ASE F1 Compressed Natural Gas Vehicles

Before servicing a CNG vehicle, what is the FIRST step a technician should take during a pre-service vehicle inspection?

• AVerify the vehicle and CNG fuel system are correctly identified and review service history
• BDefuel the high-pressure cylinders immediately
• CReplace the pressure relief device
• DPressurize the system to 4,500 psi for testing

A CNG vehicle's full service (settled) pressure for a light-duty NGV fuel system is rated at approximately:

• A150 psi
• B3,600 psi
• C900 psi
• D10,000 psi

During a pre-service walkaround, a technician notices a CNG cylinder label is unreadable due to damage. What is the correct action?

• AEstimate the manufacture date from the vehicle model year
• BContinue service because the label is not required
• CTreat the cylinder as suspect and require a qualified inspection before further service
• DSand the cylinder surface to reveal hidden markings

Which document standard establishes requirements for the design, installation, and use of compressed natural gas vehicular fuel systems in the U.S.?

• ANFPA 70E
• BOSHA 1910.95
• CSAE J1772
• DNFPA 52

A pre-service inspection of a CNG vehicle should include verifying that the manual shutoff (1/4-turn) valve operates correctly. The primary purpose of this valve is to:

• AIsolate the cylinder(s) from the rest of the fuel system
• BRegulate fuel pressure to the injectors
• CVent the cylinder to atmosphere automatically
• DIncrease storage pressure during fast fill

Which of the following is the correct relative density characteristic of natural gas (methane) that affects pre-service safety planning?

• AIt is heavier than air and pools at floor level
• BIt is lighter than air and rises and disperses
• CIt has the same density as air and stays stationary
• DIt is heavier than gasoline vapor and sinks

During pre-service inspection, mounting brackets and straps securing a CNG cylinder should be checked for:

• AElectrical continuity to the battery
• BCompatibility with gasoline
• CCorrosion, looseness, and proper torque
• DThe presence of a pressure gauge

A CNG conversion vehicle is brought in. Pre-service review shows no EPA or CARB certification documentation for the conversion system. The technician should:

• AIgnore certification because conversions are exempt
• BIncrease the storage pressure to compensate
• CRemove the conversion system without authorization
• DNote the compliance concern and inform the customer before proceeding

Tech A says a CNG vehicle pre-service inspection should include checking for the required CNG fuel-system identification decal. Tech B says the fuel filler receptacle area should be inspected for damage and the proper dust cap. Who is correct?

• ABoth Tech A and Tech B
• BTech B only
• CTech A only
• DNeither Tech A nor Tech B

A Type 4 (all-composite, plastic-lined) CNG cylinder is identified during inspection. Compared to a Type 1 cylinder, a key advantage of Type 4 is that it:

• ACosts less to manufacture
• BIs significantly lighter for the same capacity
• CRequires no inspection during its service life
• DCan be refilled past its expiration date

Match the CNG cylinder type to its construction: A cylinder with a metal liner that is hoop-wrapped with composite only around the cylindrical section is a:

• AType 1 cylinder
• BType 3 cylinder
• CType 2 cylinder
• DType 4 cylinder

The high-pressure regulator in a CNG fuel system primarily functions to:

• AIncrease pressure from the cylinder to the injectors
• BStore fuel during fast-fill events
• CDetect methane leaks in the engine compartment
• DReduce high cylinder pressure to a lower, usable working pressure

A pressure relief device (PRD) commonly used on CNG cylinders is a thermally activated device that:

• AVents cylinder contents when exposed to high temperature, such as fire
• BOpens when cylinder pressure exceeds 500 psi
• CRegulates fuel pressure to the engine
• DCloses automatically to trap gas during a fire

When inspecting CNG high-pressure stainless steel fuel lines, the technician should look specifically for:

• AColor fading from sunlight only
• BKinks, abrasion, corrosion, and secure clamping
• CCompatibility with diesel fuel
• DElectrical resistance under 1 ohm

The fuel lock-off (solenoid) valve in a CNG system is designed to:

• ARemain open at all times for constant flow
• BIncrease injector pulse width
• CShut off fuel flow when the ignition is off or during certain fault conditions
• DCool the high-pressure regulator

A CNG fuel filter (coalescing/particulate) in the fuel system primarily protects against:

• AExcessive cylinder pressure
• BCylinder over-temperature
• CElectrical short circuits
• DContaminants, oil carryover, and moisture reaching downstream components

The CNG fueling receptacle on a U.S. light-duty vehicle is typically a:

