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Air Care News
October 2001

Denver metropolitan area on verge of meeting all 
federal air pollution standards

A third consecutive summer of forecasting high concentrations of ground-level ozone pollution concluded last month with 13 ozone alerts issued between June 1 and September 15.

Preliminary results suggest that the Denver metropolitan area will be in compliance with the one-hour federal ozone standard at the end of this year. If so, Denver would be in compliance with all National Ambient Air Quality Standards.

"This is a real victory for our community," said Christopher Dann, Public Information Officer for the Colorado Department of Public Health and Environment. "We are moving forward with plans to ask the federal government to designate the Denver-metropolitan area and all other remaining areas of the state that have carried a dirty air label as ‘attainment’ areas. Within a couple of years, the whole state should receive a clean bill of health from the U.S. EPA."

Ground-level ozone has presented a tricky problem for air quality planners since 1997, when the EPA announced a new, tougher standard. Although legal wrangling that ensued after the 1997 announcement means the new standard currently is not enforceable, regulators have known for years what criteria the Denver metropolitan area would have to meet to be in compliance.

A particularly bad summer in 1998 put the area out of technical compliance with the standard, which is based on a three-year average of readings. The summer of 2001 will replace 1998 in the three-year calculations at the end of the 2001 calendar year, and indications are that the area will move back into compliance.

Ozone is a lung irritant that is formed when hydrocarbons and volatile organic compounds react in the presence of sunlight. Ozone is a problem during the summer months because of the warmer temperatures and other atmospheric conditions favorable for ozone formation.

Ground-level ozone is not to be confused with the naturally occurring ozone layer high in the atmosphere that protects the earth from the sun’s harmful rays.

The Colorado Department of Public Health and Environment and the Regional Air Quality Council have coordinated ozone education and outreach activities since the summer of 1999. Activities include the ozone alerts, gas cap testing and replacement fairs at area employers, and the "Put A Cap On Ozone" program at emissions testing facilities in the enhanced area during the summers of 2000 and 2001.

Thousands of Denver metropolitan area motorists received free gas caps this summer and last to replace faulty ones, courtesy of Envirotest Systems, Corp. and NAPA Auto Care. A passing gas cap has been a mandatory part of the emissions testing cycle in the enhanced area since June 2000.

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Dirty MAF Sensor = High NOx
by Ron Leavitt, Emissions Technical Advisor, British Columbia AirCare Program

Mass Air Flow Sensor (Hitachi Style)

The Hitachi style MAF sensor (figure1) is used on the GM 3800 and 3300 engines, Ford products, some Audis and some Isuzus. It is a hot wire type sensor, which uses a platinum wire with a glass-ceramic coating over its center section. It does not use a burn-off cycle on vehicle shut down, which may be why it has problems with contamination.

Figure 1: Hitachi MAF Sensor

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It consists of a screen to break up air turbulence, a thermistor to measure the temperature of the incoming air, a heated element and electronic module (figure2). A small passage inside the sensor directs part of the airflow past the thermistor and the heated element. The module measures the electrical current required to maintain the heated element at a constant temperature.

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Figure 2: Sensor Elements

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The output signal may be analog (Ford) or digital (GM) which is then converted by the PCM into a mass airflow rate measured in grams per second.

MAF Sensors (Bosch Style)

The Bosch style system is used on 5.0L and 5.7L Firebirds, Camaros and Corvettes, most European cars and some Asian imports. This design of sensor uses an exposed platinum sensing wire and a "burn-off"

Diagnosing contaminated mass airflow sensors

MAF sensors can get contaminated from a variety of sources -- dirt, oil, silicone, spider webs, potting compound from the sensor element itself, etc. When an MAF sensor gets contaminated, it skews the transfer function such that the MAF overestimates airflow at idle (causes the fuel system to go rich) and underestimates airflow at high airflow (causes the fuel system to go lean). This means that Long Term Fuel Trim will learn lean (negative) corrections at idle and rich (positive) corrections at higher airflow. If a vehicle is driven at WOT (Wide-Open Throttle) or high loads, the fuel system normally goes open loop with a rich mixture to provide maximum power. If the MAF is contaminated, the fuel system will actually be

Real World Example: 1993 Buick Century 3.3 Liter

This vehicle was booked in for diagnostics following a customer complaint. The catalytic converter had been replaced by the repair shop, but this had not corrected the high NOx emissions.

The first clue this was a lean A/F ratio problem was the final test result, which was very similar to the retest result of .1056 g/km [0.17 g/mi] HC, 1.6447 g/km [2.65 g/mi] of carbon monoxide and 2.5314 g/km [4.08 g/mi] of NOx. The clue here is that the NOx was actually higher than the carbon monoxide!

The second clue was the Diagnostic Trace Report (figure 4). If you look at the sections of the driving trace where accelerations occur, you will see the NOx goes high and carbon monoxide remains low. This is just the opposite of what you should expect! There will usually be some degree of enrichment and high carbon monoxide during accelerations to improve throttle response and reduce NOx emissions.

The third clue was Long Term Fuel Trim (LTFT) at high idle. I hooked up my scanner and checked fuel trim and airflow at exactly 2500 RPM while also observing the O2 sensor on my lab scope. I noted the indicated airflow for future reference which was 14.1 g/second.

At this point it was time to stop checking and start repairing. Removing the MAF sensor on the Buick is so easy, it's comical. Just remove three screws and the sensor module is in your hand. A close visual inspection showed there was a fuzzy build-up on the sensing element. This required a bit of scrubbing with a soft toothbrush and some throttle body cleaner. Just spraying the sensor wire did not remove the fuzzy build-up that appeared to be small fibers from the air filter. I followed this with some brake cleaner and compressed air to make sure there was no residue on the sensor that would attract more dirt.

With the sensor module reinstalled, it was time to see if the cleaning improved the transfer function of the sensor. Reproducing the test conditions from my baseline, I took the RPM to exactly 2500 again. The indicated airflow was 17.6 g/second, an increase of almost 25 percent!

The long-term fuel trim values were now close to the nominal 128 value with a reading of 124. A second not 505 test at the Technical Center confirmed the NOx had been reduced by 40 percent and the vehicle fast-passed when taken through the inspection lanes for an official I/M240 test.

Ron Leavitt is an Emissions Technical Advisor, British Columbia AirCare Program

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Clean Cities event features alternative fuel vehicles
by Christopher Dann

A variety of alternative fuel vehicles were on display at Bandimere Speedway in Morrison last month at an event sponsored by the Colorado chapters

Representatives from auto manufacturers and the alternative fuel industry were on hand to provide updates on the expanding infrastructure of alternative fuel stations across the country, plans for new alternative fuel vehicle models, and vehicle tax rebates and incentives.

The morning event concluded with a special "Ride and Drive" on the track at Bandimere Speedway during which participants were invited to drive some of the many vehicles on display.

The event, Alternative Fuel Vehicles: Advancing the Choice, was hosted by the Denver Metro and Weld/Larimer/Rocky Mountain National Park Clean Cities programs.

For more information on to Clean Cities Program, e-mail the Denver Metro chapter at denvercleancities@yahoo.com.

Christopher Dann is the public information officer for the Air Pollution Control Division at the Colorado Department of Public Health and Environment.

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