Preventive Maintenance and Inspection Flashcards

During a preventive maintenance inspection on a heavy-duty diesel truck, you discover that the coolant recovery tank is consistently losing fluid between service intervals, but there are no visible external leaks and the engine is not overheating. Which of the following is the MOST likely cause requiring further diagnosis?

Answer: A minor internal head gasket leak combusting coolant without producing visible white exhaust smoke at operating temperature

A minor internal head gasket breach can allow combustion gases to enter the cooling system without producing noticeable white exhaust smoke, especially if the leak is small enough that coolant burns off gradually. This causes slow coolant loss from the recovery tank without visible external leaks. The combustion gases also raise system pressure intermittently, which can push coolant into the overflow. A faulty pressure cap would typically cause overheating, not just tank depletion. Air ingestion would produce gurgling and temperature fluctuation. Electrolytic corrosion causes component degradation, not net fluid loss.

A 310T technician performing a brake inspection on a tractor-trailer combination finds that the pushrod stroke on one of the rear axle service chambers is at 2.1 inches with the brakes fully applied. The chamber is a Type 30 long stroke unit. What is the correct course of action?

Answer: No adjustment is needed; 2.1 inches is within the 2.5-inch maximum allowable stroke for Type 30 long stroke chambers

Type 30 long stroke service chambers have a maximum allowable stroke of 2.5 inches (compared to 2.0 inches for standard Type 30 chambers). A measured stroke of 2.1 inches is within the acceptable limit for a long stroke unit, so no adjustment is required at this time. It is critical for technicians to correctly identify whether a chamber is standard or long stroke before determining compliance — mistakenly applying the standard 2.0-inch limit to a long stroke chamber would result in unnecessary adjustments. The chamber does not require replacement based solely on stroke length within specification.

During a PM inspection of a truck's compressed air system, you perform a governor cut-out and cut-in test. The compressor cuts out at 125 psi and cuts in at 100 psi, but you also notice that with the engine off and all valves closed, system pressure drops from 125 psi to 110 psi within 3 minutes. Which conclusion is MOST accurate?

Answer: The system fails; the allowable pressure drop for a combination vehicle is no more than 4 psi per minute with the engine off and brakes released

For a combination vehicle (tractor-trailer), the air leakage standard with the engine off and brakes released is a maximum of 3 psi per minute — not 5 psi/min as some may misremember. A drop from 125 to 110 psi over 3 minutes equals 5 psi per minute, which exceeds the allowable limit and indicates an air leak requiring investigation. The governor cut-out (125 psi) and cut-in (100 psi) values are within acceptable ranges. The leakage test is performed with brakes released, not applied, for the base system check. This is a distinct test from the brake application leakage test.

A technician inspecting a truck's exhaust aftertreatment system notices that the Diesel Particulate Filter (DPF) has undergone three forced regenerations within a single 10,000 km service interval. The system has no active fault codes and passive regeneration appears to occur normally. What is the MOST appropriate next step during this PM?

Answer: Investigate upstream engine and fueling system health, as excessive active regeneration frequency can indicate high soot load from engine-related issues

When a DPF requires more frequent forced (active) regenerations than expected, it signals that the filter is accumulating soot faster than passive regeneration can handle — even if no fault codes are active. This pattern points toward upstream engine issues such as faulty injectors causing incomplete combustion, excessive blow-by past worn rings, a malfunctioning EGR valve stuck open, or poor fuel quality. Simply cleaning the DPF or accepting the frequency as normal without root-cause investigation ignores the underlying engine performance problem that is creating excessive soot. Premature DPF replacement without identifying the cause will result in recurrence.

During a steering system inspection, a 310T technician measures 0.038 inches of radial play at a tie rod end. The vehicle manufacturer's specification states that tie rod end replacement is required when radial play exceeds 0.030 inches. However, the tie rod end in question was replaced during the last PM interval 6 weeks ago and shows no visible damage or grease purging. What should the technician do?

Answer: Replace the tie rod end immediately and inspect the adjacent steering knuckle bore and tie rod tube threads for damage causing premature wear

A tie rod end that has failed within 6 weeks of installation indicates either a defective replacement part, improper installation, or — more critically — a problem with the mating components such as a worn tapered bore in the steering knuckle or damaged thread engagement in the tie rod tube. Simply replacing the tie rod end a second time without investigating the root cause will result in the same rapid failure. The measurement of 0.038 inches exceeds the specified 0.030-inch limit regardless of how recently the part was installed, so replacement is mandatory. There is no break-in tolerance for safety-critical steering components.

A technician is performing a PM on a vehicle equipped with a tandem drive axle and notices that the inter-axle differential lock (power divider) engages and disengages normally, but the driver reports that when the lock is engaged on low-traction surfaces, the vehicle still experiences spin-out on one side. During inspection, all axle shaft seals appear intact, wheel bearing preload is correct, and no fault codes are present. What is the MOST likely cause?

Answer: One of the single-axle differential units (standard open differentials within the tandem) is allowing wheel spin, which the inter-axle lock does not prevent

This is a commonly misunderstood drivetrain concept. The inter-axle differential lock (power divider lock) equalizes torque distribution BETWEEN the forward rear axle and the rear rear axle — it does not lock the individual wheel-end differentials within each axle. Each tandem axle still has its own standard open differential. If one drive wheel on either axle loses traction, that axle's open differential will allow the spinning wheel to receive all of that axle's torque while the opposing wheel receives none. To prevent this type of spin-out, separate rear differential locks or traction control intervention is required. The inter-axle lock is functioning correctly as described.