The delayed acceleration started after traveling to Maryland from Michigan for the holiday. I'm going to start from the beginning so this may take a while.

For reference:
0172: System too rich ON 2.
0302: Rear rotor misfire detected.
2096: Target A/F feedback system too rich ON 2.
Reset ECU: Both pedal pump and hold trip odo until 'test'.
Relearn procedure: All loads at all RPMs so ECU can relearn fuel trim. Search for it.
Power estimates are based on equilibrium (power=drag) speed assuming a 0.3 drag coefficient, and height*width frontal area. (~0.017 lbs/(ft/s)^2)

'05 GT 6MT ~38000 miles.

Bought car 10/28 as is; everything seemed okay on <30m test drives.

Beeping and blinking CEL (misfire I believe) a week or so later after high RPM pulls. CEL went away; didn't pay it much mind.

Likely unrelated: 10 days later, trans locked up in 6th gear. Drivable in 1-5 but bad noises and metal-laden fluid. Replaced trans <~12/1.

Beginning of December: Notice high RPM/low load misfires. CELs 0302, 2096. Reset ECU (pedal pump & hold down trip switch). Clear codes. Go through relearn procedure. Power is okay.

A few days later: CEL returns, can hear misfires. Clunking noise in powertrain at high RPM, low load. Broken apex seal or ruined cat came to mind. RPMs limited to ~7000. Engine sounds nasty. Read codes 0302, 2096, 0172. Freeze frame on highway, TPS ~20%. Ordered Coils from Advance. Cleared codes, reset ECU, did relearn procedure. RPMs not limited after restart. Power is okay. Old coils appear to be originals.

Note: ~16MPG average so far. ~20% city medium acceleration, cruise at ~50. ~80% highway, cruise control at 70mph.

The next day: Receive and install coils. Car runs bad but not terrible. Reset ECU and do relearn procedure. Car runs well, only slightly better than before. Power is okay. Very few audible misfires at high RPM. No CELs.

~12/15: CEL returns: 0302, 2096. Freeze frame again at normal driving (~60mph, ~20% TPS). Car runs fine. Clean MAF. Reset ECU, do relearn procedure.

~12/23: Travel to MD. 525 miles. First time the car has gone more than ~50 miles continuously. 74 mph cruise control 100% highway no traffic. Windows up, AC off. Brand name 93 octane. 19.5-20.5mpg. Car starts missing near the end of the trip. No beeping. CEL still on from earlier.

~12/27: Install new plugs and NGK wires. Idle started oscillating at ~1hz from 800 (smooth) to ~ 500 (rough) right before changing plugs and after a 40 mile trip. Rotor 1 plugs corroded at the wire end. Plugs look original and old. No oil or white patina on the tips. Reset ECU, relearn procedure. Idle is fine. Power is okay but seems to have gradually decreased from when car was first received, revs a bit smoother. No beeping. CEL still on from earlier (can't clear codes in MD).

~12/30: Low RPM, low TPS power starting to decrease. Add Lucas UCL to tank.

~12/31: Low RPM, low load (low TPS), power intermittently gone (est << 50HP during these periods.). Car accelerates normally >6000 RPM. Car accelerates normally if pedal is held open (>50% TPS) for 0-5 seconds. RPMs at power increase are not consistent. Cat is glowing after 1hr of city driving.

Note: The cat may have been glowing before this. I looked but did not realize there was a heat shield everywhere except the rear O2.

Note 2: Deliberately burn fuel to dilute Lucas UCL (if that was the problem). Problem persists after original UCL/gas mix is <25% of tank.

Today: Travel back to MI and make the following observations: 77mph cruise control (speed oscillates between 74-80mph when power is lost and cruise control has an underdamped response). 15-17 mpg. Mileage makes sense relative to previous trip. Car is near WOT for >~75% of the trip. Behavior: Car looses power at set speed when load is light. Cruise control holds throttle at WOT to regain speed. Full power kicks in after 5-10s and speed overshoots and settles at set speed. Rinse and repeat after engine is lightly loaded again.

Equilibrium speed (WOT, zero acceleration or deceleration) varies between 65-75 mph (6th gear) which corresponds to ~27 hp@3250rpm to ~40 hp @ 3750rpm. Based on some stock dyno graphs, these WHP numbers would suggest about half the engine is not functioning. This may mean one rotor is doing nothing or both may be not working fully, I don't know.

Power is reduced to the aforementioned after any period of low or light load (downhill, level ground, lift throttle). Power is okay if medium to high load is maintained. Power is okay after 7000RPM no matter what. Power is okay after holding throttle open for 5-10 seconds at any RPM in any gear.

Note: WHP is estimated at 180-200 @ 8500RPM based on longitudinal acceleration, speed, and aero drag. Checked in both 2nd and 3rd gear.

Note2: HP in crap-mode is better in colder weather (50-30° F MD-MI) and after letting car sit but not cool down.

Later today: Pulled codes 0172,2096,0302.

My thoughts:

Crap coil: Power is fine at any RPM as long as the engine is loaded.

Injector/Fuel system clog: Car is fine >7000RPM where this would show.

Wrecked cat: It probably is, but the problems moved up the powerband (not down as one would expect with a flow restriction) and don't exist at high RPM or high load. No noticeably bad smell in cabin.

Bad fuel: Consistent behavior over multiple tanks, brands, states.

Intake valve: no code, can achieve full power at any RPM.

Note: I have talked to a dealer and it looks like they are going to try to diagnose (steal my money) before looking at the cat under warranty so I would like to try whatever possible before giving it to them.

Your thoughts are appreciated. I'm trying to solve this ASAP as this is my DD and I would rather not wreck an engine.


Keywords: low power, surging power, glowing cat, p0302, p0172, p2096.

 

too much to read right now but from just reading the title - you need a new cat .

 

"Note: I have talked to a dealer and it looks like they are going to try to diagnose (steal my money) before looking at the cat under warranty so I would like to try whatever possible before giving it to them."

I do think it is bad but not sure it is causing the current problem. High RPM/high load operation is fine.

The 'gist':

Car operates fine at >7000 RPM. Car operates fine under medium and heavy load at any RPM in any gear. Power is halved or worse at 0-7000RPM at light or no load. It takes 5-10 seconds before the power comes back. Full power is achievable at any RPM in any gear but massively delayed if the starting RPM is <7000.

 

Update: The problem gets more noticeable as the car warms up. The car feels fine for the first 30 seconds or so after starting it cold.

 

Update: Disconnecting front O2 forces car into open loop mode where it is okay, although probably running a little rich. Dealer says CAT internals have no sign of wear.

 

What scan tool are you using to pull codes? Can you get the Mode 6 data on the cat?

 

I just borrowed one from O Reilly's. I am getting a cable/software to hook up to my laptop. What do you advise I look for with respect to the CAT? They said it looked great but it still worries me.

 

If your cat is glowing, it is bad.

 

You need a new cat. I had the same problem, delayed acceleration and the car died. Sat on the side of the road for about 15min, started back up no problem. The mazda dealer couldnt find anything wrong with it, no CEL no lights or anything. I took chance and bought a resonator race pipe with the cat on it (about $325) from Racing beat and it solved the problem. I strongly recommend that. If you have the racing beat exhaust to go with it (which i do) its 10x better. Diagnosing a bad cat is next to impossible. but if your car has bad accelartion after running for awhile, it the cat. replace it and have it installed by mazda, bout an hour worth of labor