ASE H1 CNG Engines (Transit Bus) Feed

Question

A transit bus with a Cummins L9N CNG engine cranks but will not start. There is no spark on any cylinder. Which check should the technician perform FIRST?

General Engine Diagnosis

Question · Easy

Question

Technician A says a stoichiometric CNG transit-bus engine runs at an air-fuel ratio near lambda 1.0 so a three-way catalyst can work. Technician B says these engines always run lean-burn for emissions. Who is correct?

General Engine Diagnosis

Question · Easy

Question

A CNG transit bus has a rough idle and a stored cylinder 4 misfire code. Compression and ignition on cylinder 4 test normal. What is the MOST likely remaining cause?

General Engine Diagnosis

Question · Easy

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Question

During a cylinder balance test on a CNG transit-bus engine, disabling cylinder 2 produces almost no change in engine rpm. This indicates that cylinder 2:

General Engine Diagnosis

Question · Easy

Question

A methane gas detector mounted in the engine compartment of a CNG transit bus repeatedly alarms with no visible leak. Which characteristic of methane should the technician keep in mind during the search?

General Engine Diagnosis

Question · Easy

Question

A CNG transit-bus engine sets a knock-related code and the ECM is retarding ignition timing. Which condition is the MOST likely root cause?

General Engine Diagnosis

Question · Hard

Question

Which tool is the MOST appropriate for safely locating a small CNG fuel leak on a transit bus before road testing?

General Engine Diagnosis

Question · Easy

Question

A CNG transit bus exhibits low power under heavy acceleration but idles normally. Long-term fuel trims are near zero and no misfire is stored. Which area should the technician investigate FIRST?

General Engine Diagnosis

Question · Easy

Question

Technician A says a HEGO/lambda sensor reading stuck near a fixed voltage can prevent proper closed-loop fuel control on a stoichiometric CNG engine. Technician B says the lambda sensor only affects emissions and never affects driveability. Who is correct?

General Engine Diagnosis

Question · Easy

Question

A relative compression test performed with a scan tool on a CNG transit-bus engine shows cylinder 5 significantly lower than the others. What does this MOST likely indicate?

General Engine Diagnosis

Question · Easy

Question

Before opening any high-pressure CNG fuel line on a transit bus, the technician must FIRST:

General Engine Diagnosis

Question · Easy

Question

A CNG transit bus engine has an intermittent stumble that the driver reports as worse in cold weather. Freeze-frame data shows the event occurs at low coolant temperature. Which is the MOST likely contributor?

General Engine Diagnosis

Question · Hard

Question

The driver reports a popping noise from the intake on a CNG transit bus during acceleration. What does an intake-side backfire MOST likely indicate?

General Engine Diagnosis

Question · Easy

Question

A CNG transit-bus engine intermittently shuts down. The technician suspects a fuel-pressure problem. Which scan-tool data PIDs are MOST useful to monitor during the fault?

General Engine Diagnosis

Question · Medium

Question

A CNG transit bus has elevated tailpipe NOx on an emissions check. The three-way catalyst is functional and fuel trims are normal. Which condition most directly raises NOx on a stoichiometric NG engine?

General Engine Diagnosis

Question · Hard

Question

A CNG transit bus engine has a steady ticking noise that increases with rpm and is loudest at the valve cover. Which is the MOST likely cause?

General Engine Diagnosis

Question · Medium

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Technician A says you can use an OEM scan tool to view individual cylinder misfire counters on a CNG transit-bus engine. Technician B says misfire monitoring is impossible on gaseous-fuel engines. Who is correct?

General Engine Diagnosis

Question · Easy

Question

A CNG transit bus loses power and the ECM reports a derate. The high-side tank pressure reads only 300 psi. What does this MOST likely indicate?

General Engine Diagnosis

Question · Easy

Question

During cylinder head inspection on a CNG transit-bus engine, the technician finds receded exhaust valve seats. Why are NG engines particularly prone to this?

Cylinder Head Valve Train

Question · Hard

Question

A CNG transit-bus cylinder head is being checked for flatness. The straightedge and feeler gauge show warpage beyond the manufacturer limit. What is the correct repair decision?

Cylinder Head Valve Train

Question · Medium

Question

Technician A says variable or hydraulic valve lash on a CNG transit-bus engine must still be verified to specification during head service. Technician B says valve lash never affects emissions or misfire on NG engines. Who is correct?

Cylinder Head Valve Train

Question · Medium

Question

A worn valve guide on a CNG transit-bus engine is MOST likely to cause which symptom?

