Clogged Cats

[Marccus]

I did a search but still have some questions:

My '89 (302,000 mi) starts up "rough" and stalls when I'm at idle, otherwise it runs fine.

I do get vibration from the exhaust. The y-pipe is tightly attached but if I tug and pull on the tail pipe / muffler area, the "rattling" will stop. It appears to be coming from where the resonator pipe separates into the two muffler systems.

The pipe at this point (and/or other locations) seems to be just close enough to the heat shield to cause intermittant rattling.

I just thought this was caused by worn out exhaust pipe / muffler hangers, but maybe it's due to high backpressure in the exhaust due to carbon buildup on the cats.

Someone suggested clogged cats.

How do my cats get clogged with carbon?

If they are clogged with carbon, that means I'm running VERY rich and there is not enough O2 for complete combustion of the fuel.

There should be excess O2 to ensure complete combustion, and the products of combustion (CO2, CO, NOx, H2O, very small amount of unburned HC's) should not clog the cats with carbon.

I passed the smog test here in California last October, although I was at the limit of the HC and CO.

But I get no codes on O2 sensors, although it has been a LONG time since I replaced them.

In my industry we typically "decoke" cracking furnace tubes using steam/air, which has to be done VERY carefully or you'll melt through a tube (Inconel) because of localized hot spots. The carbon / O2 reactions is extremely exothermic and it's tricky to pull off the decoking so the tubes aren't damaged.

Can you "decoke" the cats to burn off the carbon?

If not, are my cats toast and do I need to replace them?

I have the PP high flow cats Y-pipe. If I ruined this I'll be ****** as it's too expensive to replace. :cuss:

Should I start with replacing the O2 sensors as a first step?

Perhaps this will correct the fuel / O2 ratio but will the carbon burn off?

I still get pretty good fuel mileage (22 mpg combined city and highway) and burn about 3 to 4 quarts of oil every 3,000 miles.

Any and all suggestions appreciated.

 

[rangerj]

I am not sure exactly what the chemical processes are that go on inside the cats, but I do know I have never seen one "clogged" with carbon. I have seen them melt down inside, and I am told that running a rich mixture for an extended time will cause this.

Take your car to a reputable exaust shop and have your exaust tested. They can tell if the amount of back pressure is excessive, and if the problem is in the cats or somewhere else in the exaust, say the muffler.

 

[Marccus]

Thanks chief.

I still have the original cat back system, having only replaced the Y-pipe with a PP hi-flow. I did have to replace the resonator by taking one off a junked SHO and "mending" it at the pipe right before it splits to the two mufflers. So there are a lot of areas where to look for vibration, backpressure, etc.