the SLC LS3 hot catalytic converter mystery and Howard's infinite wisdom

Well, if you did not come from my build thread, let me give you the background . I got my SLC to go cart stage, fired engine up and it runs like crap and overheats. It turns out I plugged the crank position sensor into the oil temp plug on the oil pan. Dumb, Dumb, Dumb. Check Engine light did come on.

Overflow tank is defective and the cap does not seal , water overheats and boils over .

Both problems solved, tank cap modified , crank sensor plugged correctly, engine runs well and does not overheat . And then after running for a couple of minutes my son Miguel yells , shut it down , the catalytic converter on the driver side is glowing red , the entire thing looks like is about to melt .

OK , talked to a bunch of people, has to be a fuel problem, ( one of the cylinders probably the culprit )unburnt fuel being dumped into the cat seemed to be the most popular explanation. But why there was no code being activated , check engine light is off the entire time. NO CODES

Replaced all injectors, Cat still hot. some plugs looked terrible, (probably from the engine misfiring when the crank sensor was plugged incorrectly )

Replaced all spark plugs, Cat still hot .

Checked all the coils , Cat still hot

Oxygen sensors may have gotten messed up when the engine was running badly . Replaced oxygen sensors. Cat still Hot

checked every wire going to the coils , and injector, one by one , all the way to the engine ECU , at least I know I have a good harness, LOL

Cat still hot , now what? I am out of options , time to call Howard.

Here is the actual conversation
Me: Howard I got a hot cat. H: did you check all the fuel stuff? Me: yes, thoroughly new injector, new plugs . H: did you check the electrical stuff and the coils? yes , wire by wire, Me: most people I have talked to feel is fuel related , unburnt fuel H: I just don't believe this a fuel problem .
It has to be an exhaust leak. Me: really? the mid pipe and resonator were assembled by Fran at the factory , (initially I thought I was going to go cat less) I just added a cat and exhaust .
H; have you checked the bolts? have you checked the gaskets ? Me: Gaskets??? what Gaskets? Fran did not put any gaskets between the manifold and the mid pipe. I did not even know those existed .

So I ordered the donut gaskets , there was actually a small gap since the flanges did not fit perfectly , they fit a lot better on the other side and the situation aggravated by a failed flange bolt on the driver side , that is probably why the passenger side cat did not get as hot .

Hot Cat problem solved, AN EXHAUST LEAK . too much oxygen getting in , I'm guessing , not an expert on catalytic chemistry.

THANK YOU Howard . I should have started with the phone call. . Even my friend super Dave, a retired master mechanic was stumped. Replacing a bunch of parts without understanding the problem is an exercise in futility .

The Cat was getting hot, the engine was running fine and no Codes where activated, fuel or electrical problem unlikely . Live and learn .


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interesting how the cat also prevented you from recognizing what would be the most obvious symptom - the smell of a super rich bank. Well done figuring it out.