bigman

Basically, the Colorado emissions program website listed a table that can help indicate whether the cat is working based on a generic understanding of what the exhaust gas is like before the converter and comparing it with the gas coming out the tailpipe. Unfortunately, the measurements are in grams per mile and PA does parts per million. So, if anyone knows a ballpark conversion I'd be happy to have it. Here's the link if anyone wants it http://www.aircarecolorado.com/repai...t99.htm#Simple

Dan

Vader

Man, that just proves how pompous and ignorant some of these bureaucratic agencies can get. That's like comparing cubic furlongs per decade to atomic mass per cubic meter per minute. Why the F*CK don't these morons just adopt the federal standard and leave it alone?

If you really want to convert, you would have to gather data in two forms. You would need to know the total mass (NOT volume) of gas out the tailpipe ÷ the molecular mass of the target contaminant(s) = ppm. To relate this to grams/mile in the Colorado scheme, you would have to know the total mass of tailpipe gasses per mile (how the hell they can figure that out is beyond me, since it can vary so widely with load and speed, and even density altitude and temperature) ÷ the total mass of the target contaminant(s). I can understand how the contaminant mass could be measured, but how can any reasonable technician realistically expect the volume of total tailpipe gass per mile to be consistent?

Is that thin air in the mountains starting to kill some dendrites? I'm thinkin' so. Someone in the bureau needs a vacation, or perhaps an education in scientific method.

bigman

If I remember correctly, the table is based on work done at Colorado State University, and I'm also thinking that the latest federal test measures in gpm based on the mileage driven during the test. Of course, I could be hallucinating that part. I spent some time on Friday trying to find a relationship between the feeder gas numbers and the emissions limits and then applying the ratio to the ppm numbers, but the CO of 20 grams per mile is the limit and it's nearly 10 times the limit of the HCs, which is completely screwy compared to PA where the limits are CO .71ppm and HC is 128 ppm. So, basically I've given up decided that the cat with 158K miles on it is probably still up to the job on CO & HC, but can't cut it on NOx anymore and I'll be replacing it. If the EGR was a problem, the computer should be throwing a code.

Dan

Vader

Not necessarily. From what little I understand of the catalyzed reactions, the palladium is supposed to limit the NOx emissions, but is only effective to a limit. If the gasses entering the cat are excessively high in oxides of nitrogen, the cat won't convert all of the molecules. If your NOX is high, you may have combustion issues that the cat cannot handle. A failed EGR may not necessarily set a DTC. The ECM may THINK the EGR system is working, but there could still be problems. What engine/fuel system are you running?

bigman

Bone stock TBI. I've got some more troubleshooting to do on the EGR system, but with the valve off, by moving the valve, I got air flow out of the vacuum port. I don't remember where I read it, but I found in a couple of places talk of cats that still did a fine job on HC and CO, but weren't managing the NOx well. Of course, I also found another place today that mentioned that EGR is the primary method of reducing NOx, and that the cat is secondary. So, back to testing the EGR system.

Dan

Vader

Dan,

The TBI system uses an EGR valve with no diagnostic temperature sensor. The system monitors the MAP signal, and expects to see an increase in MAP (decrease in vacuum) when the EGR solenoid is commanded open. It also monitors the EGR solenoid electrical load (coil resistance) by measuring current (voltage drop across a resistor in the sinking output circuit). Lack of a coil load will set a DTC right away, and lack of a MAP change after a timeout will set a DTC. Obviously, the ECM is detecting a MAP change, but there still may not be enough exhaust gas flow through the EGR ports in the heads and intake to control NOx. Ususally, the blockage is in the intake passages for the EGR. Like a restricted artery, it may be flowing, but just barely enough to satisfy the "brain".

It is possible that the cat is failing as well, but before replacing the cat, clean the EGR passages thoroughly. You can remove the valve, then use whatever you can fit into the passages to break the deposits loose. Coat hangers, screwdrivers, scrapers, etc. Once the chunks are loose, a vacuum cleaner should do the rest. Even if the cat is toast, the EGR system will be at 100% for the new cat, and it doesn't cost anything to clean the passages. A new cat can be pricey and time-consuming to replace, and you'll be frustrated if you find you didn't need one.

bigman

Thanks for the advice. I do plan to work on the EGR ports possibly replace the manifold with a Holley TBI. But, at $50 a pop for emissions testing and a day of hassling with getting a ride etc. I'm inclined to pick up a 3" cat for $130 and figure that's worth it in not having to fail the emissions test a bunch more times.

Dan

Vader

Sounds like you've found the perfect excuse to install the 3" system from front to back. A new cat certainly won't hurt.

Vader

And for future reference, I realized I misinformed you. The palladium in the cat is supposed to help oxydize the HCs, and the rhodium is supposed to convert the NOx gasses. I was thinking they were the reverse, but I'm not a chemist by profession. The platinum just helps to oxydize everything and sustain the reactions, and the ceramic is only there as a substrate to hold all the catalyzing metals.

Just thought I'd clear that up.