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Discussion Starter #1
Hi Everyone,

I recently purchased a classic Saab 900, 1989 auto. It has the LH2.4 but curiously the throttlebody has a dashpot. It's backed completely off so it doesn't do anything so I'm assuming it's been pulled from an LH2.2 engine.

My question is, apart from the dashpot are there any other significant differences in the throttle body. It's got the correct TPS which is adjusted correctly, but does idle up at 950 which is a little higher than it should. As the 2.4 has no idle adjustment and the 2.2 does, I'm beginning to wonder about the stop position...

Thanks,
Lee
 

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I believe it was fitted/adjusted so if you dropped of the throttle sharlpy it would stop the engine form virtually dying through lack of air, so it consequently cushions the blow of foot off accelerator quick, and keeps butterfly open so if it's not touching anytnything it's been adjusted wrongly..
 

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Well like he says. It's not supposed to have one on an engine that late.

If the idle speed is too high it's probably getting more air than it should. Check for leaks and if you can't find any then adjust the throttle stop.
 

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Discussion Starter #4
Ripped off that Dashpot and removed the throttle stop screw entirely. The LH2.4 is supposed to handle the smooth transition from high revs to idle when the throttle is snapped shut.
 

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Ripped off that Dashpot and removed the throttle stop screw entirely. The LH2.4 is supposed to handle the smooth transition from high revs to idle when the throttle is snapped shut.
Why did you remove the throttle stop screw? You should not let the butterfly slam shut using only the throttle body to stop it. Over time I can see a nice groove developing.
 

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Discussion Starter #6
It's the best I can do with what I have. The LH2.4 throttle body does not allow any air to pass when idle, the bypass valve controlled by the LH manages that. So if I have the throttle stop screw holding it open a little bit isn't that going to give the idle control some issues? The throttle body on an LH2.4 has no throttle stop screw so I don't know how the inside differs, but you are right, there should be a rubber surface to mate against in order to make an air-tight seal and not wear anything.
 

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It's the best I can do with what I have. The LH2.4 throttle body does not allow any air to pass when idle, the bypass valve controlled by the LH manages that. So if I have the throttle stop screw holding it open a little bit isn't that going to give the idle control some issues? The throttle body on an LH2.4 has no throttle stop screw so I don't know how the inside differs, but you are right, there should be a rubber surface to mate against in order to make an air-tight seal and not wear anything.
The butterfly is cracked ever so slightly using the throttle stop set screw. The AIC is a valve that meters the additional or reduction of air required to keep the rpms steady. The ECU reads the RPM and commands the AIC to open a certain degree. In this case the valve actually shuttles open and closed metering the amount of air. So when there is a load on the engine. Say the coolant fan kicks on, the ECU see the drop and gives more duty to the AIC allowing more air to recover the engine RPM. I think you might have a vacuum leak and the AIC is doing all it can to reduce the amount of incoming air. Have you cleaned the inside of the AIC? They do get gummed up and the valve sticks. Carb cleaner and triflow.
 

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Discussion Starter #8
The butterfly is cracked ever so slightly using the throttle stop set screw. The AIC is a valve that meters the additional or reduction of air required to keep the rpms steady. The ECU reads the RPM and commands the AIC to open a certain degree. In this case the valve actually shuttles open and closed metering the amount of air. So when there is a load on the engine. Say the coolant fan kicks on, the ECU see the drop and gives more duty to the AIC allowing more air to recover the engine RPM. I think you might have a vacuum leak and the AIC is doing all it can to reduce the amount of incoming air. Have you cleaned the inside of the AIC? They do get gummed up and the valve sticks. Carb cleaner and triflow.
The AIC has been cleaned twice now. I think the issue is that the Throttle Body (TB) is the wrong one. The TB on an LH2.4 ECU has no throttle stop screw nor any means to adjust idle. The manual (FWIW) states the butterfly on the LH2.4 TB closes completely. The AIC does adapt when the fans come on, big drop in rpms, then recover. Vacuum gauge reads around 18Hg pretty constantly at idle.

I could test your theory by removing the AIC and blocking the ports on the TB and intake manifold ports. If it idles under no load the same, then you must be right air is getting in some other way. Thanks for the suggestion.
 
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