the closed throttle position. From memory I think it should be less than 400mv but that needs confirming. If above then the throttle pot screw mounting holes should be slotted to achieve this figure'.

Where would i place my multimeter probes to get the readings for the throttle pot? Plus I'm afraid you've lost me slotting the screw mounting :? :? :?


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PostPosted: Mon Dec 02, 2019 1:13 pm 
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Joined: Fri May 17, 2019 7:08 am
Posts: 27
Car Model: Saab 900 16v Turbo Convertible
Point in case. this morning I dropped my kid off at nursery, about 1 mile away and then continued with some errands about another 20mins driving, purposefully not driving with a heavy foot and everything ran smoothly, no stalling at all even when taking foot off accelerator when coming to a stop. It is just 2 degrees here at the moment so I'm not sure if that affects the heating of the engine rubbers etc so not opening hoses /connectors as if it were warmer and creating a leak?? She was running really well which is why it is all so frustrating.


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PostPosted: Mon Dec 02, 2019 8:56 pm 
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Full Pressure Turbo

Joined: Fri Jun 16, 2006 9:26 pm
Posts: 604
Don't worry about the throttle pot at the moment, get the throttle stop set up first. Use a G clamp or something to clamp off the hose from the idle valve to the throttle housing, make sure it really is blocking the hose off and not letting any air past. This may stall the engine completly but don't worry just adjust the stop until the engine can maintain an idle speed of about 600 rpm. You may need to adjust the cable to allow adjustment of the stop. The engine needs to be up to operating temp with no electrical loads. Switch the engine off before you unclamp the hose and restart it, if it doesn't settle down to a steady 850 rpm switch it off and start up again. This base setting should give an engine which won't stall as even with a completely shut idle valve the idle speed shouldn't drop any lower than 600 rpm. Only do this AFTER the throttle housing has been cleaned! (it may not be necessary)

Also worth checking the fuel pressure as a well known 'tweek' is to crush the fuel pressure regulator to increase fuel pressure. This arguably has performance benefits but also upsets low revs and low load running and can result in an engine where the revs fall away very rapidly when the throttle is closed and dip below the stall rpm before the idle valve has a chance to catch it. Fuel pressure should be 3 bar with the vacuum hose disconnected (at atmospheric).


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