HELP - muffler causes backfire, bucking

Well, I've overposted but everytime I have a problem "fixed" it all comes back to the muffler. I have changed the precat 02 sensor fyi. My 89 4.0 aw4 seems to run great with the side blown out of the muffler. I change a few things, it runs great, I replace the muffler and it starts bucking and backfiring again. Which leads me back to the 02 sensor which I already changed. I also changed the air temperature sensor because it was clearly fried. I am thinking the map sensor, would this be off with a muffler installed? The thing runs great with no muffler, but as soon as I install one (I am on my fourth brand new one in as many months) it starts bucking and backfiring. It seems to be just as I am letting off the gas pedal or right around 2500 rpm which is 55 mph or right after a stop and I am just starting to accelerate. I cleaned the ground from the engine to the firewall and it ran great, getting almost 20 m.p.g. I ran like that for a week (no muffler because it blew up again) and did not want to hear the noise anymore so I put on a high flow muffler and bam! backfiring intermittently as well as bucking at cruising speeds. I am thinking map sensor as i've swapped most of the other sensors for new. any clues?

thanks for your time,
dave hoose
[email protected]

Do your muflers keep "blowing" out? Could you be dumping excess fuel in the exhaust system and then having it ignite?
Mine ran rough when the MAP vacuum line was not connected properly, black smoke, too rich and the cat and muffler got so hot that carpet started steaming. Connected the vacuum line back to where it should have been, which got the MAP working properly and have not had a prob. since.
You don't want to keep throwing parts at this but seems like the MAP is about the only thing left, can you borrow one just to check it out b-4 you go and by a new one ([email protected] $39@NAPA).

dizzymac said:

Do your muflers keep "blowing" out? Could you be dumping excess fuel in the exhaust system and then having it ignite?
Mine ran rough when the MAP vacuum line was not connected properly, black smoke, too rich and the cat and muffler got so hot that carpet started steaming. Connected the vacuum line back to where it should have been, which got the MAP working properly and have not had a prob. since.
You don't want to keep throwing parts at this but seems like the MAP is about the only thing left, can you borrow one just to check it out b-4 you go and by a new one ([email protected] $39@NAPA).

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Sounds like we are on the same page, I just could not understand why it would run so well without the muffler, but it must cause enough back pressure to mess up the map and cause the rich condition. I inspected the vacuum line yesterday and it's in great shape. According to the renix man. I have to find 4.5 to 5 volts between c and a connectors with key on, then termina b same volts, and .5 to 1.5volts warmed up - so I am trying this next. (at least I have a good multi-meter). thanks for your input, if it's bad at least not another $100 or more part!