Vaccum hose diagram for 97 chevy tahoe 5.7 liter

You may have carbon plugging the passages on the intake side of the valve. Remove the valve and start the engine. It should die or run very rough. if it idles normal then you have blockage that needs to be cleaned out.

could be a vacuum leak at a hose or gasket. with the engine running spray a little carb spray at the intake manifold area. if the idle picks up you are close to the vacuum leak. dont spray on the exhaust manifold area, be careful

If your vehicle has one, and I think it does, check your EGR system. If the egr valve is full of carbon, it can be partially open, creating a vacuum leak. If the egr valve solenoid is bad, it could be opening the valve at the wrong time, also creating a vacuum leak. Unplug the egr vacuum hose, plug the open end of the hose and start it and see what it does. A picture of the egr is below.

had a similar hard-running problem with a 96 ES 3.8 V6; pulled the front valve cover and rocker-arm tube over #5 had split and no longer controlled the i/o valves. had to replace and re-machine #5 valves in the head.

Check the PCV valve and the PCV hose where the elbow is near the TB. It may be stuck open and the hose may have a crack, Squeeze it while engine is running. Also are you sure its not brake related. The check valve on the booster can fail. Does the RPM change when you apply just the brake ??
Also spray some brake cleaner around the base of the TB looking for a RPM increase. Spray around any vacuum source.

Have you checked for vaccume leaks?
You've got lots of parts in there, Has the problem been the reason you have replaced all of those, or did it come up while doing some of the repairs?

This is a common complaint on these cars. I own a 96 myself.
Assuming that you engine is fine and runs smoothly under normal operation, and just is rough "feeling" at idle while stopped; then I would suggest changing the motor mounts...the mounts may not be broken, but the rubber isolators get worn and soft, and transffer normal engine vibration into the chasis of the vehicle. Mine was doing the same after changing the mounts good as new again.
Please note...don't use cheap mounts, by the better grade/quality mounts.

Btw, I’m available to help over the phone in case u need at https://www.6ya.com/expert/sean_54c3a3dc48a7773c

Do a cooling system pressure check for a blown head gasket.

it is possible.

Check if fuel or spark problem,..all engines need both to run...use starting fluid in air intake ,..if it runs very shortly-fuel pmp?,,ck for spark at spark plug,work your way back..also ck timing belt-intact? Kev-MN