Since there seems to be a small debate brewing (or is it a difference of opinion?) I figured it would help NGAROVER with his fouled plug thread if we took this debate to a new home. ANY experts in this area please correct me if wrong and chime in.

From what I can tell the major sticking point concerns the OBDII, 4 sensor setup. Here are a few fun facts.. aren't all facts indeed fun?

-Federal Test Procedure (FTP) standard dictates that OBDII EFI (ALL post 1996 NAS vehicles) systems shall iluminate a check engine light if the system whether by electronic or mechanical fault causes the exaust gasses (primarily H2O, CO2, N and some O) compisition to rise above 1 1/2 times greater than the FTP standard.

-CC's facilitate chemical reactions which break down exhaust gas compounds into basic chemical components when passed over precious metals like platinum, palladium, rhodium and cerium. OBDII standards required manufacturers to supply CC's which convert exhaust emissions into water, carbon dioxide, nitrogen and oxygen since these are deemed to be harmless and natural.

-In order to acheive this FTP standard O2 sensors are fitted in the exhaust system. NOT JUST upstream as before '96. Now in order to keep the EPA happy CC efficiency must be maintained and monitored through the use of secondary O2 sensors downstream of the CC's. What good would this standard be were it not for emmisions testing or monitoring? CC's generally last anywhere from 70k miles to 100k miles (or longer) depending on engine efficiency and tune state. According to EPA standards, CC's must be replaced when they no longer are efficient at converting "dragons breath" into "Unicorn farts". This is why EPA required CC monitoring sensors and implimented testing (in certain states).

- Another fun fact is that Catalytic Converters, unless self-heated with electric coils like BMW and some other late-model manufacturers, rely on exhaust gasses to heat them up in order to become effective catalysts / chemical reaction chambers. This means our Disco CC's don't do a damn thing for the first 5 to 10 minutes from when the engine is turned on.

-As technology has advanced a bit since the Disco I was designed and built advances in monitoring and CC construction have altered the emmisions landscape... HOWEVER, since we are primarilly concerned with '94 - '99 Discos we can save that discussion for another thread.

The Disco I EFI systems (14CUX and Gems) rely on Engine temp (coolant), fuel temp, air flow and exhaust gas OXYGEN content monitoring to maintain "proper" air:fuel mix. Upstream O2 sensors cause the mix to switch back and forth between rich and lean states in order to maximize emissions efficiency. Downstream O2 monitoring data is required by EPA to keep us running with good cats...that is all there is to it in our case. This is not that case for ALL cars, but again we're talking Disco's here.

Next time you are VERY bored try a fun experiment... unplug your downstream O2's for a few minutes and see what happens... then try disconnecting your upstream O2's and you will see the difference. and for even more exciting fun with all sensors connected try disconnecting your coolant temp sensor for a few minutes. Watch the O2 sensor codes appear. This is because the ecm will richen the mix...this will be sensed by the upstream O2's.

Sorry OKD but I must disagree with some things you said in a previous post.

O2's do not read temperature...As far as the EFI system richening the mix in order to heat the cats... I highly doubt Rover would design a system which would damage the CC's intentionally through programming. A rich mix will dump more unburnt gas into the CC and we know that is VERY destructive to the CC's. And, FACT, a rich mix, in relation to a Lean mix, will create a relatively cooler burn so this theory is a bit backwards. This is why an over-lean mix state can cause an engine to seize.

Anyone else want to add their knowledge... not opinion please...? I'm certainly no EXPERT...

I just found this link that can probably explain this crap better than I can...

http://www.aa1car.com/library/o2sensor.htm

Let the facts fly...

 

Fact cc EPA and o2 sensors suck

 

I will accept those statements as facts...

 

"And, FACT, a rich mix, in relation to a Lean mix, will create a relatively cooler burn"

Agreed. One fact verified. Next!

 

X2 on that statement.

