Catalytic Converter Question

The previous owner of my '93 Jeep Grand Cherokee had the catalytic converter replaced about a year ago. Consequently, I don’t know who did the work.

Recently the car started to sound like the muffler had fallen off, so I took a look underneath. There is a hole in the cat towards the rear on the driver’s side. The hole is about 3/4" (maybe smaller), threaded, and is spewing exhaust and noise. I assume there used to be a bolt in the hole.

So I have a couple of questions…

What is the purpose of this hole? The threads are raised, and it looks like someone cut a hole in the cat and then welded these threads on. It doesn’t look like how a factory might do it, but then again I don’t know anything about catalytic converter production methods so I could be wrong about that.

Is this a standard enough “feature” that I should be able to find a replacement bolt designed to fit, or should I get out my thread gauge and go down to the hardware store?

The bolt has been missing for about 1 1/2 to 2 weeks. Is there any possibility that the cat is now damaged and should be replaced? (Besides the obvious foreign debris that might have gotten inside.)

Thanks for the help.

Actually, this happens a lot more often than you might believe. The “bolt hole” really is a fairly common feature, although some reconditioned CC’s have had it installed post-factory. Every once in a while, they either work themselves loose after a lot of hard driving or get knocked loose by contact with road debris and/or whatever surface you happen to be driving on. It happened to my pickup a couple of years ago after some rather intense off-road excursions.

Depending on your local laws, you have two basic options. You can either get the CC replaced, or you can just have a piece of flat stock welded over the hole, closing it. Getting the CC replaced is certainly the more earth-friendly course, and quite likely the only legal option in NYC, since I believe that emissions testing is part of the vehicle inspection process there. Of course, if emissions testing isn’t a concern, but money is, just welding the hole closed won’t do any damage to your vehicle. In fact, you might actually gain a few (usually not significant) horsepower now that your exhaust has a more direct route out through the system.

You’re in luck.

Last I heard, If a cat-con (or a replacement) fails in a manner other than tampering, the law states that a vehicle’s warrantee allows for a cat-con to be replaced at no expense to the owner, for up to ten years.

Go to Meineke or some other muffler shop to verify, though.

That hole maybe for the oxygen sensor. Somepeople use this hole to see how much the converter is clogged by letting it open and running the engine at higher rpm to see the flow.

Sorry, but you’re not.
The 10 year provision came in post 1993 (the year of the car in question) IIRC in '93 the longest emissions warrenty in the US was California at 7 years 70,000 miles for items that cost over $500. Any changes in the law for new cars is NOT retroactive to earlier cars.
Secondly, the converter has been replaced, with what I am sure is an aftermarket unit. (A factory unit would not have a fitting where it is not needed. Aftermarket units designed to fit a bunch of different applications would) NO factory warrenty would cover the replacement of somebody else’s part. The replacement of the converter by one not sold by the car maker voids that portion of the warrenty.
A real life example customer replaces his spark plugs himself. Car does not run correctly, and brings it in have it fixed under the warrenty. Problem is traced to brand XYZ spark plugs. Factory plugs installed, problem is gone. Customer is presented bill. Not a warrenty issue, as the cause of the fault was the lame assed plugs that the customer installed. Had the customer replaced the plugs using factory plugs, then it would have been a warrenty issue.

Lastly tampering is not the only exclusion. Physical damage is also not covered. If you run over a large rock and damage the converter it won’t be covered under warrenty. Same as if you run into a brick wall the paint on your front fenders isn’t covered.

But by all means go by Mineke and ask, they could probably use a laugh.

The hole is for air injection. The car’s air pump injects air into the exhaust, to help burn off pollutants. Right here is a nice sight with a basic description of the system, including a diagram. Depending on the car, the air might be injected into the exhast stream at the exhaust manifold(s), or into the converter.

You have two possible scenarios:

  1. Your converter is a generic aftermarket unit; not designed specifically for your car—and it is equipped with the air injection hole, which your car doesn’t need. The installer likely plugged this with a bolt, as you suspect. You can replace the bolt (if the threads are good), or go to your auto parts store, and purchase this sort of putty that forms into a hard, strong, metal-like substance. I forget the name of this putty, but it’s handy to have around for other things too.
    (The converter runs very hot—make sure to let it cool down before you work. If it’s really icy cold out, maybe run the engine for 30 seconds just to get the converter to “room” temperature. Also, I’d try to let the putty cure before subjecting it to high heat.)

  2. Your car does have air injection going right into the converter, it broke off, and you simply haven’t noticed the pipe nearby. If this is the case, I suppose you could still plug the hole with putty and forget about the air injection…but this would be unfair to the environment, probably illegal, possibly bad for the converter, and you’ll likely fail your next inspection. It would be best to repair the connection and attach the pipe properly.

In any event, unless the bolt-hole you describe is physically damaged beyond repair, there is no reason to assume that the converter itself needs replacing.

Thanks for all of the information, especially Chris.

I double checked the shop manual and there’s no air injection into the cat. So it looks like scenario 1. I happen to have some of that “metal putty” so I’ll give that a shot tomorrow.

be careful with the “metal putty”- what you probably need is not a bolt but a pipe plug, and you can get them at any hardware store. The “putty” will most likely burn off at cat temperatures, and start a fire, or poison the convertor, or both. Look at a 1/2" pipe plug at your local hardware store, and see if this doesn’t look like it will fit.