Bad Catalyst? Corrosion Strategies?

[Danh]

The car in question is a 2008 Mazda MX 5 (nee Miata). 26,000 miles, summer use only, stored winters in a garage in northern Wisconsin. This is a vacation home that isn’t visited from November to May.

Yesterday I noticed a rattling sound from beneath the car, sensitive not to road speed or load but more to harmonic vibrations at certain engine speeds. Car runs fine, no CELs. Put it up on ramps and noticed the catalyst is rusty (nothing new) but would also produce a very pronounced rattling sound when banged with a fist just downstream.

So two questions for the group:

1) This seems to me like a disintegrating catalytic converter despite the low mileage. This would be a first for me and is surprising, especially as the car is never short-tripped and is other wise well cared-for. Any other possible diagnoses? (FWIW, the catalyst is an integrated part of a rather elaborate assembly with no separate heat shield to come loose.)

2) The car is stored in a garage with a concrete floor. I appreciate water vapor can penetrate the concrete so have been parking the car on a large vinyl sheet. Nonetheless the catalyst was pretty well rusted. This part of the country does have extreme temperatures and it’s not uncommon to have rusty brake rotors in the spring. Any better strategies to avoid rust in the future?

Thanks. This is too bad: normally when you spend an amount with a comma in it you’d like your car to at least look different...

 

[kschachn]

It's the catalyst itself that is rattling and not a heat shield?

 

[Danh]

Think so. The rattle sounds internal and the heat shield itself seems secure.

 

[slacktide_bitog]

Are you near a Krown or other rustproofing shop? If so, doing that annually will stop the rust.

Rust will take down the cat after all those years. Sitting is the worst thing for a car to experience.

 

[Danh]

Are you near a Krown or other rustproofing shop? If so, doing that annually will stop the rust.

Rust will take down the cat after all those years. Sitting is the worst thing for a car to experience.

Would Krown or other rustproofing be OK to apply to a surface that gets as hot as a catalyst?

 

[slacktide_bitog]

Would Krown or other rustproofing be OK to apply to a surface that gets as hot as a catalyst?

The oil will burn off, and anyway, the real rust problem is at the flanges. That's where it breaks off. You might see smoke and smell it for a few days.
You could always skip the cat itself and spray around it. That will work too, as long as you spray the flanges and the immediate area before and after them.

 

[eljefino]

Sometimes, stuff happens. Cats are made of decent quality stainless to meet the long EPA warranties. They're one of the last things under the car to rot out. I don't see its internal failure as being a care problem. How's the rest of the car, does it have odd flash-rusting of parts that normally don't look all that bad? Hard to describe, but it's more under-hood stuff and less rocker panel/ suspension, and seen by me on cars parked outside.

As for the garage, how much rain do you get per year? Is the concrete pad at grade with, well, a swamp? Concrete is porous but it goes both ways, it can drain so long as more water doesn't fall on it and there's somewhere for it to go. Another problem is how it can be a cold-sink, causing condensation to form on it. Perhaps parking on some 4x4s will get the car 4 inches higher, which might be of slight benefit.

 

[partspro]

i think the issue is the actual catalyst material inside

 

[Driz]

In all the decades of old junkers in the rust belt I’ve never seen any of mine with a rotted cat. Almost all had the heat shield mount let go and rattle a lot. The one I did see had something break loose inside and rattle. That was an 03 Silverado with low miles but hammered to bits plowing snow. I just chopped a hole in the side scooped out the wrecked honeycomb and migged it back shut. Problem solved And still running great 3 years later.
I’d jack it up and look REAL CLOSE at those shield mounts , put a gloved hand on it ect while running and make sure it’s internal before replacing it which othwis is likely your only option.