I have a 96 F250 w/a 351w, I can hear a buffering or low wizzing sound coming from the smog pump…(I know its from the pump cuz I took the hose off and could really hear it). In addition, I notice that even after the truck is warmed up I push in the clutch and the rpm’s will stay at about 1100-1200 then slowly go back down to 900ish. But if I just let start it and it runs it will sit around 900 even after I rev it up…? I am wondering, A. is the smog pump supposed to make that sound, ( I can hear it in the cab when driving and initially thought is was a small exhaust leak), and B. could there be any relation between the smog pump and the odd idle? Thank you for you knowledge in advance!!
The smog pump has a muffler. So if the smog pump makes any noise the muffler is the first thing to check. But the smog pump also has check valves that allow air to only flow in one direction into the exhaust system from the pump, but prevents exhaust gasses to flow back into the smog pump. If either of the pump check valves are defective and is allowing exhaust gasses to pulse back into the smog pump, the smog pump can become noisy.