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Automotive Industry

Trouble Shooter

Motor, Sep 2005 by Seyfert, Karl

NO^sub x^ was the last addition to the list of monitored exhaust gases. Misconceptions regarding the conditions that promote its formation still exist.

Cracking Fort NO^sub x^

I am working on a 1995 Jeep Wrangler with a 4. OL engine that has failed to pass a smog test due to high NO^sub x^. The engine has an afiermarket header and, a Flowmaster exhaust system, which were installed by the previous owner. The tires also are larger than stock. It has no EGR valve and tlie timing us not adjustable. The vehicle is originally Jrom Canada, but it has a BAR sticker on it, indicating it has met the emissions re(juirements for our state.

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I tried running water through the intake to clean the carbon out of the cylinders. I also installed a NO^sub x^-reducing cat. After these changes, it barely passed the 15-mph emissions test and still failed- the 25-mph test for NO^sub x^.

The exhaust readings at cruise are as follows: CO, .01%; HC, 12 ppm; CO2, 15.0%; NO^sub x^, 917 ppm.

I'm thinking about restricting the exhaust because the system that's on the vehicle has very low backpressure. The customer has no driveability complaints and there are no diagnostic trouble codes stored in memory. Finally, I was told by another shop about an article in MOTOR concerning a jeep with too much NO^sub x^. I think the fix was to enlarge the Hall effect generator inside the distributor, but I haven't been able to find the article.

Dan McAuliff

San Jose, CA

First off, Dan, I believe the article you're referring to is actually three Driveability Corner columns written by Mark Warren. The first appeared in the April 2003 issue, and was followed by February and April of 2004. While the columns did deal with a driveability problem on a Jeep 4.OL, the problem was entirely different from what your customer is experiencing.

Second, a lack of backpressure caused by the aftermarket exhaust system should not cause an increase in NO^sub x^, at least not on a vehicle that's capable of adjusting its own air/fuel ratio. Let's consider the things that can cause NO^sub x^ to increase to an abnormally high level. Oxides of nitrogen (NO^sub x^) are formed when the combustion chamber temperature exceeds 2500°F (the ignition point for compressed nitrogen). The engine must normally be under some kind of load before it develops such high combustion chamber temperatures.

Many emissions systems rely on an EGR valve to lower combustion chamber temperatures. Adding a small amount of inert exhaust gas when tlie engine is under load lowers the temperature in tlie combustion chambers just enough to reduce the formation of NO^sub x^. I've always thought this mediod was similar to feeding manure to a horse. Automotive engineers must agree because many vehicles now manage to control NOx without the help of an EGR valve. The Jeep you're working on is one such vehicle.

But everything else must be working properly if the EGR valve crutch is removed. As you correctly surmised, combustion chamber deposits can artificially raise the compression ratio and encourage the formation of NO^sub x^. Overaclvanced ignition timing can also cause NO^sub x^ formation. Since the Jeep does not have an externally adjustable ignition system, we should be able to rule it out as a possible cause in this case. Jury-rigging the ignition in an attempt to retard the timing is not an acceptable solution.

Check with the owner to make sure the PCM hasn't been "chipped" or modified in some other way. One quick and dirty way to increase engine performance is to change the spark advance curve. This used to be accomplished by changing the advance springs in the distributor. Now it's done by changing the PCM's timing decisions. Too much advance at the wrong time, caused by an aftermarket chip modification, could be responsible for the excessive NOx problem.

A lean air/fuel mixture can also encourage NO^sub x^ formation. The lean mixture can be caused by insufficient fuel pressure or volume, or the engine management system may actually be commanding the mixture lean. This could occur if the PCM is receiving bogus information from an oxygen sensor that has a lazy response rate or has become biased rich. Your emissions readings didn't include an Oo reading, which would have been helpful in determining whether the engine is running rich or lean under load.

Be sure to consult Sam Bells article on lambda in this issue ("What Goes In..."). The lambda calculation is a great way to determine whether an engine is running rich or lean, without having to concern yourself with other influences.

Last of all, an engine that's running hotter than normal will encourage NO^sub x^ formation. A faulty thermostat or a clogged cooling system could be keeping the Jeep's operating temperature just shy of overheating. Adding a load pushes the combustion chamber temperature to the point where NO^sub x^ formation increases to an unhealthy level. The engine should be producing almost no NO^sub x^ at idle. If the NO^sub x^ reading is high at idle, run through the NO^sub x^-influencing factors I've listed until you find the cause.

Automotive Industry

Trouble Shooter

Motor, Sep 2005 by Seyfert, Karl

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