Pre-Conditioning

It's An Essential Part of Enhanced Emissions Testing

The vehicle was a 1993 Saturn with 96,610 miles on the odometer. The vehicle failed the initial I/M 240 emissions test for carbon monoxide (CO) only (32 grams per mile using a 20 gpm emissions test limit). The very next day, the customer took the vehicle into her local repair facility for diagnostics. The repair facility performed an engine diagnosis routine and found nothing wrong with the vehicle. The customer immediately drove the vehicle back for an I/M 240 retest and the vehicle passed (14 gpm and 96,632 miles on the odometer). Needless to say the customer was extremely upset with the inspection process, blaming the inspection lane driver and test equipment for the initial failure.

Inspection and Maintenance programs are intended to separate vehicles with excessive emissions from relatively clean vehicles. However, inadequate preconditioning of some vehicles prior to emissions testing can cause inaccurate or inconsistent test results. It is important to understand that some test-to-test variability is normal with regards to emissions testing of vehicles. There is nothing wrong with results if a vehicle first tests at 56 gpm CO and retests at 49 gpm CO. This is normal test-to-test variability. However, it is an issue when a vehicle initially tests at 32 gpm CO and then passes on the retest with no repairs performed. These types of vehicles are often labeled as "false-fail" vehicles.

What factors can explain the test-to-test variability in these cases?

The test lane driver? It is easy to blame the lane driver, but in fact, the driver is a small factor in test-to-test variability. During the transient test, each driver has a small mile per hour (mph) "window" in which to drive each vehicle. That is not to say that you will not get some test variability between different drivers. Some drivers can drive the trace a little better than others. Overall, however, the difference in test readings is relatively small.

The test lane equipment? Again, it is easy to blame test equipment. The State of Colorado requires Envirotest Systems (the contractor) to constantly monitor equipment operation and perform gas calibration on a routine basis (hourly, daily, monthly) following a strict protocol and holding the equipment to tight calibration tolerances. Lanes that do not meet the "quality assurance" protocol are shut down. An extensive quality assurance program is also performed on each lane by specially trained Department of Revenue personnel. If the equipment is a variable, it is very small.

So what is the greatest variability in test-to-test results? It is the vehicle itself! The vehicle is constantly changing before and during testing. These changes are due, in part, to a number of factors, such as:

1. Excessive wait time, accompanied by an extended engine idle time:
Excessive wait time (accompanied by an extended idle time) may force some vehicles to go into an open-loop, fuel-rich strategy. This computer control strategy is designed to protect the catalytic converter by flooding it with fuel, thus rendering it inoperative. The converter will be virtually inoperative until the vehicle re-enters a closed-loop, fuel-control operation strategy.

2. Excessive wait time with engine off:
If the vehicle has sat in the waiting area for an extended period of time with the engine off, a number of things may have occurred within the vehicle itself:

Catalytic converters lose their heat. The converter is the most effective emission control device on a gasoline-fueled vehicle. Converter operation is based on heat. During an engine-off period, engine exhaust temperatures drop and so do converter temperatures. Converter temperatures may drop so low that the converter literally stops functioning. If a vehicle has high mileage and a marginal converter it may take an extended preconditioning period to get the converter to light-off and become operational.

Vapor canisters become charged with fuel vapors. Canister charging with fuel vapors is a normal function, especially during hot ambient conditions. When does the canister release the stored fuel vapors to be burned by the engine? Many computer control strategies are employed by manufacturers to purge the canister. Most computer-controlled vehicles are programmed to purge the evaporative canister during an off-idle, transient driving condition, such as during the I/M 240 test. Canisters that are loaded with a significant amount of gas vapors can enrich engine operation, causing higher than normal carbon monoxide and hydrocarbon (HC) readings. It may take a long, extensive driving pattern by the customer (or service facility) to fully evacuate a charged canister.

Crankcase dilution can be another source of fuel vapors during transient testing. Extended oil change intervals, overly rich mixtures or a ruptured fuel diaphragm can contribute to crankcase dilution. This is especially true during the cold weather season, when vehicles may have extended warm-up intervals.

3. Insufficient operating temperature -- cold start enrichment:
Virtually all gasoline-powered vehicles are designed to use a rich air-to-fuel ratio during cold engine operation. If the engine is not fully warmed-up before entering the emissions testing sequence, high CO and HC emissions can result. Lane inspectors are trained to look at a vehicle's temperature gauge (if equipped) to confirm engine temperature before testing. However, this does not assure adequate converter or engine temperatures.

The State of Colorado has tried to address the test-to-test variability issue in two specific ways:

By establishing a vehicle precondition protocol or procedure. In Colorado, a "second chance" test is performed when a vehicle fails the initial test by less than two times the emissions test limits (or standards). In other words, if the emissions test limit for CO is 20 gpm and the vehicle initially tests at 32 gpm, the vehicle gets a second chance test. The initial emissions test was used to precondition the vehicle. However, in the case of a charged vapor canister in combination with a high mileage, marginal converter, the initial test may not be enough time to fully precondition a vehicle. Therefore, this vehicle may fail the initial test and pass the retest with no repairs performed.

There is also a secondary benefit to a second chance test -- the driver gets to "learn" the vehicle and the driver typically does a better job of following the second driving trace.

By establishing emissions test limits that take into account such things as vehicle test-to-test variability and "marginal" vehicles. Marginal vehicles are those vehicles that typically have high mileage and marginal catalytic converter operation. The tighter the emissions limits, the greater the number of marginal vehicles that fail.

Should a service facility perform diagnostics on a vehicle if they suspect that the vehicle has been falsely failed? Yes, absolutely! Diagnostics is never a waste of money. Diagnostics gives the customer and technician a better idea of vehicle condition and often does identify a problem with the vehicle. The issue is that most diagnostic procedures used by the service repair industry do not go far enough and should include dynamic diagnosis procedures; i.e., driving computer check and a catalytic converter "functional" check. The days of static, in-the-stall, diagnostics are long gone, especially when diagnosing a transient failure.

False-failing vehicles are part of I/M testing programs, but they must be keep to a minimum. The State and Envirotest have gone to great lengths to address this critical vehicle testing issue. It is important for test results to be consistent and accurate. However, when you combine all the variability factors with a marginal, high mileage converter, it becomes easier to understand how a vehicle can fail the initial test and pass a retest without receiving any repairs.

Written by Gary Cagle, Ph.D., Colorado Department of Public Health & Environment, Air Pollution Control Division, Mobile Sources Section.

Back to Fix It! page.

For more information, call the Air Care Colorado Hotline at 303-456-7090. Se habla Español. Site updated 1/31/03.

Site created by Envirotest Systems Corp., a division of Environmental Systems Products Inc. ©1998-2003, Envirotest Systems Corp. All rights reserved.
Feedback, questions & site problems: email Webmaster