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Old 07-25-2012, 11:53 AM
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How to I know if my Catalytic Converter is bad?

78 F150 California truck. How can I test if my cat is bad? Exhaust smells horrible.
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Old 07-25-2012, 12:05 PM
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78 F150 California truck. How can I test if my cat is bad? Exhaust smells horrible.
Unless you have a precat and post cat o2 sensor with scan tool hooked to computer, you dont, so as far as I know you will need a gas sniffer. The cat can make that smell from a bad state of tune or bad fuel. Antifreeze makes a funky smell too, as it destroys the cat. Are you due a inspection or something? You can do a back pressure test if you feel like it is getting blocked up. Really no way to check for proper operation without measuring input and output gases though.
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Old 07-25-2012, 12:19 PM
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It's past smog many times before and the exhaust has always smelled bad like a strong toxic smell. Makes your eyes water just being within a 30' radius. I have never smelled a truck/car like this.

No O2 sensors on this truck so I don't know how you could do the back pressure test.
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Old 07-25-2012, 12:25 PM
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We would drill a hole in the cat, and weld it up after we were done.
If it is passing smog, the cat is working (according to Cali anyway) LOL
A bad smell means its converting unused fuel, or antifreeze, or oil using the catalyst process. Do you have a missfire, or a overly rich carb setting, or oil being burned.
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Old 07-25-2012, 12:34 PM
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No oil being burned, truck runs good except a slight mis at idle that I could never cure-new everything minus carb rebuild. Truck runs great. Muffler is oem and has a large hairline crack in it. Would the cat get overly hot if it was clogged?
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Old 07-25-2012, 01:02 PM
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78 F150 California truck. How can I test if my cat is bad?

Exhaust smells horrible.
Betcha it smell like sulfer. Very common when the converter is failing.

Does the truck sound like a vacuum cleaner headin' on down the road?

If so, the charcoal is working its way past the converters wire meshed screen and is plugging up the muffler.

When cats were first introduced, the NHTSA performed tests. One was parking vehicles with the engines running with dry leaves on the ground below the converter.

NHTSA timed how long it took before the leaves caught fire.
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Old 07-25-2012, 07:57 PM
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You can tell if it's plugged. Little to no exaust coming out the pipe.

There is also a test with a lazer thermometer. But it's not an exact science. You read the incoming exaust temp and compare it with outgoing exaust temp. The outgoing temp should be hotter on a good converter.

If the temp going out is within 20 degrees of the temp going into it, then it's bad. within 50 degrees it's worn, and 100 degrees is ideal. this is on a warmed up converter.

Failing all that, a smog shop should be able to do a pretest for you and give you your tailpipe emissions. Real high HC is a sign it may be a cat, or excessive fuel problem.
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Old 07-25-2012, 09:34 PM
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My cat went bad in my DD as it began to rattle. It sounded as if a heat shield under neath was coming lose. It didn't have any odd, or different smells that I noticed. I remember in the 70's when I worked at the Ford garage most all the cars smelled of sulfur, or rotten eggs when they were running.

Every so often we would take the latest and greatest models to the mall across town early in the morning, and they made sure we got them to their locations for diplay and din't want the sulfur or rotten egg smell in the mall anymore than necessary. Usually we took in LTD's, Thunderbirds, a few Fairmonts, and on occasion a Lariat packaged dent side.
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Old 07-25-2012, 09:34 PM
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