Truck Repair: engine idle, intake manifold gasket, vacuum leak

Question
I have a 1990 Chevy Cheyene truck with a 4.3 litre engine. It runs fast at idle and when I put it in gear it runs very rough. It sounds terible. As though valves were chattering and it has no horsepower. When idling it does not clatter. I was told it is going into "limp mode" and I'll probably need to replace a sensor. Most likely the oxygen sensor. However I was always lead to believe I could locate it on the Y-pipe. It's not there. Your opinion on the poor performance and to tell me where to find the sensor..Thank You

Answer
Hi Roger,
My opinion is you have a vacuum leak.
EGR valve sticking, broken hose, or leaking brake booster. Maybe intake manifold gasket.
The sensor could be anywhere from the exhaust manifold, to the catalytic converter.
Check for the vacuum leak first.
Sensors are not used, and limp mode doesn't happen till after the engine is warmed up to operating temp.Van
Van