92 CV 4.6L - loss of milage after exhaust work

I am the owner of that 92 Crown Vic. After I replaced the oil filter mounting block gasket, which took care of the external oils leaks, I was driving the car up to a funeral in PA. The car got its usual great
highway milage - between 20 and 23 MPG, depending on driving conditions, etc. When I got up to PA, I got stuck in traffic on I-81 around SCranton while on the way to a funeral home and decided to take a detour through town. I headed into dwontown Scranton and hit a pothole and the car went from being a quiet sedan to sounding like a NASCAR racer. WHen I was able to stop and take a look, the Y-pipe cracked right in front of the muffler.
I took it to a muffler shop the next day, and they fixed it right up. They used a torch to melt the rusted old bolts off the clamps, then had to expand the pipe at the muffler to accomodate the new pipe. Then the mechanic went at the flanges behind each of the convertors with a ball peen hamer to clean off the old gaskets - which made me grimmace because I thought that might not have been good for the cat or the O2 sensor. Then they hammered at the back of the muffler to get the Y-pipe to fit into the pipe at the front of the muffler. The car was quiet again, but I noticed over the rest of the trip that the milage had gone down - now that I am home again, the milage is under 18-19 MPG. Now I did put on a new PCV valve and wasn't able to check the larger of the two vaccuum lines from the PCV valve to what I presume is the vaccuum manifold to see if it was clogged. I haven't driven enough miles to see if the oil has leaked out.
But I wonder why my MPG has gone down since getting the new pipe installed? Could the hammering have messed up the original cat - perhaps damaging the mesh in the cat body? Or did running the car with no muffler for over a day mess up the O2 sensors?
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USENET READER wrote:

When the drivers side cat on 'my '92 Grand Marquis went last year at about 190k miles, it suddenly started making rattling sounds at certain rpm ranges, usually just over idle. The matrix must have cracked and dislodged itself. I didn't drive it noisy long enough to ascertain any difference in mileage, but I had just made a 1500 mile trip and experienced normal fuel consumption. Mileage on the way back with the new convertor was the same as before. The car is equipped with factory duals. Stan K.
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