• AStandardized NGV1 receptacle rated for the system service pressure
• BSAE J1772 electrical connector
• CQuick-disconnect diesel coupling
• DVented gravity-fill port

During equipment inspection, a check valve in the CNG fill line is found stuck open. The most significant consequence is:

• AEngine will not crank
• BStored high-pressure gas could backflow out of the fueling receptacle
• CCylinder pressure gauge reads zero permanently
• DInjectors will over-fuel the engine

Tech A says CNG injectors are designed to meter gaseous fuel and generally require higher flow capacity than equivalent gasoline injectors. Tech B says CNG injectors should be inspected for proper electrical connection and for leaks at their seals. Who is correct?

• ATech A only
• BTech B only
• CBoth Tech A and Tech B
• DNeither Tech A nor Tech B

A CNG cylinder pressure/contents gauge or fuel-level sender derives the displayed fuel level primarily from:

• AA float inside the liquid fuel
• BThe vehicle's odometer
• CEngine vacuum readings
• DCylinder pressure and temperature (since CNG is stored as a gas)

While inspecting a Type 3 CNG cylinder, the technician notes a deep cut in the composite overwrap exposing fibers. The correct disposition is to:

• ACondemn or have the cylinder evaluated by a qualified inspector per cylinder standards
• BApply tape and return to service
• CReduce fill pressure by half and continue use
• DSand the area smooth and repaint

The U.S. federal motor vehicle safety standard that governs the integrity of CNG fuel containers is:

• AFMVSS 301
• BFMVSS 304
• CFMVSS 208
• DFMVSS 126

A pressure relief device vent line on a CNG cylinder must be routed so that, if it activates, gas is discharged:

• AInto the passenger compartment for monitoring
• BBack into the cylinder
• CTo a safe location, typically outside and upward, away from ignition sources
• DInto the engine air intake

When inspecting CNG system fittings, which thread/connection practice is appropriate for high-pressure gas service?

• AUse standard pipe dope rated for water lines
• BReuse compression ferrules indefinitely
• CUse Teflon tape on every flare fitting to add sealing
• DUse connections and sealants approved for high-pressure gaseous fuel service

A CNG vehicle has multiple cylinders connected by a manifold. During inspection, the purpose of an excess-flow valve in the system is to:

• ARestrict or shut off flow if a downstream line ruptures and flow becomes excessive
• BIncrease fuel flow during acceleration
• CRegulate injector timing
• DHeat the regulator

To perform a leak test on a CNG system, an electronic combustible-gas (methane) detector is generally preferred over soap solution because it:

• AWorks only on liquid fuels
• BCan detect very small leaks and locate them in hard-to-reach areas quickly
• CAdds pressure to the system
• DEliminates the need to pressurize the system

When using a bubble (leak-detection) solution on CNG fittings, the technician should use a solution that is:

• AAny household dish soap mixed strong with water
• BPetroleum-based for better cling
• CSpecifically formulated as a non-corrosive, gas-approved leak-detection fluid
• DA chlorinated cleaner for visibility

Tech A says a CNG leak test should be conducted with the system pressurized to operating pressure. Tech B says all leak-detection bubble solution must be wiped off after testing to prevent corrosion. Who is correct?

• ATech A only
• BTech B only
• CNeither Tech A nor Tech B
• DBoth Tech A and Tech B

A faint odor and a slight hissing are noticed near a CNG high-pressure fitting. The technician's safest immediate action is to:

• AEliminate ignition sources, ventilate the area, and isolate the cylinder valve
• BStart the engine to burn off the leak
• CTighten the fitting while the system is fully pressurized
• DSpray water on the fitting and ignore it

After repairing a CNG high-pressure connection, the technician must:

• ASkip retesting if the fitting looks tight
• BRe-pressurize and leak test the repaired joint before returning the vehicle to service
• CDrive the vehicle at highway speed to confirm the fix
• DPaint over the joint to seal it

The lower flammability limit (LFL) of natural gas (methane) in air is approximately:

• A0.5%
• B25%
• C5%
• D50%

When leak testing, the technician should check the high-pressure side connections in which sequence relative to system pressurization?

• ATest before any pressure is applied
• BTest only the low-pressure side
• COnly test after fully defueling the system
• DPressurize the system first, then methodically test each joint from cylinder outward

A CNG cylinder valve O-ring at the receptacle/fill side is found weeping gas. The correct repair is to:

• AReplace the O-ring with the manufacturer-specified part rated for CNG service
• BAdd more thread sealant on the outside
• CHeat the area to expand the seal
• DIncrease system pressure to force the seal closed

Why is odorization relevant when relying on smell to detect a CNG leak on a vehicle?

