1991 Chevy Caprice open loop and closed loop?

[quote:1a67d63587="diablovox"]Computer problem
1991 Chevy Caprice V8 Two Wheel Drive Automatic 215000 miles
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i changed the muffler on my car and it started giving me bad mileage.. my mechanic hooked up something to the car comp. and he says that the car starts running on an "open loop" after running on a "closed loop" because of a backpressure problem and i need to get the original muffler back.. is he right?[/quote:1a67d63587]

When you say "i changed the muffler" do you mean that you replaced the muffler? If so, did you buy a recommended replacement part? Sounds like your mechanic is saying that you have installed the wrong type of muffler. You don't necessarily need the old one back, but you will need a suitable replacement.