Mechanic.com.au Engine Repair Quiz ANSWERS

3 April 2018

Did you have a go at the Mechanic.com.au Engine Repair Quiz?

Check below for the answers to see how you went and find out if you were the lucky winner!

Which of the following creates a flapping sound near the front of the engine? *

Timing belt tension too tight

Drive belt too tight

Drive belt too loose

Timing belt tension too loose

A loose timing belt will cause a flapping sound near the front of the engine.

An engine noise sounds like a knock at the side of the engine that is louder when the engine is cold and goes away or is reduced when the engine reaches operating temperature. Which of the following is the MOST likely cause? *

Cam bearing

Main bearing

Piston slap

Wrist pin

Piston slap is a knock heard at the side of the engine which is louder when the engine is cold and goes away or is reduced when the engine reaches operating temperature.

All of the following are reasons why you must disable the fuel injection system and ignition system when conducting an engine compression test, EXCEPT: *

Prevents damage to the ignition system during the test

Prevents fuel injection into the cylinders

Prevents a shop engine fire

Prevents inaccurate readings

The purpose for disconnecting the fuel injection and ignition systems is SAFETY related. Reading accuracy is very important but not what was asked.

A power balance test is being performed on an engine. Technician A says to note the engine RPM before and during the test for each cylinder. Technician B says to record the RPM drop for each cylinder. Who is right? *

A only

B only

Both A and B

Neither A nor B

Both technicians are correct. Tech A is right because engine RPM (Revolutions Per Minute) is an indicator of when that cylinder is contributing to the engine power and if it is balanced. You note the RPM before and during the test so when the cylinder is shorted out you can tell if it changes. Tech B is right because when the engine speed (RPM) does not drop or change, then that cylinder is not having an effect on engine operation or balance.

What engine seal is pictured in the below illustration? *

Rear main

Camshaft Plug

Throttle Valve

Valve

A valve seal is shown in the picture.

Technician A says the closed coil end of a valve spring should go against the cylinder head. Technician B says all valve springs use shims to control free spring height. Who is right? *

A only

B only

Both A and B

Neither A nor B

Technician A is a correct. A closed coil end of a valve spring should go against the cylinder head. Technician B is wrong because all valve springs do not use shims to control assembled height. It is also wrong because it should state assembled height.

All of the following statements are correct when adjusting valve lash on engines with overhead camshaft cylinder heads, EXCEPT: *

Shims may be used to make adjustments.

Clearance is measured between the camshaft and follower or rocker on most engines.

The follower or rocker must be on the base circle of the camshaft when measuring.

The engine must be cold for all engine manufacturers

You must check the specific OEM procedure for adjusting valves either hot or cold.

The surface of the cylinder head has just been machined. What will have to be done to the valve train? *

Lengthening the push rods

Increasing valve spring tension

Grinding the valve stems

Shim the head

Grinding the valve stems will have to be done because the distance has been reduced between the rocker and the pushrod, so you grind the valve stem to compensate.

A technician hears a knock or thumping at the side of the engine block that is louder when the engine is hot and goes away when the affected cylinder is shorted out. Which of the following could be the cause? *

Main bearing

Wrist pin

Rod bearing

Piston slap

Rod bearing noise is a thumping noise or knock at the side of the engine block that is louder when the engine is hot and goes away when the affected cylinder is shorted out. A is wrong because main bearing noise is constant and cannot be shorted out. A main bearing noise is generally deeper sounding than a rod bearing. Also, a main bearing makes an evenly spaced single knock while a rod bearing generally makes a double knock. B is wrong. A wrist pin noise can be diagnosed by removing ignition from a cylinder one by one. Should the noise in the engine immediately double in frequency, then it's a wrist pin. Should the noise merely diminish a bit, then probably a rod knock caused by a loose bearing. D is wrong because piston slap noise disappears once the engine is warm. Piston slap could be caused by the piston skirt deforming under stress.

An engine is making a knocking sound that changes with RPM, and the noise disappears while a cylinder is being tested during the cylinder balance test. Which of the following could be the cause? *

Rod bearing

Main bearing

Piston slap

Burnt Valve

A bad rod bearing will make a knocking sound with changes in RPM and disappear when the cylinder is shorted out.

Technician A says old antifreeze may cause corrosion build-up in the cooling system. Technician B says some original equipment manufacturers (OEM) recommend that the coolant be changed at specified intervals. Who is right? *

A only

B only

Both A and B

Neither A nor B

Both technicians are correct. Antifreeze or engine coolant contains corrosion inhibitor that wears out over time, so this is the reason it is recommended that the coolant be replaced and the system flushed at a service interval. The freeze protection of the coolant never changes.

Which of the following steps is LEAST likely to be necessary when refilling a cooling system? *

Bleeding the air out of the system

Mixing tap water and distilled water

Mixing the proper ratio of water to coolant

Check the radiator cap for proper fit

Distilled water is the least likely required in a cooling system because it has no effect on the cooling ability of the solution.

A technician is using a laptop computer and manufacturer software to test the ECT (Electronic Controlled Throttle) circuit that has a stored DTC P1121 (Electric Throttle Control Actuator). Which of the following customer concerns could be caused by a defective ECT return spring malfunction? *

Rough Idle

Low Power

High Idle

Detonation

The ECM has a stored DTC P1121 (Electric Throttle Control Actuator). When the malfunction is detected, ECM enters fail-safe mode and the MIL lights up. While the engine is in fail-safe mode and malfunction A of DTC P1121 is detected, the engine speed will not rise more than 2,000 RPM and thus cause a low power complaint. The ECM controls the electric throttle actuator by regulating the throttle opening around the idle position.

Technician A says the voltage required maintaining spark is referred to as the spark line and the duration of the spark line is based on total primary circuit resistance and coil voltage available. Technician B says problems with the burning (called fuel propagation) will show up within the spark line. Who is right? *

A only

B only

Both A and B

Neither A nor B

Technician B is correct because problems with the fuel/air burning (called fuel propagation) will show up within the spark line. Technician A is wrong because the voltage required maintaining spark is referred to as the spark line and the duration of the spark line is based on total secondary circuit resistance (not primary circuit resistance) and coil voltage available. A typical firing voltage for a DI system is 8-10 kilovolts (kv). The spark line, which indicates the amount of time current flows across the spark plug gap, should also be smooth and level. Normal current flow usually lasts 1.2-2 milliseconds. Short or uneven spark lines, like high-firing voltages, indicate high secondary circuit resistance.