• ANatural gas is naturally pungent and always easy to smell
• BPipeline natural gas is odorized with a sulfur compound, but odorant fade and personal se…
• CCNG cannot be detected by any instrument
• DOdorant increases the flammability of methane

A technician performs a system leak-down (pressure-hold) test. A noticeable pressure drop over the hold period with no temperature change indicates:

• ANormal operation
• BThe cylinder is overfilled
• CA leak somewhere in the pressurized system requiring localization
• DThe regulator is functioning correctly

Apparent fuel pressure on a CNG cylinder gauge can change significantly with ambient temperature because:

• AThe cylinder physically expands and contracts dramatically
• BThe gauge needle warps in heat
• CThe fuel chemically reacts with the cylinder liner
• DGas pressure is directly affected by temperature (gas laws), so the same mass shows diffe…

A technician must defuel a CNG vehicle before tank removal. The preferred method is to:

• AUse an approved defueling/evacuation system that captures or safely discharges gas to a p…
• BOpen the cylinder valve and vent gas into the shop
• CDrive the vehicle until the tank is empty
• DDrill a hole in the cylinder to release pressure

After locating and repairing a leak at a compression fitting, the technician should torque the fitting:

• AAs tight as possible by hand and then add two more turns
• BTo the manufacturer's specified torque value using a calibrated wrench
• CUntil the threads strip to ensure sealing
• DOnly finger tight to avoid cracking

During emissions verification, a dedicated CNG vehicle running properly is expected to produce, compared with an equivalent gasoline engine, generally:

• AHigher particulate and higher CO2 per unit energy
• BNo measurable emissions at all
• CLower particulate and typically lower CO2 per unit energy
• DOnly nitrogen as exhaust

The primary octane-related performance advantage of CNG (methane) as an engine fuel is its:

• ALow octane rating, requiring lower compression
• BInability to support combustion
• CTendency to detonate at low loads
• DHigh effective octane rating (~120+), allowing higher compression and knock resistance

A bi-fuel CNG/gasoline vehicle shows a noticeable power drop only when operating on CNG. A common, expected reason for slightly reduced peak power on gaseous fuel is:

• AGaseous fuel displaces intake air, reducing volumetric efficiency
• BCNG has more energy per unit volume than gasoline
• CThe engine cannot run on CNG at all
• DCNG always doubles horsepower

During emissions performance verification, an oxygen (O2) or air-fuel ratio sensor on a CNG engine is used to:

• AMeasure cylinder storage pressure
• BProvide feedback for closed-loop fuel control to maintain near-stoichiometric mixture
• CDetect methane leaks at the cylinder
• DRegulate high-pressure fuel

A CNG engine sets a lean fuel-trim/misfire code that worsens as cylinder pressure drops near empty. The MOST likely cause is:

• AExcess fuel from a stuck-open injector
• BA faulty odometer
• CFuel delivery falling off at low cylinder pressure due to a weak regulator or restriction
• DToo much engine oil

Tech A says a properly functioning three-way catalytic converter on a CNG engine helps reduce CO, HC, and NOx. Tech B says unburned methane (HC) is generally harder to oxidize in the catalyst than larger hydrocarbons. Who is correct?

• ATech A only
• BTech B only
• CNeither Tech A nor Tech B
• DBoth Tech A and Tech B

When verifying emissions, the technician finds high tailpipe CO and HC with a rich CNG mixture. A plausible cause is:

• AA high-pressure regulator delivering excessive pressure or a leaking injector enriching t…
• BA lean injector with no fuel
• CAn open cylinder valve only
• DCorrectly adjusted closed-loop control

During performance verification, why might a CNG vehicle show reduced range compared with a similar gasoline vehicle on a full tank?

• ACNG has higher volumetric energy density than gasoline
• BCNG stores less energy per unit of onboard storage volume even at high pressure
• CThe engine is more efficient on CNG so it uses no fuel
• DCNG range is unaffected by storage pressure

A CNG engine exhibits hard starting when cold but runs fine warm. A CNG-specific cause to investigate is:

• AFrozen gasoline in the fuel rail
• BWorn spark plugs are impossible on CNG engines
• CA regulator or pressure-control issue affected by low temperature, or first-stage heating…
• DCNG cannot be used in cold weather