I've replaced the exhaust down pipe because I was told by two reputable exhaust shops that there was a hole in the pipe underneath the heat shield. I got the pipe of of a long standing internet auction site. The shops also told me that they had no source to order the pipe.and when confronted with " I have the pipe coming" said " sorry we don't install customers parts, too much trouble they never fit" . So I borrowed a friend's garage and put it in myself. Fit perfically.. When attaching the down pipe to the middle pipe found that I didn't have to replace it after all. The rear rubber hangers had perished. The pipe had sagged pulling the joint apart. Once the rear pipe was back in place, all is good. So much for expert professional investigation. Now for my new problem/ question. Driving home from the shop the engine would hunt on deceleration. Drop a couple hundred rpm, then back up. Idles normally, 8 or 9 hundred in drive no CEL. Nothing changed other than the pipe. O2 censors reused in the same positions, clipped to original wires. Is it possible that the mid cats are plugged? Which after the hangers failed pushed the mid pipe off of the down pipe? Or something else? I did have to jump start it, but it was hunting 10 miles down the road with the engine warm turning in to the yard. Idled normally at shut off? Sorry for the long ramble. Felt you might need background of the problem.

 

Could you have connected the O2 sensors to the wrong sockets, ie transposed them?

 

Agree with @countyjag . That's first thing I'd check.

Use OBD to check the STFT readings. If O2 sensors are the wrong way around then STFT will be MAX +ve on one bank and MAX -ve on the other (Max is about 24% I think).

 

O2 sensors placed in the original positions. Connected to the original sockets.

 

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Thank you everyone for your input. I took her for a thirty mile drive today. Seems to have cured itself. Go figure. Fickle kitty!