ASE S2 Diesel Engines (School Bus) Feed

Question

A school bus with a Cummins ISB diesel engine cranks normally but will not start, and the technician confirms there is no fuel reaching the high-pressure pump. Which check should be performed FIRST?

Fuel System

Question · Easy

Question

Technician A says low engine compression on one cylinder can cause hard starting and a miss. Technician B says a cylinder cutout (power balance) test can help isolate a weak cylinder on an electronically controlled diesel. Who is correct?

General Engine Diagnosis

Question · Easy

Question

A school bus diesel produces excessive black smoke under acceleration but idles cleanly. The MOST likely cause is:

Fuel System

Question · Easy

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Question

During a cylinder compression test on a school bus diesel, one cylinder reads significantly lower than the others. Adding a small amount of oil through the injector bore raises that cylinder's reading noticeably. This indicates:

General Engine Diagnosis

Question · Easy

Question

A school bus engine has low power and the ECM logs no active fault codes. The technician should:

General Engine Diagnosis

Question · Hard

Question

Blue-gray smoke from the exhaust of a school bus diesel, most noticeable at startup and during deceleration, MOST likely indicates:

General Engine Diagnosis

Question · Easy

Question

A cylinder leakage (leak-down) test on a school bus diesel shows air escaping from the engine oil fill/crankcase. This indicates a leak past the:

General Engine Diagnosis

Question · Hard

Question

A school bus diesel has a steady knock that increases with engine speed and is loudest at light load, with no related fault codes. The technician should suspect:

General Engine Diagnosis

Question · Hard

Question

Technician A says white smoke at cold start that clears as the engine warms can be normal unburned fuel from cold combustion. Technician B says continuous white smoke with a sweet odor and coolant loss indicates coolant entering the combustion chamber. Who is correct?

General Engine Diagnosis

Question · Easy

Question

A school bus engine is consuming engine oil but shows no external leaks and the exhaust shows blue smoke under load. Compression is good. The MOST likely cause is:

General Engine Diagnosis

Question · Hard

Question

Engine oil analysis on a school bus diesel shows elevated coolant (glycol) and a rising sodium/potassium level. This MOST likely indicates:

General Engine Diagnosis

Question · Hard

Question

A school bus diesel cranks slowly and is hard to start on cold mornings only. After warming, it starts normally. The technician should check the:

General Engine Diagnosis

Question · Hard

Question

Which scan tool data set is MOST useful when diagnosing a low-power complaint on a common-rail school bus diesel?

General Engine Diagnosis

Question · Easy

Question

A school bus diesel surges at idle and the rail pressure on the scan tool fluctuates widely. Technician A says a sticking fuel pressure regulator/control valve can cause this. Technician B says a leaking injector or air in the fuel can cause it. Who is correct?

Fuel System

Question · Hard

Question

During an engine inspection, the technician finds metallic debris in the engine oil and bearing material in the filter. The BEST next step is to:

General Engine Diagnosis

Question · Easy

Question

A school bus diesel has excessive crankcase pressure (high blow-by) measured at the oil fill. This MOST likely indicates:

General Engine Diagnosis

Question · Hard

Question

A school bus diesel runs rough and misses at idle but smooths out under load. A cylinder cutout test shows little change when one cylinder is disabled. This cylinder is MOST likely:

General Engine Diagnosis

Question · Hard

Question

A coolant odor and white residue are found around a school bus diesel's exhaust outlet, and the surge tank shows combustion bubbles during cranking. The MOST likely cause is:

General Engine Diagnosis

Question · Hard

Question

When road-testing a school bus to verify a low-power complaint, the technician should:

General Engine Diagnosis

Question · Easy

Question

A school bus diesel has a high-pressure fuel leak that sprays atomized fuel near the engine. The technician should:

General Engine Diagnosis

Question · Easy

Question

A school bus diesel will not reach rated power and the scan tool shows the engine is in a derate. Technician A says an emissions-system fault such as a clogged DPF or SCR problem can command a derate. Technician B says low DEF level or quality can also command a derate. Who is correct?

