Ok I'm a automotive technician going on 35 yrs. I've got a 03 monte carlo with the 3400 car only has 41 thousand miles on it and it is a one owner car. Problem is car runs 3grand at idle now with saying that I've checked and I've blocked off every external vacuum there is now here is the kicker. When I first started checking the car oh also no codes and ecm reads perfectly. Back to checking car at first I found pvc valve stuck closed so I replaced it with a new one no Change removed it along with all other vacuum lines and plugged it and all then I started car now only hose I did not unplug is the vent that comes off the back valve cover and goes to the intake boot. I used old pcv valve on front valve cover and engine still runs 3thousand runs at idle. Also I pulled the vent tube from the intake boot and the engine is pulling internal vacuum. Anyone have any suggestions?

 

It's possible there's an intake gasket leak

 

TB is for sure 100% closed? Have you checked out the IAC?

I'd probably block off the air inlet completely then use some smoke to try to figure out where the air is going in. Something else has to be pulling a good bit of air (assuming TB is closed and pcm has shut the IAC all the way to try to compensate) so you should see smoke getting pulled in.

Does the crank case vacuum seem abnormally high? There will likely be a little due to PCV, even at idle - but if theres serious suction in there, then that likely points towards your problem. I wonder if something like the LIM to head gasket failed (the GM 60 degree engines had some intake gasket problems through the years) and the engine is sucking from the valley. Its just a bit confusing as you say the PCM reads fine- as you'd think fuel trims would be strongly positive given all of that air would be coming in post MAF.

Thats why I questioned the IAC / TB closure since it would almost have to be coming through the MAF since you said the ECM reads perfectly (which I assume includes fuel trims).