03 mazda mpv
 

Can use some help in diagnosing my issue, recently did the alternator & right front ball joint on my minivan. It sat for a year in my garage before I got around to doing this pains taking job. Once I finished this I went to get new tires for it put some high test in it along with fuel injection cleaner.
After I ran for a while it began to run really rough with no power & back firing. Going up a hill it just puttered along, barely making it up the incline.
Was getting late somewhat dark I looked underneath & noticed that my flex pipes were red hot & burn't out now, exhaust is really loud.
There is no check engine light on I brought to local shop & he put diagnostic machine on & got no errors. He said he can't do anything without any readings to tell him what is going on.
I was going to have the flex pipes replaced but he recommended not doing so until the backfiring is taken care of. Any idea what caused fire to come out where the flex pipes used to be.
Thanks in advance,

That is plain crazy! To torch your exhaust system as you describe would need an exhaust valve burnt or stuck open, with an injector pouring gas into the engine. I just can't imagine how many diagnostic codes that ought to have thrown. In any case there's absolutely no question that the rear O2 sensor ought to have put the check engine light on, both because of raw flaming gas going into the exhaust, and after the burnout, because of the amount of air leaking into the burned out exhaust duct(s).
You could start with looking at the spark plug condition and color, then run a static and dynamic compression check. I think your troubles are sufficiently serious that you should forget about your local mechanic and head for a well equipped recommended Mazda dealer. Your guy is dead right about not fixing the exhaust plumbing until you've find out why all this happened and get it fixed.

Thanks for the response to my issue with raw gas coming out of exhaust where flex pipes used to be. I was hoping to avoid going to dealer but it looks as though I'm @ their mercy.
regards,
mazdampvgreen