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These 10 free CNG-FSI questions are organized by exam domain, so you can see how each part of the Compressed Natural Gas Fuel System Inspector blueprint is tested. Reveal the answer and explanation under each question.
A ceiling-level detector indicates methane in an enclosed CNG service bay while a floor-level detector reads zero. Both instruments are methane-capable and have passed their checks. The released gas is at room temperature. What best accounts for the difference?
Correct answer: B - Buoyant methane can collect near the ceiling before the room is thoroughly mixed.
A CNG service van weighs 4,300 kg on a scale, but its manufacturer-assigned gross vehicle weight rating is 5,000 kg. Under the current FMVSS 304 container-label inspection advisory, which periodic interval applies, in addition to inspection after an accident or fire?
Correct answer: D - At least every 12 months, because the GVWR exceeds 4,536 kg.
The only unresolved finding on an Agility PRD vent is a missing protective cap. The checks required by the applicable service instructions have found no moisture, debris, tube damage, PRD corrosion, or leakage. The PRD and vent arrangement are correct. What closes this finding?
Correct answer: C - Install the specified cap and record correction of the finding.
While inspecting a refuse truck, you hear a hiss beside the CNG fuel module. A verified methane detector finds no methane at the fuel connections checked. Compatible leak-detection liquid produces continuing bubbles at a nearby air-brake fitting, but none at those fuel connections. How should these results be interpreted?
Correct answer: B - A pneumatic leak is identified; the CNG inspection still needs to be completed.
One end of a CNG cylinder is fixed; its opposite support allows the longitudinal travel specified by the manufacturer. The travel, fasteners, isolators, cylinder surfaces, and connected lines all meet their inspection criteria. A technician suggests locking the sliding support to eliminate movement. What should the inspector recommend?
Correct answer: C - Retain the approved movement allowance and record the finding.
Two fully wrapped CNG cylinders have the same service-pressure rating and metal valve bosses. One is recorded as Type 3; the replacement may be Type 4. Which manufacturer specification resolves the construction type?
Correct answer: D - The material forming the gas-tight liner
An impact inspection of a Quantum Type 4 cylinder finds a shallow resin scratch and localized wall flattening. Earlier photographs, taken at the same settled pressure, show a smoothly curved wall at that location. No leak is detected. Which interpretation best fits the evidence?
Correct answer: A - The changed wall contour indicates possible structural damage despite little surface material loss.
Depth measurements for one cylinder gouge differ substantially. Both gauges pass their checks, but one inspector used a raised resin run as the reference and the other used a different part of the curved surface. Before assigning a damage level, how should the disagreement be resolved?
Correct answer: C - Repeat the measurement using the manufacturer's prescribed undamaged-surface reference.
A Luxfer composite AF cylinder is governed by the October 2018 manual. Its abrasion table defines Level I as depth below 0.010 in; Level II requires depth between 0.010 and 0.030 in AND area below 1.0 in². Abrasion exceeding the Level II limits is Level III. A rectangular wear patch measures 1.6 in by 0.75 in, with a maximum depth of 0.60 mm. Using 1 in = 25.4 mm, which classification AND supporting reason are correct?
Correct answer: D - Level III: the 1.20-in² patch exceeds the middle-band area allowance.
An installation review identifies a 200-psig-rated component downstream of the regulator on a vehicle with 3,600-psig CNG storage. The approved overpressure protection limits this branch to 150 psig and has been verified. The component also meets the manufacturer's fuel, temperature, and location requirements. Its pressure rating is:
Correct answer: A - Acceptable because it exceeds the protected branch's 150-psig maximum.
The CNG-FSI exam also covers these domains. Drill them in the full free practice test:
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