FREE CNG Engines and Fuels Questions and Answers

How can a vehicle equipped with a compressed natural gas system be identified?

It is marked with an NGV sign.

It has a natural gas tank.

The engine compartment must be checked.

The vehicle manual book must be checked.

Correct! Wrong!

Explanation:
Vehicles equipped with Compressed Natural Gas (CNG) systems are often marked by an NGV (Natural Gas Vehicle) sign. This sign helps easily identify vehicles that use natural gas as fuel.

What must be done first to repair an EPA-certified CNG system that has failed emissions due to a failure of an expensive system component?

The system must be modified.

The system manufacturer must be checked for warranty.

Find a replacement component from a different manufacturer.

The part must be replaced and the customer must be charged to ensure timely repair.

Correct! Wrong!

Explanation:
When an EPA-certified CNG system fails due to a high-cost component, the first step should be to check with the system manufacturer for warranty coverage. This ensures that the repair is conducted according to manufacturer specifications and may provide cost-saving options for the customer.

When checking the fuel line connections after a compressed natural gas conversion, which of these should be used?

Propane

Nitrogen

Carbon Dioxide

Non-Ammonia Soap

Correct! Wrong!

Explanation:
Non-ammonia soap is commonly used to check fuel line connections after a compressed natural gas conversion because it can help detect leaks by producing bubbles when applied to the connections. Nitrogen, propane, and carbon dioxide are not typically used for this purpose.

What is the natural gas vehicle (NGV) engine regulator used for?

To measure NGV gas from the tank.

To proportion NGV gas from the tank.

It is used for NGV gas residual from the tank.

To regulate NGV gas pressure from the tank.

Correct! Wrong!

Explanation:
The NGV engine regulator is responsible for regulating the pressure of natural gas from the tank before it enters the engine. This ensures that the gas is delivered at the appropriate pressure for efficient combustion in the engine.

What should a technician do if stress corrosion cracking is evident in a hoop-wrapped Type-2 cylinder?

Use an approved coating to fix the damaged area.

Document the damage on the inspection form.

The cylinder must be removed from service.

Stress corrosion cracking is normal.

Correct! Wrong!

Explanation:
Remove the cylinder from service. Stress corrosion cracking poses a significant safety risk, so it's essential to take immediate action to prevent any potential hazards.

A transit bus's engine compartment makes a puffing noise during operation. Which one of the following might be the cause?

The air intake is leaking.

The water pump is faulty.

It’s just a normal noise when operating.

The exhaust manifold gasket is leaking.

Correct! Wrong!

Explanation:
When an exhaust manifold gasket is leaking, it can cause a puffing noise from the engine compartment due to the escaping exhaust gases.

What is the compress natural gas engine mixer used for?

To allow gas flow to enter the intake.

To allow gas flow to enter the exhaust.

To control gas flow before entering the intake.

To blend air and gas before entering the intake.

Correct! Wrong!

Explanation:
The mixer in a compressed natural gas (CNG) engine blends air and gas before entering the intake manifold of the engine. This mixture is essential for proper combustion within the engine.

The technician finds that the vehicle won't take fuel after replacing the CNG fuel tank. Which one of these might be the reason?

The manual lock-down valve(s) failed to open.

There’s a pinched O-ring on the receptacle line.

There’s an open circuit on the in-tank solenoid valve.

None of the above.

Correct! Wrong!

Explanation:
If the manual lock-down valves are not opened properly after installing a new CNG fuel tank, it can prevent the vehicle from accepting fuel.

After three hours, a 350-mile typical range CNG transit bus stops running. 375 psi is displayed on the high-pressure fuel gauge. The following can be the cause, except_____________.

defective engine ECM wiring

defective high-pressure regulator

stuck cylinder flow control solenoid

low CNG fuel cascade system pressure

Correct! Wrong!

Explanation:
If the high-pressure fuel gauge shows 375 psi, it suggests that the high-pressure regulator is functioning properly, as it's maintaining pressure within the expected range. Therefore, the correct answer is: defective high-pressure regulator.

There is a no-start condition in the dedicated CNG vehicle. The following could be the cause, except _______________.

recurved timing

no fuel pressure

no ignition source

All of the above.

Correct! Wrong!

Explanation:
While the other options could potentially cause a no-start condition, recurved timing typically wouldn't directly lead to such an issue. Timing adjustment usually affects engine performance but doesn't directly prevent the engine from starting.

Normally, the tank shown in the picture below has a closed solenoid valve. What must be done to allow the cylinder to be vented?

Cut the wires for the tank solenoid valve.

Ground the tank solenoid valve's housing.

Remove the relay and install a jumper wire across terminals 86 and 87

Remove the relay and install a jumper wire across terminals 30 and 87

Correct! Wrong!

Explanation:
Remove the relay and install a jumper wire across terminals 30 and 87 to allow the cylinder to be vented.

CNG Practice Test 2025
Welcome to our comprehensive guide on conducting a CNG test 2025 to ensure optimal safety and compliance with regulations.…
FREE CNG Systems Questions and Answers

FREE CNG Engines and Fuels Questions and Answers

How can a vehicle equipped with a compressed natural gas system be identified?

What must be done first to repair an EPA-certified CNG system that has failed emissions due to a failure of an expensive system component?

When checking the fuel line connections after a compressed natural gas conversion, which of these should be used?

What is the natural gas vehicle (NGV) engine regulator used for?

What should a technician do if stress corrosion cracking is evident in a hoop-wrapped Type-2 cylinder?

A transit bus's engine compartment makes a puffing noise during operation. Which one of the following might be the cause?

What is the compress natural gas engine mixer used for?

The technician finds that the vehicle won't take fuel after replacing the CNG fuel tank. Which one of these might be the reason?

After three hours, a 350-mile typical range CNG transit bus stops running. 375 psi is displayed on the high-pressure fuel gauge. The following can be the cause, except_____________.

There is a no-start condition in the dedicated CNG vehicle. The following could be the cause, except _______________.

Normally, the tank shown in the picture below has a closed solenoid valve. What must be done to allow the cylinder to be vented?

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