A medium-duty truck with a large-displacement gasoline engine has a rough idle and low power. A cylinder compression test shows one cylinder reading 90 psi while the others read about 155 psi. Which of the following is the MOST likely cause of the low reading?
General Engine Diagnosis
Question · Easy
Technician A says a cylinder leakage (leak-down) test pinpoints where compression is escaping. Technician B says air heard at the throttle body or air intake during a leak-down test indicates a leaking intake valve. Who is correct?
Introduction & ASE T1 Exam Overview
1.1 ASE T1 Exam Format, Scoring, Fees & Certification Requirements
Study guide
During a running compression test on a gasoline truck engine, the snap-throttle reading is much lower than the cranking compression reading on the same cylinder. This MOST likely indicates:
Question · Medium
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A gasoline-powered medium truck has white smoke from the exhaust, coolant loss with no external leaks, and a sweet smell. The MOST likely cause is:
A truck gasoline engine has a steady knocking noise that increases with load and is loudest at idle when warm. Disabling the suspected cylinder's ignition reduces the noise. This MOST likely indicates:
Which condition would MOST likely cause low oil pressure at idle but acceptable pressure at higher RPM on a gasoline truck engine?
1.2 Class 4–8 Gasoline & LPG Truck Engines and ASE Question Strategy
A gasoline truck engine consumes oil but shows no external leaks and produces blue smoke mainly on deceleration after a sustained pull. The MOST likely cause is:
A no-start gasoline truck engine cranks normally. Which step should be performed FIRST to logically narrow the cause?
Technician A says a vacuum gauge reading that is low and steady can indicate late ignition/valve timing or an intake manifold leak. Technician B says a needle that drops regularly indicates a burned or leaking valve. Who is correct?
A gasoline truck engine has an exhaust restriction (suspected plugged catalytic converter). Which symptom set is MOST consistent with this condition?
Chapter 1: General Engine Diagnosis — Complaints, Leaks, Noises & Exhaust
2.1 Concern Verification and Service Information
When performing a wet compression test, oil is added to a cylinder and the reading rises significantly compared to the dry test. This indicates:
A gasoline truck engine surges and has an intermittent rough idle. Spraying carb cleaner or propane near the intake manifold gasket changes engine speed. This indicates:
A truck gasoline engine has a slight, regular ticking that increases with RPM and is loudest at the valve cover. This is MOST likely:
A gasoline truck engine fails an emissions test for high hydrocarbons (HC) and a misfire is present at idle. Which is the BEST initial diagnostic approach?
2.2 Fluid Leak Isolation and Gauge Verification
During cranking, a gasoline truck engine produces uneven, low compression on adjacent cylinders that share a head gasket land. The MOST likely cause is:
A gasoline truck engine has black exhaust smoke under load and the spark plugs are sooty. This MOST likely indicates:
A gasoline truck engine overheats only under heavy load while highway cruising is normal. The cooling system holds pressure and the fan operates. The MOST likely cause is:
Technician A says an oil analysis showing high coolant (glycol) content suggests an internal coolant leak. Technician B says metallic particles in oil analysis can indicate bearing or component wear. Who is correct?
2.3 Lower-End Noise and Cylinder Cut-Out Evidence
A gasoline truck engine has a hard hot-start condition but starts fine cold. Fuel pressure is normal. Which is the MOST likely cause?
A propane (LPG) fueled medium truck engine lacks power and the operator reports it 'runs lean.' Which check is MOST appropriate FIRST?
A gasoline truck engine has low power across the RPM range, normal compression, and correct ignition timing. A backpressure test shows 4 psi at 2500 RPM (spec is under 1.5 psi). The MOST likely cause is:
A gasoline truck engine has a misfire only under heavy load. Spark plugs, coils, and injectors test good when checked at idle. Which condition is MOST likely?
2.4 Valvetrain, Accessory and Vibration Diagnosis
Before condemning an internal engine component for a performance complaint, the technician should FIRST:
A gasoline truck engine has a bluish-gray smoke at startup that clears after a minute or two of running. The MOST likely cause is:
A gasoline truck engine cranks but will not start. Spark is present and strong, but a noid light does not flash during cranking. The MOST likely problem is in the:
An engine performance complaint involves a vibration that increases with engine RPM regardless of load. Which is the MOST likely cause?
2.5 Oil Consumption and Blue Exhaust Smoke
A gasoline truck engine has a steady miss at idle that disappears above 1500 RPM. Cylinder balance testing identifies one weak cylinder. Compression and injector on that cylinder are good. The MOST likely cause is:
Question · Hard
Which tool is BEST suited to confirm that combustion gases are leaking into the cooling system of a gasoline truck engine?
During cylinder head inspection on a gasoline truck engine, a valve shows a burned, eroded margin on the exhaust valve face. The MOST likely cause is:
Cylinder Head Valve Train
Technician A says excessive valve guide clearance can cause high oil consumption. Technician B says worn valve guides can cause a vacuum leak that leans the mixture. Who is correct?
2.6 Coolant Vapor, Rich Mixture and Exhaust Sounds
When checking a cylinder head for warpage on a gasoline truck engine, the technician should use:
A gasoline truck engine has noisy hydraulic lifters that quiet down after the engine warms and oil pressure builds. The MOST likely cause is:
During valvetrain inspection, a rocker arm shows a galled, grooved contact pad. The technician should:
A valve spring on a gasoline truck engine measures below the minimum free height and fails the load test. This condition will MOST likely cause:
Chapter 2: General Engine Diagnosis — Vacuum, Balance, Compression & Leak-Down Testing
3.1 Vacuum Principles and Controlled Dynamic Tests
After a valve job, a gasoline truck engine still has a single-cylinder misfire. A leak-down test on that cylinder shows air escaping from the tailpipe. The MOST likely cause is:
When inspecting a camshaft on a pushrod gasoline truck engine, one lobe is visibly worn down compared to the others. The MOST likely result is:
A gasoline truck engine with a charge-air or intake-duct collapse has low power and a whistling intake noise under acceleration. Technician A says a collapsed or restricted intake duct reduces airflow and power. Technician B says a torn intake duct downstream of the MAF can let unmetered air in and lean the mixture. Who is correct?
Fuel Air Induction Exhaust
When measuring cylinder bore wear on a gasoline truck engine, the largest diameter is usually found:
Engine Block
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