Bad Cats?

[TicoSHO]

I’ve searched the Forum for a couple of days and haven’t found much about catalytic converters. I’m hoping you can help me out.
My problem is when the car gets to temp, the RPMs start going up and down (1500-500) and then it dies. My codes came back with bad O2 sensors. That seemed to by the logical answer. Not having any money the SHO has been sitting in the driveway for 10 weeks. A buddy’s cousin just came to town and he came over to look under the hood.
He had me start the car and then went right to the pipes, held his hands in front of them, and told me I had bad cats. He said that bad cats would cause the Comp to throw O2 codes. Told me that all I need to do is take off the cats and knock out all the honeycomb crap inside.
Can that my problem? Can cats go south slowly? (My problems started maybe once every two days and then got to several times each time I took the car anywhere) Is that a job that a non mechanic can do? Would it be easier just to get new cats or a whole new pipe setup?
Also Florida does not have emissions anymore.
Thanks for any help you can provide.

 

[SHO#7]

Your friend thinks your converters are bad because of heat? A converter is supposed to get hot. To me it sounds like one of a couple things. First, what is your exact code(s). Both oxygen sensors or just one. You may have a vaccum leak, making the car throw lean codes, or you may just have a bad o2 sensor. I have seen bad ones do strange things, but not make them idle that lopey. You may have an idle air control valve heading south. They are nice and common on Ford products.

But to answer your question. Bad cats will not set an o2 code. The o2 sensors are before the cats. The o2 sensors do not know if the cats are woking or not.

Mike

 

[TYSHO]

Driving around with bad o2's can **** the cats.

 

[Rob94]

Tell your friend to pound sand. What is he, the Great Carnac??

 

[SHO#7]

Driving around with bad o2's can **** the cats.

Agreed, but IMHO an erratic idle does not make for a good case of bad cats.

Mike

 

[TYSHO]

Agreed, but IMHO an erratic idle does not make for a good case of bad cats.

Mike

Erratic idle, meaning there is a vacuum leak causing for lean conditions, in which the o2's send the signal back to dump more fuel, therefore causing for a rich condition killing the cats.

 

[TicoSHO]

Help me. It kills me every morn to go to work and get into a lavender kia rio instead of my SHO. I seeth every time a little import with nothing but exhaust goes blowing by me on the way. HELP! I'm almost ready to try drastic measures.
I got guy who said he could come over and cut the cats out with a recip saw . He then said he would just put straight pipe in there. I made tenenative plans for him to come over next weekend and knock it out.
I've done some searching on the forum and i haven't seen anything that could go wrong except that the exhaust will sound a little raspy and a small drop in torque.
Now on to the questions.
Are the O2 sensors infront or behind the cats? (I've only been under it once and that was with a shaky jack, didn't want to spend to much time under there)
I think this dude might only be able just cut the cats out. Will a shop put in the pipe knowing you just had your cats cut?
Is all this going to worth it? Or would I just be better off getting Rainbow Muffler putting two new cats put in for $450? (Not easy money to come up with, but possible)
Thanks for any help you can give.
I also wanted to say what a great website and you can't find a better bunch of entusiasts for any car.
PS: sorry for any misspells. It's hard to see thru all the tears :cry:.

 

[AFSho]

Erratic idle, meaning there is a vacuum leak causing for lean conditions, in which the o2's send the signal back to dump more fuel, therefore causing for a rich condition killing the cats.

Not to discredit you or anything, i just want to clear up any misconceptions about the purpose of 02 sensors and EGO feedback.

when your car is idling the ECU is operating in open loop mode. Feedback from the 02 sensors is ignored, simply because it is too eratic to be of any value. In most cars, closed loop ECU operation does begin until around 1200-1800 rpm. At these engine speeds, there is sufficient exhaust flow over the sensors to provide steady readings. (watch the AF readings of your car while on a dyno to confirm this).


The ECU primary method of delivering the correct amount of fuel comes from hard coded "lookup tables". By knowing the amount of air entering the engine (MAF) and the engine speed the ECU goes to a lookup table to determine the volumetric effeciency of the engine in its current state. From there it calcutes the correct injector pulsewidth to yeild a proper air fuel ratio.

That being said, these hard coded values are "safe" values, meaning they will never produce an AF ratio so lean it will harm the motor. These lookup values are safe values no matter where are how the engine is operating. The purpose of closed loop operation and EGO feedback is so when the vehicle is under cruise the mixture can be leaned out to provide cleaner emmissions and better fuel mileage while making sure the mixture does not become dangerously lean.

When the ECU detect a changing TPS value (acceleration) the ECU comes out of closed loop and defaults back to the lookup tables. Depending on how fast the tps reading is changing (volts/sec) the ECU enriches the mixture by an acceleration multiplier, this allows a smooth transition from one lookup bin to another. Once engine activity has stabilized, the ecu will resume closed loop.

Now, a vacuum leak after the MAF will indeed cause a lean condition, and an occilating idle. But since the ECU is in open loop, it will not correct for the lean condtion because it doesnt know there is one. The reason the idle occilates is because as the mixture leans due to the leak, the rpms will rise. This will trigger an acceleration enrichment and/or a bin change. The mixture will richen and the rpms will fall. Then the cycle repeats. However, the mixture will never be so rich as to burn the cats.

If you are burning cats at idle, you have some form of an injector leak. period.


Sorry for the lengthy explanation, but i hope it helps.