To follow up on some of the comments: the title should be “poor acceleration when warm” instead of “…hot”
The check engine light came on when I unplugged each of the O2 sensors. I took the car to a car parts retailer to check the codes; the code indicated an issue with the O2 sensor. I disconnected the battery, check engine light went off and hasn’t come back on.
For clarity, prior to disconnecting the O2 sensors the check engine light was not on, but I had been experiencing the poor acceleration issue for several weeks.
The acceleration issue is most noticeable when accelerating from a stop after having driven at ~70 mph for 20 minutes. I take off from the stop and shift through the gears, and when initially shifting into 3rd it feels like I’ve hit the brakes. Performance is poor in all gears but is more conspicuous in the higher gears.
I also notice a bit of a front-to-back rocking feeling particularly on the highway (although I notice it at lower speed, most obvious when ascending a hill) as if I’m very subtly changing the amount of pressure I’m putting on the gas pedal. At low speed I notice the exhaust note change slightly corresponding with the rocking feeling.
When I first purchased the car I drove it about 200 miles. I didn’t notice the acceleration issue but I think that may have had something to do with how gingerly I pressed the gas pedal for fear of the horrible noise that would come from the huge hole in the side of the muffler. However, I have the impression that whatever the issue is, it’s gotten worse.
On this particular car the manifold has a catalytic converter and there’s an O2 sensor on the mk
manifold before and after the cat. The exhaust also has a cat inline which a mechanic told me could be removed and straight piped.
One last piece of information which could be completely unrelated, but for the sake of completeness… a previous owner replaced the stock 6 speed transmission with a 5 speed. Why this was done has baffled many but the mechanic who replaced the manifold assured me it looked like it was done professionally. Perhaps a side effect of the transmission swap is that my cruise control no longer works.