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Air Care News
October 2000

Gas cap replacements lead to emissions failures?
by Doug Decker

In June 2000 gas cap inspections became a mandatory part of the emissions testing process. With the new standard came some new emissions failures.

Evaporative hydrocarbons (HC) are not something we have ever measured, since HC is not emitted from the tailpipe. It is, however, easy to estimate based on fuel volatility, ambient temperatures, and the number of vehicles with bad caps. By one estimate, 60 percent of all fuel evaporative systems failures are caused by bad gas caps.

So what's the impact for the repair shop? For the most part, none. Of all the vehicles that failed for faulty gas caps and had replacements installed, almost all of them passed their "after repairs" (gas cap replacement) inspections.

This is what you'd expect. If only the gas cap seal failed and the rest of the evap system was intact, replacing the cap should return the evap system to fully functional status, causing mild enrichment for a few minutes during and after warm-up as the canister purges off.

This mild enrichment is measured by the O2 sensor and compensated for by fuel trim. Once the canister is purged of the stored HC, the purge function may or may not continue (depending on the strategy), with fuel trim taking up the slack.

The sharp reader will note that I said "almost all of them passed..."

In the first month, there were 27 vehicles that initially passed the tailpipe and failed the gas cap check test, which then failed the tailpipe after having the gas cap replaced.

First, let's put this in perspective: out of approximately 3,000 gas caps replaced in 30 days, less than one percent had further problems.

Twenty-six of these vehicles failed for CO and one failed for HC after their gas caps were replaced. This should come as no surprise, considering the relatively passive nature of the evap system (we rarely deal with it) and the fact that it intentionally causes enrichment and enleanment. These were passenger cars and light/medium trucks between the model years 1983 and 1994.

Let's look at the common failure modes of a fuel evap system.

Open system (gas cap missing or system leak)

Without slight pressure build in the fuel tank, vapors escape through the filler neck and into the atmosphere. The system relies on tank pressure to force fuel vapors into the canister. Without a sealed gas cap, tank and vent lines, the canister will not load.

System will not purge

If the system will not purge, it usually will not create a symptom. Even if there is HC in the canister, it will not get to the intake to be burned. Usually, there are no emissions or driveability effects, but a complaint of fuel smell. With a sealed system, the canister soon becomes saturated with fuel. Potential causes for lack of purge include: control system problems; purge line leaks; restrictions to purge lines, canister or fresh air supply; and kinked/misrouted lines.

System purges all the time

If a malfunctioning purge valve or control circuit causes constant purging of the canister, it means an uncontrolled vacuum leak, which may become an uncontrolled fuel source (you GM carbureted experts know all about this one!). Drive-ability and emissions symptoms will vary, but with an open system (bad cap), the canister never loads.

For the purpose of our discussion, let us consider the results of closing an open system in conjunction with the purge failures. This is our situation with the 27 cars that had caps replaced and then had a resulting emissions failure.

If an open system will not purge, then the same system will also not purge when closed. We have no failure until the canister saturates and starts to smell up the garage. The system that will not purge has closed a calibrated vacuum leak and this enrichment should be reflected in long term fuel trim.

If an open system purges all the time, it will usually purge only fresh air because there is no pressure build to force the vapor into the canister. This enleanment may be compensated for by fuel trim, but likely will be compensated for in other creative ways as well (like BBs, screws or misrouted vacuum lines). When this system is sealed with a new gas cap, it becomes an unregulated and intermittent fuel source that will vary the mixture. Again, fuel trim may or may not be able to compensate, but the BBs and screws are really starting to work against it now. It may create excess CO on warm-up until the canister purges out and then revert to a lean condition afterwards.

So, what was really wrong with the 27 vehicles? Based on analysis of second-by-second emissions data on back-to-back I/M 240 tests, most of the 27 vehicles were purging properly, but had multiple problems and were "marginal vehicles" to begin with. The majority of these vehicles had passed tailpipe tests and failed cap pressure checks at 12-25 grams CO. When the cap was replaced most failed tailpipe at 20-35 grams CO. There were a few anomalies, but marginal vehicles were the trend, with additional fuel from proper purging pushing them beyond what fuel trim could control and over the I/M standards.

These vehicles were all repaired through normal channels and eventually passed, but no repair data or repair cost information was recorded for any of the 27 vehicles (ahem!). Consequently, we were not able to determine the extent and cost of the actual repairs.

Based on conversations with some of the vehicle owners and technicians we contacted, our suspicions were confirmed that repairs involved multiple systems and components, generally not the evap system, and in some cases, repairs were expensive.

So, the short answer is that, yes, a new gas cap could cause a tailpipe emissions failure -- but only if the car was already broken. Remember that just because a vehicle passes, doesn't mean it's clean.

Fix the problem, not the symptom!

Doug Decker is an environmental protection specialist at the Colorado Department of Public Health and Environment.

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Study demonstrates cost-effective repairs can reduce emissions

This article was excerpted from The Analyzer; the Wisconsin Department of Transportation's repair industry newsletter. Wisconsin has an I/M 240 emissions program similar to Colorado's.

Wisconsin's Gateway Technical College, under the direction of automotive instructor Steve Kukawka, recently completed a study that examined the repair and retest process of a specific engine family of vehicles. The main purpose of the study was to identify effective repair strategies for a common group of vehicles which the aftermarket repair industry works with on a frequent basis.

