I've seen people blame a million things on "definitely" plugged catalytic converters and I've checked a few. None were ever at all plugged up until this car's.
There was always something funky about my car's exhaust - the individual exhaust pulses were VERY intense and staccatto. Most cars, and now mine, are sort of gentle puffs that you could cover up with your hand for a second. Until today, they were incredibly "pulsey" and turbulent. Each one would just about blow your hand off the exhaust pipe. That super intense and pulsed characteristic of the exhaust is something I've never seen mentioned when it comes to plugged cats. With the new cat, that weirdness is gone and it feels like any other car's exhaust pulses. Power and noise are both better too.
Here's the story:
My car's exhaust has been annoyingly buzzy/droney and I was trying to get it back the quiet mellow factory sound as well as maximize the mpg. I've been getting 19.5mpg on my 6ish mile commute (3 on the freeway) to and from work in fairly cold temps. I took it out on one long highway trip and got 25mpg - probably partly due to the locking torque converter in the AW70L and due to mainly staying between 50 and 70 mph except for crawling along at about 15mph for about 20 miles in the back woods. I also replaced the hose for the warm air "quick warmup" and re-enabled the airbox flapper for winter (I removed a screw I had holding the flapper against the hot air inlet side.
Along the way, I found it had a Walker Quietflow3 (walker #22430) muffler. It has a smaller volume and smaller inlet and outlet than stock - plus it had a tip that necked the outlet down to about 1.75 inches. I cut the tip off, then I replaced the muffler with one from my GLE project car (it's getting a rebuit b230ft and I have an exhaust for it), and then replaced my aged Regina Titania (Walker oxygen sensor at 227k miles. None of those things made the exhaust pulses feel normal, and I am planning to check on the valve clearances (they may be negative at this point).
When I replaced the O2 sensor, I stuck my borescope in the O2 sensor port just to check. I didn't expect to see anything unusual and I was very surprised. Out of a diameter of about 4 inches, only the center 1-1.5 inches was open. The rest (about 80%) was plugged with carbon. I happen to know this car had been driven for a LONG time with the MAP sensor disconnected due to a broken 90-degree soft elbow breaking but looking fine. Also the oil/flame trap had a leaky hose at the oil trap. I replaced that with about an 18 inch long length of 5/8 ID hydraulic hose over 6 months ago and it still looks great. Hopefully it was just the bad mixture forever because I can see from old emissions tests that the CO and HC values increased over time. One of the last straws for me to get a new O2 sensor was my CO detector in my garage going off *all day long* with my garage door shut from me just starting the car and driving away a few mornings.
Today, I had a shop install a WALKER 15037 catalytic converter - which is listed as the right one for my car on Walker's website. I got it from Rockauto for about $65, which is amazing for a 10-inch rectangular cross-section cat. It had a garbage "mini" cat that was only about 4 inches long worth of catalyst material.
The oxygen sensor I used is the Walker 35033015 - https://www.rockauto.com/en/moreinfo.php?pk=11204549&cc=1288425&pt=5132&jsn=11. There is another one that says "25023811 OEM Based" that I avoided. The one I used has been great from the beginning with no issues and immediately improved power and throttle response (but not mpg).
Anyway, that's my story and my notes on plugged cats. A symptom may be that the exhaust pulses are very individual and the force is very high for each pulse.
Happy bricking and almost happy new year!
Will
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