Engine Troubleshooting Overview

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Revision as of 09:49, 12 December 2006

Introduction

Trouble shooting is only a process of elimination and provided the procedure is carried out correctly and systematically an accurate diagnosis of the trouble can be made in the minimum amount of time.

For an internal combustion engine to run there are three basis requirements, these are ignition, fuel and compression. There are other factors of course, but as a rule an engines failure to start can be attributed to a fault in one or more of these three systems.

The most likely cause of an engine wont start issue is an ignition or electrical failure first followed by fuel, with mechanical or compression failure the least common.

Before getting into ignition troubleshooting it is worth checking, is there fuel in the fuel tank? If yes begin the ignition/electrical diagnosis.

Ignition and Electrical Systems checks

Switch on the ignition and check for warning lamp illumination.

Operate the starter switch and check that the starter rotates the engine at a steady speed.

Switch on the headlamps and check for good light intensity.

Should the lamps not illuminate or the starter motor not turn the engine, carry out the following action:

(a) Remove the battery terminals and clean both terminals and posts. Refit the terminals and tighten firmly but not excessively.

(b) Check that the earth lead from the battery to the engine and body frame is not broken and that the connections are clean and secure.

(c) Check that the lead from the battery to the starter motor or starter solenoid is intact and with a clean and secure connection.

(d) Where necessary carry out repairs to (b) and (c)

Again carry out the check procedure. Should the starter motor still not operate, or the lamps illuminate, then one or more of the following faults may be the cause:

• No starter motor operation or lights: Battery flat or defective.
• Lights illuminate but no starter operation: Starter motor drive jammed in mesh with flywheel ring gear. Starter motor or solenoid defective. Ignition/starter switch faulty.
• Lights dim and starter motor operation sluggish: Discharged battery or fault in starter motor. Battery flat due to broken fan belt or defective alternator. Faulty battery due to cell breakdown.

If the battery and starter motor operation prove satisfactory but the engine still fails to start continue as follows:

(1) Disconnect a spark plug lead and position the terminal within 3 mm of the cylinder head. It may be necessary on leads fitted with terminal insulation to use a piece of wire, a split pin or screwdriver as an extension to the terminal.

(2) Have an assistant switch the ignition 'ON' and operate the starter switch.

(3) Check that the spark, if any, jumps the gap to the cylinder head. If there is no spark or the spark is weak or yellow, carry on with point (4). If the spark is satisfactory proceed to point (5)

(4) Disconnect the coil lead from the distributor cap and carry out the check procedure previously described for the spark plug lead in points (1) to (3). If there is a strong blue spark then the fault lies within the distributor. Should there be no spark or a weak and yellow spark on both or either of the tests, then carry out the following action.

(a) Check the spark plug leads to ensure that they are perfectly dry and that the insulation is not cracked. (b) Check the distributor cap to ensure that it is dry and clean. Examine both inside and outside of the cap for cracks or tracking, particularly between the spark plug segments in the cap. (c) Check the rotor arm for cracks, deposits or burning on the metal arm. (d) Ensure that the spark plug leads and the coil lead have dry, clean and secure connections in the distributor cap. (e) Check that the wires to the ignition switch side of the coil and from the contact breaker side of the coil to the distributor are intact and have clean secure connections. (f) Check that the carbon brush in the centre of the distributor cap interior face is clean and dry and ensure that it moves freely in and out of the locating hole. (g) Check that low tension current is reaching the ignition coil when the ignition is switched 'ON' by using a test lamp between the coil low tension terminals and a good earth. Ensure that the breaker points are open or insulated when making the test. Should the test lamp light on the ignition switch side of the coil but fail to light on the contact breaker side of the coil then it is possible that the coil primary circuit is faulty. Remove and test the coil or replace it with a known serviceable unit. NOTE: If a test lamp is unavailable the low tension lead can be disconnected from the ignition switch side of the coil and 'flashed' to a good earth in order to check for current flow. Similarly the other low tension lead can be disconnected at the distributor and 'flashed' to some other good earth to check for current flow. As in test (g) ensure that the ignition switch is in the 'ON' position. (h) Check and if necessary clean and adjust the contact breaker points. See the appropriate heading in the Engine Tune-up section. (i) Ensure that the distributor low tension leads and the earth wire are intact, clean and secure.

Again carry out the check procedure. Should there still be no spark or a weak and yellow spark, then one or more of the following faults may be the cause:

• Defective coil, capacitor or high tension leads. If the contact breaker points showed severe burning or pitting of the point faces in the previous check then this would indicate a possible capacitor failure. Renew the capacitor and contact breaker points.

(5) If the above checks prove the spark to be satisfactory but the engine still fails to start, remove all the spark plugs and check their condition and electrode gap.

Fuel System Checks

Check there is clean fuel in the fuel tank by unscrewing the fuel tank drain plug and draining a small quantaty into a clean container.

Determine if there is fuel in the carburettor float chamber. To do this, remove the air cleaner assembly from the carburettor and with the choke valve open look down the carburettor throat. While looking down the carburettor throat open and close the throttle several times by actuating the throttle linkage by hand. If squirts of fuel are seen to discharge from the accelerator pump discharge nozzle it is an indication that the carburettor float chamber is full of fuel and that the fuel system is functioning satisfactory.

If on the other hand no fuel is being discharged from the accelerator pump nozzle then proceed, checking as follows: (2) Pull the lead from the top of the coil or remove the distributor rotor arm to prevent the engine from possibly starting during the next part of the test.

(3) Disconnect the fuel supply hose at the carburettor and position the end in a suitable container. Have an assistant operate the starter and note if fuel is being discharged into the container.

Proceed to point (7) if the fuel flow is satisfactory, if not, continue with points (4) to (6).

(4) Disconnect the fuel supply hose from the tank to the inlet side of the fuel pump.

(5) Remove the fuel tank filler cap and have an assistant listen at the tank filler aperture. Blow down through the fuel pipe towards the tank and provided the pipe is clear, air bubbles will be heard in the tank.

(6) Reconnect the fuel hose to the pump and again check for fuel delivery from the fuel pump. If there is no delivery of fuel to the carburettor side of the fuel pump then one or more of the following faults may be the cause:

• No air bubbles heard on tank test: Fuel line obstructed or fractured. Fuel filter blocked.
• Air bubbles heard in tank but no delivery from fuel pump: Faulty fuel pump.

(7) Should there be a satisfactory supply of fuel at the carburettor end of the fuel pipe but the engine will not start, carry out the following additional procedures: (a) First check that flooding is not the cause due to excessive use of choke and/or accelerator when attempting to start the engine. (b) Remove the spark plugs and check for petrol saturation of the electrodes, if evident thoroughly clean and dry before replacement. (c) If flooding persists, check the float and needle valve for sticking and also ensure that the float is not punctured allowing it to fill with fuel. (d) Check the air cleaner for a choked element. If flooding is not the cause and there is an adequate supply of fuel to the carburettor, one or more of the following faults may be the cause:

• Stuck or obstructed needle valve.
• Choke valve not closing correctly.
• Air leaks around the inlet manifold or carburettor mounting flanges because of distortion or defective gaskets.
• Fractured hose or loose connections between the brake servo unit and the inlet manifold.