Gateway received a contract from the Wisconsin Department of Natural Resources to conduct the study. Repairs to the target vehicles were performed by automotive repair facilities employing WISETECH (Wisconsin Emission Technician) training program [equivalent to Colorado's FIRST& EDGE training] graduates. The study, funded through a federal Congestion Mitigation and Air Quality Improvement Program (CMAQ) grant, provided 80% of the repair costs of vehicles in the study, up to a maximum of $450 per vehicle [both Colorado and Wisconsin have a repair limit of $450.]

Study Objectives

Obtain pertinent emissions repair frequency data.

Evaluate the effectiveness of the WISETECH training program.

Provide a basis to determine Wisconsin's policy on future emissions limits and repair waivers.

Selected Group of Vehicles for Study

Analysis of Wisconsin Vehicle Inspection Program emissions limits and repair waiver failure data for light duty gasoline vehicles (LDGV) identified General Motors as the manufacturer of a majority of failing vehicles. This is due to the heavy distribution of General Motors vehicles in the fleet as compared to other manufacturers.

Other analysis identified two engine families as having high initial test and high retest failure rates. The first is the General Motors Oldsmobile carbureted 5.0L (307cid) VIN "Y" engine with Computer Command Control (CCC) installed in full size vehicles from 1980-1990. The second engine family is the General Motors Chevrolet carbureted 5.0L (305cid) VIN "H" engine with (CCC) installed in full size vehicles from 1981 through 1988. Both engine families were factory-equipped with consistent fuel systems and computer system technology throughout the model year ranges.

A total of 64 VIN "Y" vehicles were selected and 17 VIN "H" vehicles were selected for the study.

How the Study Worked

Motorists were informed of the study either through the group of repair facilities or at the emissions testing facilities on the south side of Milwaukee, Racine and Kenosha. These vehicles failed the initial emissions test. The vehicle owners took their cars to one of the participating repair facilities. Vehicle diagnosis was then performed utilizing the WISETECH diagnostic practices. If a vehicle had an engine mechanical problem, it was excluded from the study.

Once these vehicles received initial repairs, they were retested. The vehicle was given back-to-back full trace emissions tests at that time. If the vehicle failed this retest, it was returned to the repair facility and the catalytic converters were replaced. The vehicle then received additional testing. The vehicle's repair data and testing data were then compiled and analyzed.

Study Conclusions

Repair effectiveness has a direct link to vehicle retest pass rates

The study demonstrated that the effectiveness of aftermarket emissions repairs is a determining factor in the increased level of emissions test passes in southeastern Wisconsin. The group of repair facilities utilized in the study, which employed at least one WISETECH graduate, achieved a 100% I/M 240 pass rate in study vehicles at phase-in cutpoints and a 95% pass rate at the current cutpoints. This is a good indication that the procedures and techniques covered in the WISETECH curriculum are effective.

Catalyst replacement one of most effective repairs

The study also demonstrated that the catalyst replacement is necessary to achieve a high retest pass rate. Prior to catalyst replacement, less than half of the study vehicles passed the I/M 240 test at the current cutpoints. After the catalyst replacement the pass rate increased to 94% without the recommended NOx pass/fail cutpoints, and 84% passing with NOx pass/fail cutpoints in place.

Preconditioning vehicle improves emissions

Preconditioning the vehicle significantly lowered the I/M 240 emissions readings. The reductions achieved with preconditioning were greater for HC and CO than for NOx. All pollutants were reduced significantly after catalyst replacement.

Motorists typically decline repairs past $450 waiver cost limit

Many motorists want to spend the minimum on their vehicle to pass the inspection. Subsequently, if the waiver amount ($450) has been spent, the repairs stop and the application for a waiver continues. In most cases, to complete the repairs, the catalytic converter needs replacement. The cost for replacement puts the total repair cost well in excess of the waiver amount and the repair is often declined in favor of a waiver.

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Training to improve inspections
by Tom Myrsiades

The Colorado Department of Public Health and Environment (CDPHE) and the Colorado Department of Revenue (DOR) continue to receive feedback from the repair industry and consumers concerning the importance and accuracy of the visual portion of the emissions inspection. When a customer applies for a waiver they have generally failed at least two emissions inspections, endured at least one trip through a repair facility and made the necessary arrangements to meet with a DOR representative to receive a waiver. In the process of applying to DOR for issuance of a waiver, vehicles will be denied a waiver when tampered with and/or missing emissions components are observed. You can only imagine how frustrated the customer becomes on hearing this.

A continuous effort to improve the inspection process is a never-ending job. Ideally, we expect every inspection to be performed exactly perfect every time. However, perfection is always an elusive goal, consequently we will always be in pursuit of excellent performance.

To that end, the departments have developed a remedial inspector training class. This training is primarily directed toward inspectors who have missed a covertly tampered vehicle on visual inspection. The inspector may be required to attend a four-hour class known as "Inspector Remedial Training." The cost of the class is approximately $50 per student and must be paid prior to the class. Training criteria includes an overview of the visual inspection components, purpose, operation, configuration, location and much more. The definition of emissions tampering, vehicle certification and elements of design are very informative.

Earlier this year a company with several licensed inspection and readjustment stations from the Basic emissions area agreed to enroll and pay for all of its inspectors to take the class. The training is a success and feedback has been positive.

Any time you are repairing an emissions failure, be sure to verify that emissions equipment is present and operational.

When you find improper inspections on vehicles, please take a few minutes to contact the DOR, 303-205-5672 or e-mail.

We have a training program that will help.

Tom Myrsiades is an emissions compliance supervisor with the Colorado Department of Revenue.

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