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Old 10-11-2003, 04:27 PM
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Im' sorry Dmorris, but I will have to completely dissagree with almost everything you have stated.

Quote:
Originally posted by dmorris1200
Actually if you think about it, the EGR's job is to richen the mixture. [/i]
The EGR valve will not richen up the mixture. Its only function is to open when necessary, either by a vacuum signal, or electronically and recirculate exhaust gas back into the air/fuel stream to cool down the combustion temperature, which in turn will help reduce pinging, therefore resulting in a lowering of the NOX emissions. It cannot richen up the mixture. Its introducing an already burned mixture back into the engine. Its basically a neutral gas, it contains virtually no usuable oxygen or fuel.
That is why the EGR is NOT designed to work at idle and low throttle openings.
If you've ever opened an EGR valve manually at idle, what happend? The engine leans out, stumbles and usually dies if you continue to keep the valve open. Why? Because it does not contain enough oxygen or fuel to aid in further combustion.
If, in fact, it DID richen up the mixture, it would run better at idle with the valve opened and would have no negative affect at idle and low throttle openings.

Quote:

That's why when it isn't working the engine runs too lean, usually pings, and fails for NOX due to increased cumbustion temps.


Sorry, but when the EGR valve fails to open, the resulting mixture is NOT diluted with exhaust gas and and will actually be marginally richer. More importantly however, the temps in the combustion chamber will still be too high, pinging may accure and NOX polutents will be high, as the engine is designed to, and will be running on the lean side anyway.
In my previous posts, I never said that a bad EGR will cause a high CO failure. I said the engine was running too rich, fuel problem,( high CO failure) and that there was most likely an EGR failure too, ( high NOX reading) Two different problems, two different emission readings and two different "fixes".

Quote:

What's interesting about his failure is that a failed CO could often mean different things in different applications. Any poor combustion could raise CO's (incomplete burn of fuel mixture) but his high NOX also tells a story of a lean mixture causing high combustion temps.


First off, CO is burned gasoline.
High CO is NOT caused by incomplete combustion. If you have low CO readings, all it means is you have successfully burned all the fuel in the combustion process. Low CO readings are a good indicator of a properly set carb or injector system, showing that all the fuel has been properly burned and that the engine is not over rich.
If you have high CO readings, all that means is you have successfully burned a very rich mixture.

Second, HC ( hydrocarbons) is nothing more than raw, unburned gasoline.
Poor combustion will not raise the CO level. If you have poor combustion, as in incomplete combustion, the HC readings will be high, because you are not completely burning all the fuel, giving you raw fuel leaving the cylinder and exiting the vehicle. If the fuel is not being burned, it cannot turn into carbon monoxide, (burned fuel)
His HC readings were fine, showing that he basically had good combustion, just too much fuel in the mixture.



Quote:

Were his readings at idle, at elevated rpms?


No one knows for sure. The "Grams Per Mile" test does not break it down into seperate readings. But by his readings, we do know that one:
there is too much fuel being introduced into the engine at some point, causing a high CO reading.
and two:
That it is very probable that the EGR valve is NOT doing its job, for whatever reason, resulting in higher than normal combustion temps, probable pinging and giving a high NOX reading.


Quote:

High combustion temps caused by excessively lean mixtures due to a faulty EGR could also cause poor combustion events causing CO's to increase.


Again, a non-functioning EGR valve will not result in a lean mixture. It will, however, result in higher than desired chamber temps, which will increase the chance of pinging and elevated NOX readings.
Again, poor combustion will not raise the CO levels, it will raise the HC levels. ( poor combustion is the incomplete burning of the mixture, resulting in un-burned fuel, "hydrocarbons", leaving the exhaust system and giving a high HC reading.)

Quote:

There are a lot of variables to this kind of stuff, it's not always as simple as 2+2=4.


Yes, there are alot of variables in this, but without knowing where the rich mixture is comming from, ( idle or cruise), its very hard to give the proper advice.
My suggestion to try to lean out the idle mix was given because that is the simplest and easyest way to affect a change in CO without going into the carb, if so equip't.

Quote:

I agreed with the possibility of a faulty EGR as a possible cause but not because it causes a rich condtion. Rich conditions lower NOX readings


Again, I never stated that a faulty EGR will cause a high CO failure. I stated that he had 2 different problems, both resulting in the vehicle failing the test.

While its possible that a richer than normal mixture may result in a slightly cooler combustion chamber and lower NOX readings, at that point you would have too high of a CO reading, failing the test again.
Maintaining proper emissions is a balancing act between having just barely enough fuel in the engine to have COMPLETE combustion,(acceptable CO and HC readings.) but fighting high chamber temps and pinging at the same time.


My suggestion is to take your car to a shop that has a 2 gas analyzer and watch the CO reading as you open the EGR valve at idle. As soon as its opened, the CO will go down and the HC will go up. The mix is leaned/diluted and the amount of burned fuel goes down, ( low CO) and at the same time, the mix is too lean/diluted to COMPLETELY burn, giving you high HC readings, (raw un-burned fuel)

Mark

Last edited by Jmark; 10-11-2003 at 05:37 PM.
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