First off, I have a SD 305 TPI, basically stock. The idle is ok when the car is first started. Almost immediately, the idle will fluctuate. It will almost die and then catch itself. When it does die, it sounds like a sucking noise coming from the air filter (K&N on the TB). I disconnected the IAC and the idle was high but smooth. I tried to set the minimum air but the car wouldn't even run during the test.

My fuel pressure tested above 40 and does not change when the stumble occurs. I have done a tuneup to rule anything out. There are no codes. I'm not getting a misfire.

Do the above symptoms point to a bad IAC valve? Is there anything else I should check before replacing it?

 

you could have a vaccume leak somewhere. I had a big split in one of the hoses gonig to the TB and it did that . Wouldn't stay running without some pedal

 

is this a MAF car? if so.... you say that the filter is right on the throttle body..... where is the sensor? if its MAP sensor like it should be, in 1991 then yes, your probably experiencing either a bad idle air controler, or a vaccum leak problem.

 

He said its a SD 305

 

Thanks for the responses. I checked for a vacuum leak but couldn't find one. I took off the TB and cleaned it well, and replaced the IAC last night. After this it ran great. Smooth idle and good pickup.

But today it is running worse. It won't idle for more than a few seconds. When it stalls there is smoke coming from the TB. I took the air filter off and I can see it's coming from the plenum. What could cause this?