I did a 2RZ swap in my 1st Gen a few years ago and never thought it ran as good as it should. I recently took a job teaching at a local college and asked the Auto Tech guys to look at it. They went over everything and found 7-8psi back pressure in the exhaust. Ii had a Flowmaster that has been on the truck since I bought it many years ago, so off to the muffler shop to get a new Series 40 installed. WOW!!!! It's like 50hp more, well it runs as I always hope it would now. Not a common issue with after market mufflers but definitely the problem.

 

So what exactly was the problem? Clogged muffler? Heard of clogged CAT but not muffler before.

Flowmaster a don't exactly flow that well, they're chambered. Something like a turbo muffler is straight through design.

 

Well the muffler shop didn't cut it up and I didn't really care. The muffler was 10yrs old at least. They are triple chamber design but still should have little flow resistance according to the school as well as the muffler shop. It was only the 3rd time in 20yrs the muffler shop had seen an issue like that, yes cats fall apart and clog the exhaust. I guess this could have happened before I bought the truck and I just never knew the muffler had issues.