Chevrolet Repair: Rough idle engine and rough perforemance after 80km speed, engine top overhaul, mass air flow sensor

Question
Dear Van,
I have 97 model 3.1engine(V6) cheverolet lumina(sedan). More than 5 months back the engine started to become little rough idle. I checked with my mechanic, he said it is normal the engine is getting old. My car has done 100,000KM only. But recently the rough idle increased and engine is also starts shaking. While driving above 80Km speed the engine starts shaking and becomes rough. But if you give full pedal throttle the shaking again stops and when it reaches the thorttled speed again it starts shaking. I changed the following parts after advise of my mechanic:
Mass Air flow sensor, plug wires, plugs,1no. faulty fuel injector, air filter,cleaned fuel filter and also used fuel system cleaner/injector cleaner treatments.
After all above works again i tested the car  on computer car testing centre and this time it showed:       high HC Propane(ppm vol):2218

The Car Testing Centre Technician infomed me that the engine is getting extra air.
I took the car to the mechanic he checked all the inlet manifold pipes for any leakage or damages nothing found.
The mechanic is suggesting that the rough idle is due to inproper operation/wear of the valves and their seats. And he is advising to do engine top overhaul.

You are kindly requested to advise regarding above situation. Thanks!

Answer
Hello Baber,
I have a 95 Olds with the same engine, and I had a problem that sounded exactly like you describe. Violent shudder, actually if it shifted to OD under moderate throttle.
My problem fit the valve scenareo. It had a broken spring on one valve.
A compression test narowed it down...using the eratic guage readings on only one cylinder.
I replaced the spring without removing the head, and it has run great ever since...62,000 miles ago.

You can do the complete top overhaul....or locate the bad one and see if the spring is broken, and go from there. But do a compression test first.

Van