Piston Slap: The catalyst for catalytic converter failure?
Douglas writes:
My 1997 Porsche 911 check engine light has come on, apparently due to the catalytic converter being clogged. The car has 34,000 miles on it. Is it normal for this to occur with such low mileage? My mechanic cleaned it once with chemicals but that only lasted a few months and then check engine light turned back on. Any cures other than replacing the catalytic converter?
Sajeev answers:
It’s been a long time since catalytic converters were known for such failures at low mileage. I recall this happened when Malaise Era (1973–83) machinery was new, as technology has improved tremendously since the bad old days of two-way, pancake-style designs, and early three-way converters. Something else is causing the problem.
There are usually three reasons why catalytic converters fail: the converter either overheated/melted, is fouled/blocked up, or suffered some sort of internal/external structural damage. The latter is unlikely, unless you see signs of physical damage or corrosion around the exhaust system. Maybe they are damaged because someone gutted them in the past? Seems unlikely, but it can happen if you don’t know a vehicle’s service history after rolling off the showroom.
This intricate pattern of pollution-reducing materials could be fouled/blocked up by oil, coolant, or carbon soot. Or it can melt from a series of engine issues. Before getting a new set of converters for your Porsche, I’d address the following potential issues:
- Poor exhaust repair in the past, or aftermarket exhaust parts installed poorly
- Coolant leaks (bad gaskets that might let coolant into the combustion chamber)
- Oil consumption
- Ignition Misfiring
- Low compression
- Improper Air/Fuel Ratio (from weak spark, defective O2 sensors, or even failing fuel injectors)
While valve guide issues aren’t uncommon for 993s, I am not gonna even try to arm-chair these concerns. If yours didn’t come with a service history, this is the time to check everything and catch up on all maintenance needs, as per the owner’s manual. Once you check everything above, remove the catalytic converters off the car for a proper cleaning (not additives poured into a gas tank). With any luck, the problem will be solved. If not, and if your repair was obviously needed (i.e. like bad valve guides), feel confident in your repair and order a new set of converters.
But what if you didn’t find conclusive proof of some engine/exhaust issue causing the problem? I would get a second opinion from another repair shop, because something isn’t adding up here.
Have a question you’d like answered on Piston Slap? Send your queries to pistonslap@hagerty.com, give us as much detail as possible so we can help! If you need an expedited resolution, make a post on the Hagerty Community!
