A1: Engine Repair (Sample Test)

Correct answer: Both technicians are correct.
Technician A is right because RPM should be measured both before and after the test to ensure proper balance. Technician B is also correct since any drop or change in RPM is significant—it can indicate an impact on engine balance or performance.

Correct answer: Determine if all injectors have the same electrical resistance.
Although comparing readings to manufacturer specifications is ideal, an injector showing significantly higher or lower resistance than the others is likely faulty. Additional tests can then be performed to confirm the diagnosis before proceeding with repairs.

Mixing tap water and distilled water is the least critical step when refilling a cooling system. While it’s generally best to use a 50/50 mix of distilled water and coolant, using tap water in an emergency won’t necessarily harm the system. However, tap water contains minerals and impurities that can lead to scaling and corrosion over time, potentially damaging the cooling system.

In contrast, bleeding air from the system, checking the radiator cap for a proper seal, and ensuring the correct coolant-to-water ratio are essential steps. Bleeding air prevents overheating caused by trapped air pockets. A properly sealing radiator cap helps maintain system pressure. And the correct coolant mixture protects the engine from both freezing and overheating.

A stuck closed valve or a clogged crankcase breather tube may be contributing to the issue.
A stuck valve can lead to excessive pressure buildup in the crankcase, forcing oil past the seals. Conversely, a blocked breather tube can create a vacuum in the crankcase, which may also result in oil leaking through the seals.

To pinpoint the exact cause of the oil leak, a qualified mechanic should conduct a comprehensive engine inspection. This may include a compression test, a PCV valve check, and a thorough examination of the oil seals.

Correct answer: Prevents inaccurate readings
While disconnecting the fuel injection and ignition systems does enhance safety, the primary reason in this context is to ensure accurate readings. Preventing false or fluctuating data during testing is crucial, which is why these systems are disconnected.

When measuring the installed height of a cylinder head’s valve stem and noticing that one valve tip sits higher than the others, the most likely cause is a worn valve seat. A worn seat prevents the valve from seating properly, resulting in a taller valve stem height compared to the rest. This can disrupt valve operation and lead to performance issues. While factors like incorrect valve spring height, loose head bolts, or worn valve springs can cause other valve problems, a worn valve seat is the condition most directly linked to uneven valve stem height.

Variable valve timing (VVT) adjusts the timing of the opening and closing of the engine’s valves, allowing the engine to operate more efficiently at different speeds and loads. VVT systems can adjust valve timing independently of crankshaft rotation, as Technician A stated.
However, thick and contaminated oil can have a significant impact on the operation of a VVT engine, as Technician B stated. VVT systems rely on pressurized oil to actuate the mechanism that adjusts valve timing. Thick or contaminated oil can clog or restrict the oil passages, preventing the VVT system from functioning properly and potentially causing damage to the engine.
Therefore, it is important to maintain a clean and proper level of oil in a VVT engine, and to follow the manufacturer’s recommendations for oil changes and other maintenance.

Inspecting the pulleys for signs of wear and damage is the next step. Pulleys are the wheels that the drive belt/serpentine belt rides on, and they can become worn or damaged over time. This can cause the belt to slip or make noise.

When replacing connecting rod bearings in an engine, do not mix connecting rods and rod caps because the connecting rods and rod caps are machined together as a set, and they must be kept together during reassembly to maintain the correct fit and clearances. Mixing them up can lead to engine damage or failure.
Replacing the crankshaft is not necessary if only the connecting rod bearings are worn, as long as the crankshaft is still in good condition. Scratching the cylinder number on each connecting rod arm is not necessary for replacing connecting rod bearings. However, it can be a useful practice for keeping track of which rod goes with which cylinder during reassembly. Using oversized bearings may be necessary if the crankshaft or connecting rod journals have been machined undersize due to wear or damage. However, oversized bearings should only be used if recommended by the manufacturer or a qualified engine builder.

The statement “The engine must be cold for all engine manufacturers” is incorrect when adjusting valve lash on engines with overhead camshaft cylinder heads.

While it is true that valve lash adjustments are typically made on cold engines, some engine manufacturers specify that the adjustment should be made on a warm engine. This is because the clearance between the camshaft and the follower or rocker can change as the engine warms up and metal components expand due to thermal expansion. Therefore, it is important to follow the manufacturer’s specifications regarding the temperature of the engine when making valve lash adjustments.

The other statements are generally correct when adjusting valve lash on engines with overhead camshaft cylinder heads. Clearance is measured between the camshaft and follower or rocker on most engines, shims may be used to make adjustments, and the follower or rocker must be on the base circle of the camshaft when measuring.