Cylinder Head Valve Train

Question · Medium

Question

When reconditioning valves on a CNG transit-bus head, the technician measures the valve margin. A valve margin that is too thin should be:

Cylinder Head Valve Train

Question · Medium

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A CNG transit-bus engine has a bent pushrod-style valvetrain on one cylinder after a timing-related event. What is the MOST appropriate action?

Cylinder Head Valve Train

Question · Medium

Question

On a CNG transit-bus engine equipped with hydraulic lash adjusters, a noisy lifter that quiets after the engine warms up MOST likely indicates:

Cylinder Head Valve Train

Question · Medium

Question

During CNG transit-bus head reassembly, the technician must ensure valve stem seals are installed correctly because a missing or damaged seal will:

Cylinder Head Valve Train

Question · Easy

Question

A CNG transit-bus engine has a deep, rhythmic knock that is loudest under load and at low rpm and decreases when the corresponding cylinder is disabled. This MOST likely indicates:

Engine Block

Question · Medium

Question

While measuring a CNG transit-bus engine cylinder bore, the technician finds taper and out-of-round beyond the service limit. The correct repair is to:

Engine Block

Question · Medium

Question

Technician A says a CNG transit-bus engine can experience cylinder washdown differently than a gasoline engine because gaseous fuel does not wet the cylinder walls. Technician B says NG engines therefore depend even more on correct ring and oil-control function. Who is correct?

Engine Block

Question · Hard

Question

During crankshaft inspection on a CNG transit-bus engine, a journal is found out-of-round beyond the limit. The correct procedure is to:

Engine Block

Question · Medium

Question

A CNG transit-bus engine consumes oil with no external leaks and the spark plugs show oil fouling on multiple cylinders. Compression is acceptable. What is the MOST likely cause?

Engine Block

Question · Medium

Question

When installing piston rings on a CNG transit-bus engine, why must ring end gap be checked in the bore against specification?

Engine Block

Question · Medium

Question

During engine block overhaul on a CNG transit bus, main bearing bores are checked for alignment. Misaligned main bores will MOST likely cause:

Engine Block

Question · Hard

Question

A CNG transit-bus engine shows pressurized coolant escaping from the cylinder block area and combustion gases are detected in the coolant. With the head verified flat, what should the technician inspect NEXT?

Engine Block

Question · Hard

Question

A CNG transit-bus engine has a low-oil-pressure warning at idle and the technician finds the oil pump pickup screen partially blocked with sludge. What is the MOST appropriate correction?

Lubrication Cooling

Question · Easy

Question

A CNG transit-bus engine overheats only at sustained high load. The coolant level is correct and the thermostat tests good. Which is the MOST likely cause to investigate?

Lubrication Cooling

Question · Medium

Question

Why do CNG transit-bus engines place high demand on the cooling system compared with some diesel engines?

Lubrication Cooling

Question · Medium

Question

Technician A says the CNG pressure regulator on many transit buses is coolant-heated to offset the cooling caused by gas expansion. Technician B says regulator heating is only for passenger comfort. Who is correct?

Lubrication Cooling

Question · Medium

Question

A CNG transit-bus engine has low oil pressure at hot idle but normal pressure at higher rpm. Which is the MOST likely cause?

Lubrication Cooling

Question · Medium

Question

When servicing the cooling system on a CNG transit bus, the technician finds the coolant heavily contaminated with oil. What is the MOST likely internal cause?

Lubrication Cooling

Question · Medium

Question

A CNG transit-bus engine cooling fan never disengages and runs at full speed continuously. Which fault could cause this on a thermostatically controlled fan system?

Lubrication Cooling

Question · Medium

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Why is using the correct manufacturer-specified coolant important on a CNG transit-bus engine cooling system that also heats the fuel regulator?

Lubrication Cooling

Question · Easy

Question

A CNG transit-bus engine shows a fault for low air filter/intake restriction along with reduced power. Which is the BEST first step?

Air Induction Exhaust

Question · Easy

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A turbocharged CNG transit-bus engine has low boost pressure and a code for boost below desired. Compression is good. Which is a likely cause?

Air Induction Exhaust

Question · Medium

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Technician A says a leaking charge-air cooler (intercooler) boot can cause both low power and incorrect fuel control on a CNG transit-bus engine. Technician B says intake boots only affect noise. Who is correct?

Air Induction Exhaust

Question · Medium

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A CNG transit-bus engine has high exhaust backpressure measured upstream of the three-way catalyst. What does this MOST likely indicate?

Air Induction Exhaust

Question · Medium

Question

On a stoichiometric CNG transit-bus engine, the cooled EGR valve is stuck closed. Which symptom set is MOST expected?

Air Induction Exhaust

Question · Hard

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