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Failed emission test


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hey everybody. i have a '92 eldorado with the 4.9L V8. my car is failing emissions by being just a bit too high in the NOx department. the EGR valve is just over a year old, and the vacuum control solenoid is brand new. the catalytic converter isn't even a year old yet, and the oxygen sensors are just over a year old. i would have thought this car was a shoo-in to pass its smog check this year, but the state of Arizona disagrees.

what could make the NOx emissions too high? is there anything to consider other than the EGR system? the check engine light is not on, and there are no codes.

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1992 Eldorado 4.9L

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hey everybody. i have a '92 eldorado with the 4.9L V8. my car is failing emissions by being just a bit too high in the NOx department. the EGR valve is just over a year old, and the vacuum control solenoid is brand new. the catalytic converter isn't even a year old yet, and the oxygen sensors are just over a year old. i would have thought this car was a shoo-in to pass its smog check this year, but the state of Arizona disagrees.

what could make the NOx emissions too high? is there anything to consider other than the EGR system? the check engine light is not on, and there are no codes.

Remove air cleaner duct, lift the throttle lever, you'll see two EGR tubes sticking out under the throttle blades. Clean the carbon deposits inside the tubes with a piece of coat hanger. Clean the bores of the throttle body and the back sides of the throttle plates with a brush and a carb cleaner. Disconnect the negative cable for a minute to reset default computer settings (for clean throttle body). After the procedure your engine should run better and you should pass emission if the EGR valve and solenoid and all the vacuum lines are fine.

Too make sure the EGR passages are fine, start the engine, let it idle in park, press with fingers the EGR valve diaphragm down. If the engine slows down and stalls, your EGR passages are clean enough.

To test EGR valve on engine, pull the vacuum line off. Press the diaphragm down as far as you can and block the vacuum port of the valve with finger. If the diaphragm is fine it should not move back until you take your finger off.

The saddest thing in life is wasted talent

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A Google search turned this up.

NOx emissions are the result of high combustion chamber temps pure and simple. NOx readings go up whenever there are hot spots that exceed about 2400 degrees. Things like high compression and lean mixtures contribute to high chamber temps, as does over-advanced ignition timing.

There isn't much we can (or want...) to do about the high compression pressures in the 928 engine. Lean mixtures are usually caused by some failed components or by air (vacuum) leaks in the intake. Ignition timing on the S4 is managed by the brain trust in the passenger footwell, and isn't adjustable except by some re-mapping of some lookup tables in the controllers.

With these three mechanical things managed, some other factors contribute. Poor fuel quality contributes to detonation, a favorite cause of high chamber temps. Got a load of pretty poor gas? That will do it easily. Worn plugs allowing a little misfire? Not a problem unless the worn electrode is glowing and causing early ignition, and none of us true enthusiasts would leave old plugs in the engine. Oil contamination of the intake charge can cause an effective reduction in octane rating of the fuel. Adding a little exhaust gas back into the intake has the opposite effect by the way, effectively reducing the charge density in the given volume in the combustion chamber.

"The mechanic said high NO usually points to a bad EGR or cat".

The cat depends on some reasonable operating temps to get a good reaction, and a bout of sitting in traffic or waiting in line for a pass at the rollers might allow the cat to cool enough to stop the NOx conversion process. The principle component that heats the cat is HC, where the raw fuel that passes through the engine unburned finally gets a chance to do some work heating the rest of the catalyst media. With the excellent engine management and the feedback loop from the oxygen sensor keeping the HC and CO in the pipe to a minimum, the NOx reaction is probably reduced.

Now you said that the EGR relatively new. Did you rod out the EGR tubes down in the TB?

EDIT: Oops! Adallak beat me to it.

By the way, if you pull battery power, do not retest for a few day till the readiness flags reset in the computer. They already figured out we'd clear the DTC's right before testing.

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