Overheating Catalytic Converter

[mdsbch]

I've looked through several threads and can't seem to find anyone that has had problems with their cat overheating. I have an '85 Corvette and tuned it up about 2 months ago (plug, rotor and cap, capacitor, plug wires, set timing, idle speed, etc.) and it was running great. A week ago I started it up to warm up and it was very hard to start (I had to run it around 1500 rpm to keep it running) then I noticed some smoke from under neath and the cat was glowing red hot (burned a hole in some old carpet that it is parked on). I figured from doing some research that it was probably plugged. I replaced it today and the same thing it was hard starting and this time it was not only glowing red but there were like little pieces of honey comb type material that came out of the tailpipes. Looking in my Helm's manual it talks about the cat overheating if there is continuous air flow to the cat or exhaust manifold from the A.I.R. pump tube. I'm sure that someone must have had or seen this problem. It wories me that it could catch the car on fire if it isn't fixed, but am not sure where to start trouble shooting without causing a fire. Please help. Thanks.

 

[photovette]

Running real rich would do that...

 

[BlackNBlue95]

If you have chunks of media coming out the pipes the cat is toast. Now there may be another issue that caused it to fail. If the cat was plugged I would think if you raised up the R's in neutral it would stall out because it can't breath IE exhaust restriction.
-=Rick

 

[Ken]

Hmmm... the emission control system, possibly an air control valve, may not functioning properly or it could be a leaky fuel injector or faulty fuel pressure regulator, causing the engine to run rich.

Possible causes for the excess fuel entering the exhaust system are an incorrect fuel mixture, incorrect timing, corroded spark plugs, worn and cracked ignition wires, improper fuel pressure, a faulty oxygen sensor, sticking float, faulty fuel injector or a malfunctioning check valve.

A malfunctioning oxygen sensor could also be the cause for an overheating catalytic converter. [font=Verdana, Arial, Helvetica, sans-serif]The oxygen sensor measures the amount of oxygen present in the exhaust gas. Depending on the voltage generated by the oxygen sensor, the engine management system will change the air/fuel ratio to obtain the desired oxygen level present in the exhaust gas. A malfunctioning oxygen sensor sending an erroneous reading to the engine control system can cause a too rich or too lean condition. A rich condition will cause the converter to overheat and melt down from the unburned fuel being ignited while a lean condition can result in a misfire that can lead to the same result. Oxygen sensors wear out and need to be changed per your auto manufacturers time and mileage limits.[/font]

One way to test for a plugged converter is to drill a small hole in the exhaust pipe right before the catalytic converter, then install a temporary hose fitting and attach a long rubber hose with a 0 to 10 pound pressure gauge.

With the gauge inside the car, drive up a hill or accelerate and read the pressure; normal is 2 to 3 pounds, restricted 4 pounds or higher. The drawback is you'll have to weld the small hole shut afterwards.

 

[mdsbch]

I do believe it is running quite rich, because I get black exhaust at first when it starts and then it clears up. It also smells rich when first starting up. As for the chunks coming out of the exhaust, they are about a half inch long and are a honey comb like hard clay. I don't think they are coming from the cat, since I have stock mufflers (not strait through) and don't see how these chunks could be getting through the mufflers. Do the stock mufflers have this type of material in them? I will re-check all my settings (TBS, timing, etc.) as well as the O2 sensor. Could a brand new cat go bad in less than 5 minutes of running time if it is running rich (since this is a new one)?

 

[BlackNBlue95]

Stock mufflers do not.. I've see this happen before though pretty much every car I have seen has had a muffler after the cat and the chucks get through.. they just blow through the exhaust.. my guess is the cat has been degrading for a long time..

 

[Hib Halverson]

You have one or more problems relating to engine controls...but, I think you know that. The bottom line is the motor is or has been running so rich you've melted the cat. So the first thing to do is replace the cat. Forget testing for a plugged cat. That the car is blowing chunks of cat substrate out the exhaust indicates you're way past the time to "test". The cat's melted.

At the same time, you need to find out why the system is rich.

First, you need the factory service manual. If you don't have one, get one. You can properly diagnose the car without it. Run the fuel pressure tests listed in the book. This will tell you if you've got leaking fuel injectors. If the car passes those tests, the service manual will lead you to other tests and/or solutions such as a faulty O2S discussed by Ken.

The important thing to remember, is you can perform efficient repairs by throwing parts at the problem or guessing what's wrong. Unless you are a very experienced service technician, you need the factory service manual.

 

[RonJ]

One way to test for a plugged converter is to drill a small hole in the exhaust pipe right before the catalytic converter, then install a temporary hose fitting and attach a long rubber hose with a 0 to 10 pound pressure gauge.

I'm sure that would do the job Ken, but my two cents says connect a vacuum gauge to the plenum, raise the rpm to around 1500 rpm and see what the vacuum gauge does. If the reading goes lower and lower at 1500 rpm, the exhaust (cat) is plugged. No drilling, no welding!!!!

Ron ...

 

[Ken]

I just threw that out there as one method. I wouldn't necessarily use it either.

 

[WhalePirot]

they are a honey comb like hard clay.

Yup, that's CAT stuff. I gutted an old one.

Rather minor mods can overcome the ECM's ability to trim fuel and make the engine run rich. This fouls the O2 sensor and eventually can ruin the CAT, not to mention al the other issues. I ran my L83 with headers and a mild cam for years. Even after the Z-man took 30% of the fuel out of the ECM's low-end fuel tables, the car ran rich! My CAT plugged up.

Personally, I'd run with a gutted CAT or a test pipe until the richness is cleared up, which should be done ASAP. Such is NOT legal, by federal law.

 

[mdsbch]

Thanks for all the responses. I have ordered a test pipe to put on until I can get the fuel problem squared away, but meantime I need to get it back up on my ramps and am a little afraid to start it due to how red the cat was getting (don't want to start a fire). I was reading in my Helms shop manual under code 45 (rich exhaust) and it says to put it into "Field Service Mode" by grounding the diagnostic terminal and starting the engine. What exactly does this do? Does it set default settings for running the engine? If not, is there any way of setting things to a default that will allow the engine to run in a not so rich state so that I can get it back up on the ramps to get the toasted cat off without catching something on fire?

Thanks.
Mike

 

[WhalePirot]

Gee, Mike. I have a test pipe in a garage cabinet that would fit an '85, plus an extra old CAT, which could be gutted easily. Too late, I suppose.

Getting a test pipe is (I guess), like buying illicit drugs and about as illegal. I had to swear I never bought it from the place and had to pay cash. Plan ahead and used liberal amounts of, and allow soak-in of, penetrating oil on those exhaust bolts. You might want to find new gaskets ahead of time.

Disconnect power from the ECM (orange wire, at the + battery terminal, with a disconnect built in) for at least 10 seconds to erase the ECM memory. Any rich learning should be erased.