4.0 Cats Glowing


scotts90ranger

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Just to throw it out there because I see it every now and then. Just the act of adding fuel alone will NOT make a catalytic converter glow, engines add more fuel (on gasoline engines anyway) to cool off the exhaust, which works until a misfire happens... when the engine misfires from a broken spark plug or bad plug wire or similar an unburnt air fuel mixture is pushed into the catalyst which is running at say 1000F normally which will ignite the air fuel mixture and over time (minutes) melt the catalyst(s) into a blob of ceramic goo. If it was a gaseous fuel system like propane or natural gas adding fuel will definitely push the cat into thermal overload, but on gasoline it starts cooling with more fuel than stoicheometric

Back to your regularly scheduled troubleshooting :). Best of luck, I do agree with everything else above
 


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Dirtman

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Briebennett17

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Just to throw it out there because I see it every now and then. Just the act of adding fuel alone will NOT make a catalytic converter glow, engines add more fuel (on gasoline engines anyway) to cool off the exhaust, which works until a misfire happens... when the engine misfires from a broken spark plug or bad plug wire or similar an unburnt air fuel mixture is pushed into the catalyst which is running at say 1000F normally which will ignite the air fuel mixture and over time (minutes) melt the catalyst(s) into a blob of ceramic goo. If it was a gaseous fuel system like propane or natural gas adding fuel will definitely push the cat into thermal overload, but on gasoline it starts cooling with more fuel than stoicheometric

Back to your regularly scheduled troubleshooting :). Best of luck, I do agree with everything else above
So after doing some digging we noticed that all emission components are oe with the ford stamp . That puts most of them around 25 + years old .
We bought the truck from one owner at 120k
After that it went to my younger brother who drove it as his high schoolttuck / 4x4 beater . Up in till 4 years ago . My father bought it back from him and it became a grandpa truck . We then bought it from my dad as a beater for our daughter knowing we would be doing a major overhaul to get it right . We can drive the truck for around 30 -40 mins with no issues after that it will shutter and rpms hold at 950 while pressing throttle. somedays I get lucky and it will catch but mostit stalls . Yesterday it stalled while under throttle around 35 mph aNd would not restart till I pulled the o2 on upper side of bank 1
 

franklin2

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Just to throw it out there because I see it every now and then. Just the act of adding fuel alone will NOT make a catalytic converter glow, engines add more fuel (on gasoline engines anyway) to cool off the exhaust, which works until a misfire happens... when the engine misfires from a broken spark plug or bad plug wire or similar an unburnt air fuel mixture is pushed into the catalyst which is running at say 1000F normally which will ignite the air fuel mixture and over time (minutes) melt the catalyst(s) into a blob of ceramic goo. If it was a gaseous fuel system like propane or natural gas adding fuel will definitely push the cat into thermal overload, but on gasoline it starts cooling with more fuel than stoicheometric

Back to your regularly scheduled troubleshooting :). Best of luck, I do agree with everything else above
The above post makes no sense to me? I will admit I am a little slow sometimes, but you first say adding fuel will not make the cat overheat, and then later you say a miss fire adding unburnt fuel to the cat will overheat and melt the cat? Then in the last sentence you say adding fuel will cool the cat?

From my actual experience, if the engine is running too rich for any reason, it overheats the cat. I have had the carpet in the interior melt because the carb on a older vehicle was running rich at idle.
 

Dirtman

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The difference is BURNED fuel vs UNBURNED fuel. When you add more fuel to the engine it doesn't matter as long as it's all being burned during combustion. A richer air/fuel mix will indeed cool the engine/exhaust to a point when things are working properly, but when you have a misfire you obviously don't have combustion so you are dumping raw unburnt fuel into the exhaust which damages the cat (combustion starts happening in the cat).

More BURNED fuel = OK
More UNBURNED fuel = Bad
 
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19Walt93

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More fuel is not added to cool the engine. Fuel is added when needed due to engine load or temperature.
 

scotts90ranger

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More fuel is indeed added to cool the exhaust, I have MANY hours running engines on dyno's tuning for emissions, power and checking things like this for durability purposes. Best power is made around the stoicheometric fuel mixture (14.7:1 air to fuel for gasoline, around 15.5 air to fuel for propane and natural gas), but once the exhaust gets hot and you near the valve melting temperature (some engines it's around 1400F, others have inconel sodium filled valves that can handle sustained 1550F) you add more fuel which doesn't burn as efficiently (good call dirtman, I didn't think that it does burn, just not efficiently) so the temperature drops where as if you reduced fuel instead of adding it the temperature would rise. Automotive engines aren't too picky about exhaust valve temperatures, unless you are towing up hill you can't take a 400 horsepower engine and run it wide open throttle at high rpm more than a couple minutes, I'm used to marine and industrial applications where they can live at 80% load for literally hours at a time and in some industrial applications DAYS. Gasoline is easy, I shouldn't have confused things with the propane/natural gas comments, they actually make sense where the exhaust gets hotter with more fuel, I haven't figured out why but I'm sure it has to do with the chemical composition of the fuel where natural gas and propane are much simpler being CH4 and C3H8 instead of a big long string so it's a less complex chemical reaction or something...

It gets confusing, I didn't mean to go TOO far on a tangent :), they key is misfires cause the issue which are usually from ignition component failure (melted spark plug wires, worn out spark plugs, bad crank sensor signal, etc) causing unburnt air and fuel to get put into the exhaust. Old carbureted systems are odd and different, if it had an air pump and at idle chances it was misfiring and didn't have complete combustion if it was running rich so there's extra fuel and the air pump added oxygen to the fire if I had to guess... I ignore pre fuel injection emissions systems if at all humanly possible :), it's hard enough keeping everything I do in line...
 

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The raw fuel versus rich fuel mixture cleared it up for me. Thanks.
 


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