IM Testing Procedures

Inspection/Maintenance (IM)

The advances made in engine management and emission control technology over the past two decades have resulted in the steady decline of vehicle pollutants. Unfortunately, while HC, CO and NOx output per vehicle is lower than ever before, emissions reductions have been offset by the extraordinary increase in the number of cars and trucks on the road. However, the vehicle pollution problem is not as much a result of increased volume as it is a consequence of greater numbers of maintenance neglected vehicles. Inspection/Maintenance (IM) programs help weed out these vehicles by subjecting them to an annual emissions test. According to the requirements of the 1977 Clean Air Act, implementation of IM programs is compulsory in those areas of the United States unable to meet the National Ambient Air Quality Standards (NAAQS).


The two-speed idle test is a no load check of a vehicle’s hydrocarbon and carbon monoxide emissions. The test is intended primarily for checking vehicles with all-wheel drive or full-time four-wheel drive, since these systems cannot be operated on a dynamometer. In addition, vehicles with traction control systems may be given a two-speed idle test if the system cannot be deactivated. In some states, two-speed idle testing is used to check 1983 and older vehicles, as well as 1982 and newer trucks with a Gross Ve­hicle Weight Rating (GVWR) above 8500 pounds. As the name suggests, the two­ speed idle test is performed at different engine speeds while the vehicle is idle (no load applied). During the high-speed phase of the test, the engine is operated between 2200 and 2800 rpm for 30-90 seconds. This is followed by the low-speed mode, in which engine speed is reduced to a maximum of 1100 rpm and the vehicle is tested for an additional 30 seconds. In the event the vehicle fails either phase of the initial test, a ‘second chance’ test is administered based on the ‘first chance’ test results . For example, if the vehicle fails the initial high-speed test but passes during the low-speed phase, the engine will be tested again at high speed. The two-speed idle test is performed using a four-gas analyzer. Gas readings are recorded in the fractional amounts of parts-per-million (ppm) for HC, and percentage (%) for CO, C02 and 02. The test is automatically aborted if the engine stalls out, or the CO and C02 readings combined equal less than six percent. If the test is cancelled due to the latter, it indicates that one of the following conditions exist:

  1. The exhaust probe is not fully inserted into the tailpipe
  2. Air is being drawn into the exhaust
  3. Ehaust system (leak)
  4. The air/fuel ratio is too lean


Acceleration Simulation Mode is a steady-state, loaded mode test, in which the vehicle is checked for hydrocarbon, carbon monoxide and oxides of nitrogen emissions. The test is performed with the vehicle running on a variable weight dynamometer at a steady speed under a pre-determined load. Since the energy required to overcome inertia varies according to vehicle weight, the variable weight dyno matches the load (roller resistance) to the vehicle’s gross weight. The heavier the vehicle, the greater the applied load. The ASM test can be considered a distant relative of the Federal Test Procedure (FTP). During one phase of the FTP, the vehicle is required to accelerate at 3.3 mph per second. To achieve this rate of acceleration, the dynamometer applies a specific load to the vehicle based on its GVWR. For an ASM test, the rate of acceleration is far less than that of the FTP. Consequently, the applied load is also less. The two most common ASM tests are the ‘5015’ and the ‘2525.’ For the 5015 test, the dynamometer applies a load necessary to limit the vehicle’s acceleration rate to 50% of the FTP standard of 3.3 mph per second. Vehicle speed is limited to 15 mph during the test. In an ASM 2525 test, the applied load is reduced to 25% of the FTP standard, but vehicle speed is increased to a constant 25 mph. Like a TSI test, acceleration simulation uses a partial sampling method for measuring emission levels. Consequently, HC and NOx are displayed in parts-per-million, while CO, C02 and 02 are expressed as a percentage.


The gas analyzers used for two ­speed idle and acceleration simulation tests use ‘partial stream sampling’ to measure exhaust emissions. With this method, the exhaust sample is evaluated based on gas concentration levels. For example, HC and NOx readings are based on the exhaust sample being divided into one million parts. Therefore, the ppm reading shows how concentrated the gas sample is with either pollutant. Percentage readings essentially indicate the same thing using a different value. However, neither ppm nor percentage indicates the actual mass of pollutants being discharged into the atmosphere. Moreover, the sample itself is only representative of the vehicle’s total exhaust gas output. To illustrate this, consider two vehicles; one equipped with a 3.0L V6 and the other with a 6.0L V8. Let’s say that the CO reading for each vehicle is 2%. Although the CO readings are identical, the vehicle with the larger displacement engine is actually emitting twice the amount of carbon monoxide into the atmosphere.

I/M 240

The I/M (Inspection/Maintenance) 240 test provides the most accurate indication of a vehicle’s emissions output. This 240-second transient test not only measures the quantity of emissions under various driving conditions, but also uses a technique called Constant Volume Sampling to maintain a steady gas flow throughout the entire four minute period. Because emissions are measured by weight, results are indicated in ‘grams-per-mile.’ The IM 240 drive cycle is a combination of idling, acceleration, cruise, deceleration and braking . At the conclusion of the test, a detailed printout plots the test results against the backdrop of the drive cycle pattern. This allows technicians to see the vehicle’s emission output during each one-second interval of the test. For a technician diagnosing the cause of a vehicle failure, this information is an invaluable resource.


Depending on the location, enhanced emissions tests may include a check of the vehicle’s evaporative emissions control system. Purge flow is measured during the dynamometer sequence to determine the volume of fuel vapor removed from the canister. As long as the volume exceeds state requirements, the vehicle passes the purge test. The EVAP system may also be checked for leaks. This is accomplished by pressurizing the system to 0.5 psi (14 inches of water) with nitrogen. Once pressurized, the equipment monitors the rate of pressure drop. A pass is recorded as long as the pressure remains above 8 inches of water following the two-minute test period. In some states, the gas cap is also checked for leaks with a tool resembling a cooling system pressure tester.

This post was written by: Martin Hand


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Martin Hand

About Martin Hand

ASE Certified L1 Advanced Mastertech. Martin Hand has over 15 years experience in Asian and European Import Auto Repair. Specializing in electrical diagnosis, engine performance, AT/MT transmission repair/rebuild. Martin is also pursuing a degree in Computers Science & Information Systems starting at Portland Community College while he plans to transfer to OIT. Certified in Java application level programming, experienced with other languages such as PHP, Ruby, JavaScript and Swift. Martin has future plans of automotive diagnostic software development.

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