Gen III Caught on Fire - Possibly Catalytic Converter

[johnnyk_6]

A friend and I have been doing some troubleshooting on the car while I wait to get it serviced at Woodhouse next week. At this point, we've pretty much ruled out the catalytic converter.

The first theory I had was a plugged cat. Jon B helped me out and shipped an OEM replacement while I wait for the high-flows that I ordered.

Once that arrived, a buddy and I took off the old cat to look for damage but couldn't find anything obvious. Before putting the new OEM on, I used a borescope to look at the catalyst material inside each one. Here's what they looked like:

Original cat:
cat_original.jpg

The replacement:
cat_new.jpg

We put the replacement on and fired up the car. Within about a minute we started to see smoke coming from the replacement cat. I looked around with a flashlight and saw smoke coming from near the flex pipe. At this point I realized the same issue is still happening and it probably isn't the catalytic converter.

Next, we checked the O2 sensors with a multimeter. Both pre-cat and post-cat sensors have continuity. Then we fired up the car and used an OBD scanner to confirm that they are working fine.

While running the OBD test, my friend used a socket extension as a redneck stethoscope to listen to each injector. Each injector seemed to be opening and closing just fine. We also used the multimeter to check if each injector had continuity, which they all did.

The only remaining thing I could think of was the spark plug wires. The plugs were replaced about a year ago so we didn't bother to pull them out. We did however check the inside of each of the rubber boots and found one that appeared to be damaged. I stuck a borescope in each of the boots to get a closer look and here's what I found:

The damaged one:
plug_bad.jpg

What the other ones look like:
plug_regular.jpgplug_regular2.jpg

So at this point, I'm pretty sure it's a bad spark plug wire. My current theory is that one cylinder isn't getting spark so unburnt fuel is being dumped into the exhaust, either burning up and superheating the cat, or causing the O2 sensor to run that side extremely lean and causing excessive heat. It hasn't thrown a code yet, but I haven't ran it for more than a couple minutes at a time.

Could a bad spark plug boot/wire be causing the cat to get hot enough to the point that it smokes or catches something on fire? Any input is appreciated.

[usmcfieldmp]

The smoke on first start up is most likely just the oils and such burning off. If you put your bare hands all over the exhaust pipe or catalytic converter, it'll smoke a bit when you start it back up. Plus factory/manufacturing oils, etc.

Have you pulled and checked your spark plugs and read them? https://ngksparkplugs.com/en/resources/read-spark-plug

If it does actually have a pop/crackle tune on it, then that is definitely your problem as those will destroy catalytic converters. Definitely check all other avenues before assuming this is the case.

Simply running pig rich will also clog them. This was a common issue with the early Cobalt SS's when I bought mine in 2006. From the factory it was running around a 10.2:1 AFR at WOT. Ridiculously rich.

[GTS Dean]

First, check for bad plug boots and obvious signs of damaged ignition cables. If those look good, open the hood and run the engine in the garage with the door open at night with no lights on. Use a spray bottle to mist the wires with Windex or soapy water and look for sparks. I found that my 20+ year old Magnecor wires which looked perfect in every way were arcing trough pinholes on multiple cables. Gen 3-5 engines have much shorter wires than Gen 1&2.

[johnnyk_6]

First, check for bad plug boots and obvious signs of damaged ignition cables. If those look good, open the hood and run the engine in the garage with the door open at night with no lights on. Use a spray bottle to mist the wires with Windex or soapy water and look for sparks. I found that my 20+ year old Magnecor wires which looked perfect in every way were arcing trough pinholes on multiple cables. Gen 3-5 engines have much shorter wires than Gen 1&2.

Thanks for the tip. I checked all of the plug boots last night and found one that looks damaged on the inside compared to all of the other ones. Since the issue doesn't appear to be a catalytic converter, O2 sensors, or fuel injectors, I'm thinking it has to be the plug wire. I'm ordering a new set of ignition cables today. Will post another update once I get them installed.

[ViperSRT]

If you have arcing down the plug ceramic you may have to replace both plug and wire at the same time. Visible by a carbon track either on the plug ceramic or the inside of the boot.

[Johnny ViperBro]

It almost looks like copper wire inside the heat shield, never seen that. If the cat was clogged I would think you would have noticed as my bet would be it was running less than optimal. Just my 2 cents. Glad it didn’t burn!

Correct. I had a clogged Cat that had to be replaced ($2200). Car ran like a slug and smelled like unburned gas in the cockpit.

[JonB ~ PartsRack]

Correct. I had a clogged Cat that had to be replaced ($2200). Car ran like a slug and smelled like unburned gas in the cockpit.

YIKES ! ONE clogged cat for $2200? Did that include repairing whatever caused the un-burned fuel clog?



FYI, HiFlow cats for the entire car cost about 35% of that, and offer several driveability benefits.