Catalytic converters questions

[Roland Jacques]

I suspect my 2 Catalytic converters are clogged or close to it.

Is there a good way to confirm or eliminate these suspensions?
Also does anyone no what the second set of O2 sensors (after the cats) are controlling?
Is their anyway to clean a clogged cats?

2001 GMC 3500 10,000 lbs box van with 200,000 plus miles on it.

[Boltazar]

If the cats were not doing what they were supposed to do the O2 sensors behind them would tell the computer that somthing is wrong. And pop a code.

[Roland Jacques]

I know it's suppose to give a code, but i'm not trusting it right now. I was thinking there might be an easy way to confrim they are working good.

[HHOinKY]

Cut them out :p


You should be able to drop the echaust right where the converter is and look inside. If not, rev it up and see if crap shoots out the exhaust.

My dad was having noises comming from his exhaust and it turned out his converter was broken up and comming out the exhaust.

[HiTechRedNeck73]

your local muffler shop has a way to poke a hole in the exhaust tube and take a pressure reading to know if they are good or not...

I don't know the exact process, but I've had it done before... usually they don't charge to check a cat...

[Jaxom]

HiTech has it...a simple backpressure test will tell you for sure, and it only takes about 10 minutes. Most of the muffler shops around here don't charge for it.

Basically they just punch an 1/8" hole in the exhaust pipe just before the cat, hook a pressure gauge to the hole, and rev the engine. If the backpressure jumps higher than 4 or 5psi, then there's a restriction, and they'll try to sell you a cat. Removing the cat completely is illegal, but some people still do it.

Also, the downstream O2s can't detect a restriction. They only set codes if the chemical reactions inside the cat fail.

[Roland Jacques]

Awesome, thanks guys :)

[Roland Jacques]

Does anyone know if the "high flow cats deliver any MPG gains?

My trucks not subject to emision tests, and my emisions runing HHO are far lower than non hho trucks. I wonder how it would perform without any cats at all :rolleyes:

[HiTechRedNeck73]

Does anyone know if the "high flow cats deliver any MPG gains?

My trucks not subject to emision tests, and my emisions runing HHO are far lower than non hho trucks. I wonder how it would perform without any cats at all :rolleyes:

hi-flow cats reduce back pressure and allow exhaust gases to escape faster...

there is a process that some shops use to make hi-flow cats out of normal ones... basically, they take the old cat loose and ram a pipe down in the cat to breakup and reduce restriction in the cat... it's the in-between of hi-flow and no cat at all...

if your vehicle has o2 sensors after the cat, running no cat at all will throw a code on the computer and probably a CEL... the computer compares the front and rear o2 sensors to check if it's working (reduction of o2 gases detected)... without a cat, the gases will be the same and will not be within the expectations of the computer...

[Roland Jacques]

hi-flow cats reduce back pressure and allow exhaust gases to escape faster...

I seem to remember that some back presure has some power benifts when running straight gas ( i think low end torque). But when running HHO and a lean mix, id think back pressure may have no usefull purpose at all. what do you think?

basically, they take the old cat loose and ram a pipe down in the cat to breakup and reduce restriction in the cat... it's the in-between of hi-flow and no cat at all...

LOL, yeah i was thinking about of that also.

If your vehicle has o2 sensors after the cat, running no cat at all will throw a code on the computer and probably a CEL... the computer compares the front and rear o2 sensors to check if it's working (reduction of o2 gases detected)... without a cat, the gases will be the same and will not be within the expectations of the computer...
I wonder why they (the rear O2 sensors) dont see a problem when we run HHO.

[HiTechRedNeck73]

I don't know of any back pressure that is good... I could be wrong, but I've always tried to get back pressure as low as possible in my engine builds...

to answer your question about the rear sensors...

http://www.supermotors.net/getfile/678893/fullsize/graph-o2volts-idle1.jpg

the graphs above are from my Intrepid running gas... the top charts are the front (upstream) o2 sensors and the bottom charts are the rear (downstream) o2 sensors... and side-to-side, drivers side and passengers side respectively... hopefully it is clear enough, but you will notice that the chart scales are the same: 0-1.275 volts... the other axis on the chart is simply time in seconds...

from these charts, you can see the extreme changes in the front sensors compared to the rear sensors... basically, the front sensors are watching the exhaust coming out of the cylinders after a spark ignited burn... as exahust comes out of the cylinders and enters the cat, there is an amount of unburnt fuel and oxygen...

the cat, by chemical reaction, uses the heat from the exhaust, the unburnt fuel, and the oxygen to rearrange the chemical arrangement of the polutants... here is the two styles:

A two-way catalytic converter has two simultaneous tasks:
1. Oxidation of carbon monoxide to carbon dioxide: 2CO + O2 → 2CO2
2. Oxidation of unburnt hydrocarbons (unburnt and partially-burnt fuel) to carbon dioxide and water: 2CxHy + (2x+y/2)O2 → 2xCO2 + yH2O

A three-way catalytic converter has three simultaneous tasks:
1. Reduction of nitrogen oxides to nitrogen and oxygen: 2NOx → xO2 + N2
2. Oxidation of carbon monoxide to carbon dioxide: 2CO + O2 → 2CO2
3. Oxidation of unburnt hydrocarbons (HC) to carbon dioxide and water: 2CxHy + (2x+y/2)O2 → 2xCO2 + yH2O

now I can't tell you what type you have... but basically, these both reduce the amount of o2, by remixing (for lack of a better term) it with other elements to create safer exhaust gases... because of the lower amount of o2, the sensors after the cat are assumed to be of a lower voltage to reflect that... so the rear sensors should always be less than the front and I'm sure that most engine computers know that and watch for it...

here are some of the error codes associated to the rear sensors in my car: (as an example)

Sensor Shorted to Ground, input voltage below normal operating range
Sensor Shorted to Voltage, input voltage above normal operating range
Sensor Slow Response, sensor response slower than minimum required switching frequency
Sensor Stays at Center, neither rich or lean condition detected
Sensor Heater Failure, sensor heater element failure
Sensor Slow During Catalyst Monitor Test, (this is similar to the slow test above, except this is a portable smog test that actually monitors the efficiency of the catalytic converter... no all cars have this)
Catalyst Temp too High
Catalyst Temp too Low, (I've been told that these two are interolations that the computer does from the o2 sensors... I don't know for sure, but my cats don't have temp sensors so it must be...)
Catalytic Converter Overheat Detected, (I have no idea where it gets the temp)
Catalytic Converter Efficiency, catalyst efficiency below required minimum (there's the proof that it does compare front and rear...)

that is the public list of catalytic-based error codes for my car... I didn't look in my Factory Service Manual, because that list is a lot longer... the way I read all this is, you can't remove them without CEL coming on... but you can modify within reason...

[farmer]

just gotta say i removed my cat last week, fitted full stainless pipe from manifold,got it a size up from standard(can't rember exact size atm) to reduce backpressure -to see if mpg would rise. well massive power increase but no more mpg whatsoever...no less either so make of that what you will.