Trying to figure out best course of action here. Sounds like could be a transmission AND a cat problem, but obviously trying to be smart about how much $ to put into an 18 year old car.

2006 Mazda Tribute 3.0L V6, 141,000 miles

Here are the symptoms:
(No codes at all)
Initially, accelerating from stop, 1st gear seemed to slightly slip until it got close to shifting to 2nd, unless accelerating slowly. Drove fine once got into 2nd gear.

Took it to the transmission shop (which rebuilt transmission 4 years/24,100 miles ago) - says fluid seemed burnt (but fluid level was full). They said it could be just the clutches, could be a whole new transmission. They also noted rotten eggs smell and said there may also be a problem with cat. At this point hadn’t noticed any problems with power loss.

Week or 2 later, had to drive up into the mountains and on a big hill with the accelerator floored the engine would only get to ~3000 rpm and could only sustain 45mph (big straight hill, speed limit was 65). Highway driving was ok, but acceleration from a stop it has very little power to get going.

Took it to a recommended exhaust shop - they were skeptical about it being the cat with no codes present. They raised it and knocked on the rear cat, said it wasn’t plugged up. They didn’t impress me so I’m not convinced of their opinion.

Only other thing I’ve noticed is when accelerating from a stop, if I don’t accelerate at a grandma pace, as the engine gets close to shifting to 2nd, there’s a surge in RPM (~1000 more) before it comes back down and shifts into 2nd.

I’ve got a pressure gauge showing up today to check back pressure at the exhaust manifolds. With the shifting anomalies and power loss I’m aware I could be looking at new cats and a new transmission, but obviously if there are any other possibilities that aren’t $5,000+ of work I want to check that first. After reading through other posts my thoughts have been to check for a vac leak (but would that cause something this severe?), check/clean MAF, and change out the spark plugs. Does anyone have any advice/suggestions on things to check, or an order of repairs to try? I have also recorded OBD data for the last few drives as well if any of that would help…

Sorry for the long initial post, just trying to give all the info I can! Thanks for any help!

 

Some extra info in case anyone is able to suggest any possible solutions:

Smoke tested the vac system and no leaks found.
Backpressure tested all 3 cats and they all seem completely fine.
Tried flooring the car from a stop today and still having the no power issue, this also caused the O/D Off light to flash on. Stayed on for the remainder of the drive, but did not come back on when restarting the car. No codes were thrown.