I hate emissions equipment period. I even took the EGR off my 300D. (And yes it makes a big difference)

 

I think I may slightly disagree with the CC not functioning for 5 - 10 minutes. the required temp for a CC to start functioning correctly is around 400-600 degrees and I believe those precious metals inside the CC reach that temp within about 2 minutes of start up on your average V8 (outside temp shouldn't affect this much if at all) I may agree the 1200-1600 degree temps that they operate at 98% of the time isn't achieved until about 10 minutes after starting. as for BMW using heated CC's I would like to think this is to protect the rest of the exhaust system for those first few minutes from all the bad stuff that isn't filtered out (of course that's just a guess).

But everything else looks pretty good to me!

 

A rich mix will allow more unburned fuel to enter the exhaust. When it lands on the converter media, it raises the temperature of the media to an effective value. That is why you have a richer mixture when it is cold, to heat the cats up quicker.

What do you think O2 sensors measure? They are just high temp thermometers....

I agree they were added to throw a code BUT - I had a damaged cat and did not get a code. I replaced my cats and it was still the same, only I got a leaner burn. Pipe was black before. Out of curiosity and what people say on here, I was curious about the function of the rear sensors, so I took them out and plugged the holes.

With the rears in free air, it ran a richer mixture. Not as rich as having the fronts removed, but richer none the less. I have seen the live data, first hand. I know that the rears have an effect on mixture.

 

Chris... I believe we are on the same page. The peak efficiency of a Cat is reached "minutes" after startup. Whether that is 2 or 7 or 10 minutes is variable depending on a few factors... the biggest determining factor is probably age and condition of the CC, followed by engine tune then least would be (my guess) ambient temps.

Same thing applies to O2 sensors...they take time to warm up to operate. Which is why most (I think ALL but not 100% sure) OBDII system O2's are heated internally.

OKD.. I agree with you that O2 sensors are effected directly by heat BUT their output values are determined by the heat of the exhaust O2 when COMPARED with ambient O2 . The varriance in temperature between the internal electrodes generates a small voltage, between 0.01 volts and 1.0 volts. It is the constantly varrying voltage based on mix that the ECM uses as data to continue that mix fluctuation in order to acheive an everchanging efficiency.

My point is this... IF it were merely the temperature of the exhaust and Cats that was critical to the EFI system then simple temp sensors could be used and not O2 specific sensors which measure the oxygen content difference between exhaust gasses and outside air causing the voltage to flow.

Raw fuel kills CATS... is that in dispute?

 

Sorry, but this is completely incorrect: for definitive info read here:

http://en.wikipedia.org/wiki/Oxygen_sensor

While there are different technologies, they all work on a common principle of using a catalyst to create a differential voltage as a function of the amount of oxygen present on two sides of of the sensor, with external air being the reference and exhaust gas being the active component. They all require relatively high temperatures to function.

"The sensors only work effectively when heated to approximately 316 °C (600 °F), so most newer lambda probes have heating elements encased in the ceramic that bring the ceramic tip up to temperature quickly. Older probes, without heating elements, would eventually be heated by the exhaust, but there is a time lag between when the engine is started and when the components in the exhaust system come to a thermal equilibrium. The length of time required for the exhaust gases to bring the probe to temperature depends on the temperature of the ambient air and the geometry of the exhaust system. Without a heater, the process may take several minutes."

 

now you have sparked my curiosity I would think a warn out CC would get hotter faster but it would be because there is less catalyst plating on it (kind of like boiling a cup of water versus 5 gallons of water), so with that said a newer catalytic converter would take longer to work but would work more efficiently than an older one. And of course your right about the other factors, like fuel octane, spark plug condition/gap, and probably carbon build up on the cylinders, valves and exhaust manifold.

So I wonder what the rate of break down vs. efficiency would be on a CC? like do they only work half as good after a few months or are they almost as good as new right up until they throw a CEL? and with that said would a well broken in one actually be more efficient than a new one? my guess is that since they are not supposed to sell used CC's that they just start degrading from day 1