Chevrolet Repair: low intake vacuum, intake manifold gasket, vacuum lines

Question
3.4L SFi Had lower intake manifold gasket replaced. Now intake manifold vac measures 5"Hg at hot idle. Cold start will not idle - surging with no throttle to 2000 RPM then dies out unless the throttle is bumped, then back to 2000, sometimes backfires if stalls out. Runs fine at high RPM.

No codes. Checked timing chain, TPS, IAC valve, IAT, ERG, MAF, MAP, all vacuum lines, compression.

What is another possible cause of low vacuum?

Answer
Hi Scott... is it possable that you have a gasket leak...was that the reason you did lower intake...do you check intake for warpage. low vac would tell me you have a leak. take a good air intake cleaner and spray around the intake when running and see if RPM bumps while spraying.. if it does you have a leak. if all the other componants are ok, that is where i would start looking. Hope this helps