Speedy Auto > Auto FAQ > Cars > Saab

Saab: 1990 SAAB 900S turbo exhaust idea, 1990 saab 900s, vacuum leak


Question
QUESTION: I have a 900 covertible with the 5 speed manual tranny. My idle seems to raise and lower. This occurs with cold and hot engine. The idle sits at around 900RPM, then it jumps to about 1500, and returns back down. It seems the car wants to idle smooth, but something is sending a signal causing it to run high. My A/C and all switches are turned off. It just sits there and does it. Clutch engaged at stop light, same issue. I have checked all hoses, and cannot determine the problem. Any help is appreciated!
AZPITBULL

ANSWER: AZ...:

  It sounds like either a "stuck" Automatic Idle Control (AIC) valve or a vacuum leak somewhere. Since you noted that you checked hoses, consider the former.


         Hope this helps,

         Jerry

---------- FOLLOW-UP ----------

QUESTION: I also have a question about the exhaust system. I am about to buy a new BOV, along with a 3in downpipe. Once I weld the 2 oxy sensors and hook it to a new exhaust&cat I should be a bit happier with response and performance. My question is this; (I am replacing the full exhaust system cause its worn out) Do I have to stick with the 2.5 inch cat exhaust?, or can I go up to a full 3 inch to match the downpipe? Also what size would you recommend for a muffler? single/dual? what size?
thank you sir.

Answer
Before buying a new AIC, I would try "freeing it up" as follows:  Since the valve action is due to the internal rotation of a cylinder, try soaking it in carb cleaner and rotating the inner cylinder with a pointed object, taking care  not to deform the aluminum. Also, if you have a variable voltage source, applying approx. 7.0 volts to the valve will center it..you can observe the rotation as  you vary the voltage.

 An easy way to check if the AIC is at fault is to pinch or block  one of the hoses going to it, since it functions as a variable air bypass device.

 As to the exhaust question, I am afraid that I really don't have a good answer...I have only dealt with the 'stock' systems. I do understand that exhaust systems are designed with a certain back pressure effect..which would be lessened with a bigger bore.What effect that would have overall? I do not know.

         Jerry