'92 Lumina z34 idle problems

My brother has a '92 Lumina z34 and it wont idle at normal speed. It sits at about 2000 rpms and won't go any lower regardless of what he does. He recently replaced the spark plugs and wires so while he had the intake plenum off to do that he replaced all vacuum lines that he could access, so we don't think it's the vacuum.

Any suggestions as to what it might be? i know thats not a lot of info to go by but im an import guy so i don't know a whole lot about his car.

it's got the 5spd manual tranny with the 3.4L DOHC engine.

Check all the vacuum lines again. Does the service engine light come on? If it does get a paper clip, Under the dash on the drivers side there is a black connector with I think 12 holes. Thats the diagnostic port. Bend the paper clip into a U shape and plug it into the TWO TOP RIGHT plugs. Turn the key to ON but dont start the engine. Watch the ses light, it will begin to flash any codes stored in the ECM. IT will flash a code 12 three times. Flash... flash flash = 12. That means its in diagnostic mode. Then it will flash any other codes it has stored. It will flash each code three times. Write them down, if he has a haynes manual you can look in there to see what they mean. Or you can post them hear and I can look em up for you.

thanks for the info... he's going to try the diagnostics tomorrow... and he's got the haynes manual, so he can read the codes.