2001 pontiac montana

The montana has an issue, restrictions in rpms, at 1500, 2000 and 3000 rpms. I can crank it and it may only rev. to 1500 rpms with terrible miss past the 1500. That number may be the 2k or 3k. At times it would run great for about 2 weeks, then the rpm restrictions again. I had the converter taken off and pipe put on, ran great for about 1 1/2 weeks, then same thing. Replaced Throtle Position sensor, ran great for about 2 weeks. Then the restricitions again. If I try to rev past the restricion, the engine sounds and feels like it has jumped time and starts to backfire. Idle down and it is running smooth again. Only code was 0420. :runaround:

If you have no converter, a P0420 will set.......

No power, could be a blockage in the rest of the exhaust......when it won't rev past 1500, remove the second O2 sensor, creating an exhaust leak, and see if the car now revs beyond 1500 rpm's.....if it does, something may be moving in the exhaust, blocking it....