General Engine Diagnosis

Question · Medium

Question

A school bus diesel has a no-start, and the scan tool shows no engine RPM signal during cranking. The MOST likely cause is:

General Engine Diagnosis

Question · Medium

Question

A school bus diesel has fuel in the engine oil (rising oil level, fuel odor). The MOST likely cause is:

General Engine Diagnosis

Question · Medium

Question

Before condemning an electronically controlled diesel engine for a mechanical fault, the technician should FIRST:

General Engine Diagnosis

Question · Easy

Question

A school bus diesel idles roughly and has a knock that disappears when one injector is electronically disabled. This indicates the problem is MOST likely:

General Engine Diagnosis

Question · Medium

Question

A school bus diesel exhibits poor fuel economy and low power, but compression and air systems test normal. The technician should next evaluate:

General Engine Diagnosis

Question · Medium

Question

A technician suspects a school bus diesel has a mechanical valvetrain problem. Which test BEST supplements compression testing to localize an air-sealing fault to a specific path (intake valve, exhaust valve, rings, or head gasket)?

General Engine Diagnosis

Question · Easy

Question

During in-chassis cylinder head removal on a school bus diesel, the technician should torque the head bolts:

In Chassis Engine Repair

Question · Easy

Question

When replacing a cylinder head gasket in-chassis on a school bus diesel, the technician must check the deck and head surfaces for:

In Chassis Engine Repair

Question · Easy

Question

Technician A says valve lash (clearance) on a school bus diesel must be set to specification at the correct cylinder position. Technician B says incorrect valve lash can cause hard starting, low power, and valve damage. Who is correct?

In Chassis Engine Repair

Question · Easy

Question

When replacing an injector in a Cummins ISB-style common-rail school bus engine in-chassis, the technician should:

Fuel System

Question · Medium

Question

After in-chassis replacement of a cylinder head on a school bus diesel, the engine has a coolant leak at the head-to-block joint. The MOST likely cause is:

In Chassis Engine Repair

Question · Medium

Question

When replacing a rocker arm and pushrod on a school bus diesel in-chassis, the technician should also inspect the:

In Chassis Engine Repair

Question · Medium

Question

During in-chassis service, a school bus diesel cylinder liner protrusion (liner stand-out) is found to be below specification. This can cause:

In Chassis Engine Repair

Question · Hard

Question

When timing the valvetrain after camshaft or front-cover service on a school bus diesel, the technician should:

In Chassis Engine Repair

Question · Medium

Question

A school bus diesel has low compression on two adjacent cylinders, and a leak-down test shows air passing between them. This MOST likely indicates:

General Engine Diagnosis

Question · Hard

Question

When installing a new cylinder head gasket on a school bus diesel, the technician notices the gasket is marked TOP or FRONT. The technician should:

In Chassis Engine Repair

Question · Easy

Question

After replacing a school bus diesel's front crankshaft seal in-chassis, oil still leaks from the front of the engine. Technician A says the harmonic balancer/sealing surface may be grooved. Technician B says the seal may have been installed at the wrong depth or damaged during installation. Who is correct?

In Chassis Engine Repair

Question · Medium

Question

When performing in-chassis repair, why must the technician follow the manufacturer's specified fastener torque and replace single-use (torque-to-yield) bolts?

In Chassis Engine Repair

Question · Easy

Question

During in-chassis valve service on a school bus diesel, a valve is found with a burned, eroded face. The MOST likely cause that should also be investigated is:

In Chassis Engine Repair

Question · Medium

Question

A school bus diesel runs hot only at highway speed and load, but stays normal in town. The MOST likely cause is:

Lubrication Cooling

Question · Medium

Question

Technician A says a stuck-open thermostat can cause a diesel to run below normal operating temperature. Technician B says chronic under-temperature operation can increase wear, fuel dilution, and emissions. Who is correct?

Lubrication Cooling

Question · Easy

Question

A school bus diesel has low engine oil pressure at idle when hot but normal pressure at speed. The MOST likely cause is:

Lubrication Cooling

Question · Medium

Question

A school bus diesel oil cooler is suspected of leaking internally. Which symptom would BEST support that diagnosis?

Lubrication Cooling

Question · Medium

Question

When servicing the cooling system on a school bus diesel that uses extended-life coolant (ELC), the technician should:

Lubrication Cooling

Question · Medium

Question

A school bus diesel overheats and the technician finds the radiator hot at the top but much cooler at the bottom with the engine at temperature. This MOST likely indicates:

Lubrication Cooling

Question · Medium

Question

A school bus diesel loses coolant with no visible external leak and no oil contamination, and a cooling-system pressure test holds. The technician should next check for:

Lubrication Cooling

Question · Medium

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