Engine fault diagnosis such as fuel and

7.3.4 Engine fault diagnosis such as fuel and

ignition

table 1

7.3.5 Engine fault diagnosis

Symptom Possible causes

Suggested action

or faults

table 2

Oil consumption Worn piston rings

Engine overhaul

Please note that this section covers related engine

and/or cylinders

systems as well as the engine itself.

Worn valve stems,

Replace valves

guides or stem oil

(guides if possible)

Possible cause Oil on engine

seals

and oil seals

Symptom

Battery connection loose or or floor

Leaking gaskets or

Replace appropriate

Engine does not rotate

seals

corroded Build up of pressure Check engine

gasket or seal

when trying to start

Battery discharged or faulty in the crankcase

Broken loose or disconnected Mechanical

breather system

wiring in the starter circuit knocking noises bearings (big ends or overhaul engine.

Worn engine

Replace bearings or

Defective starter switch or automatic mains for example)

Good idea to also

gearbox inhibitor switch

Engine systems 103

Starter pinion or flywheel ring gear Incorrect plugs or plug gaps loose

HT leads breaking down Earth strap broken. Loose or

Idle speed incorrect corroded

Engine stalls

CO setting incorrect Engine rotates but

No fuel in the tank! Fuel filter blocked does not start

Discharged battery (slow rotation) Air filter blocked Battery terminals loose or corroded

Intake air leak Air filter dirty or blocked

Idle control system not working Low cylinder compressions

Fuel filter blocked Broken timing belt

Lack of power

Air filter blocked Damp ignition components

Ignition timing incorrect Fuel system fault

Low fuel pump delivery Spark plugs worn to excess

Uneven or low cylinder Ignition system open circuit

compressions (maybe valves) Difficult to start when

Discharged battery (slow rotation) Fuel injectors blocked cold

Battery terminals loose or corroded Brakes binding or clutch slipping Air filter dirty or blocked

Incorrect ignition timing Low cylinder compressions

Backfires

Incorrect valve timing (cam belt not Fuel system fault

fitted correctly) Spark plugs worn to excess

Fuel system fault (airflow sensor on Enrichment device not working

some cars) (choke or injection circuit)

Low engine oil level Difficult to start when

Oil pressure gauge low

Faulty sensor or switch hot

Discharged battery (slow rotation)

or warning light on

Battery terminals loose or corroded Worn engine oil pump and/or engine Air filter dirty or blocked

bearings Low cylinder compressions

Engine overheating Fuel system fault

Oil pickup filter blocked Starter noisy

Starter pinion or flywheel ring gear Pressure relief valve not working loose

Ignition timing incorrect Starter mounting bolts loose

Runs on when

Idle speed too high Starter worn (bearings, etc.)

switched off

Anti-run on device not working Discharged battery (starter may

Carbon build up in engine jump in and out)

Engine overheating Starter turns engine

Ignition timing incorrect slowly

Discharged battery (slow rotation)

Pinking or knocking

Battery terminals loose or corroded

Ignition system fault Earth strap or starter supply loose

under load

Carbon build up in engine or disconnected

Knock sensor not working Internal starter fault

Leaking exhaust manifold gasket Engine starts but then

Sucking or whistling

Leaking inlet manifold gasket stops immediately

Ignition wiring connection

noises

intermittent Cylinder head gasket Fuel system contamination

Inlet air leak Fuel pump or circuit fault (relay)

Water pump or alternator Intake system air leak

bearing Ballast resistor open circuit

(older cars)

Rattling or tapping

Incorrect valve clearances Worn valve gear or camshaft

Erratic idle Air filter blocked Loose component Incorrect plug gaps

Inlet system air leak

Worn main bearings (deep Incorrect CO setting

Thumping or knocking

knocking/rumbling noise) Uneven or low cylinder

noises

Worn big end bearings (heavy compressions (maybe valves)

knocking noise under load) Fuel injector fault

Piston slap (worse when cold) Incorrect ignition timing

Loose component Incorrect valve timing

Bearings on ancillary component Misfire at idle speed

Rumbling noises

Ignition coil or distributor cap tracking Poor cylinder compressions Engine breather blocked

7.4 Fuel system

Inlet system air leak Faulty plugs

Authors Note: Even though carburettor fuel systems Misfire through all

Fuel filter blocked are now very rare, they are still used on some specialist speeds

Fuel pump delivery low vehicles. For this reason, and because it serves as a Fuel tank ventilation system blocked

good introduction to fuel systems, I decided to include

104 Advanced automotive fault diagnosis

7.4.1 Introduction

7.4.2 Carburation

All vehicle fuel systems consist of the carburettor Figure 7.7 shows a simple fixed choke carburet- or fuel injectors, the fuel tank, the fuel pump, and

tor operating under various conditions. The float the fuel filter, together with connecting pipes. An

and needle valve assembly ensures a constant engine works by the massive expansion of an

level of petrol in the float chamber. The venturi ignited fuel air mixture acting on a piston. The job

causes an increase in air speed and hence a drop of the fuel system is to produce this mixture at just

in pressure in the area of the outlet. The main jet the right ratio to run the engine under all operat-

regulates how much fuel can be forced into this ing conditions. There are three main ways this is

intake air stream by the higher pressure now appar- achieved:

ent in the float chamber. The basic principle is that

as more air is forced into the engine then more petrol is mixed with air in a carburettor;

fuel will be mixed in to the air stream. petrol is injected into the manifold, throttle The problem with this system is that the amount body cylinder or to mix with the air;

of fuel forced into the air stream does not linearly diesel is injected under very high pressure follow the increase in air quantity, unless further directly into the air already in the engine com- compensation fuel and air jets are used. A vari- bustion chamber. able venturi carburettor, which keeps the air pres-

This section will only examine the carburettor sure in the venturi constant but uses a tapered systems; diesel and injection comes under engine

needle to control the amount of fuel, is another management later.

method used to control fuel air ratio.

Choke flap Air-correction Air vent

Idling jet

jet

Throttle valve nearly closed

Emulsion tube

High vacuum Progression High vacuum moves in this area

drilling

to main outlet

On idling, high vacuum below the throttle Fuel is drawn from the main outlet as the vacuum valve draws fuel through a separate circuit

around it increases

Choke flap

Choke flap

Pump outlet

Accelerator pump

Accelerator linkage A flap is used to partially block the barrel for

The accelerator pump squirts an enriching shot of fuel cold starts. It increases vacuum around the

down the barrel to provide rapid response when the fuel outlet and draws more fuel to provide a

throttle is opened quickly, when extra power is needed rich mixture

Figure 7.7 A simple fixed choke carburettor

Engine systems 105

7.4.3 Stages of carburation

stages to consider. This can help a lot with diag- nosing faults because it helps you ‘zoom in’ on

The basic principle of a carburettor is to mix fuel the area where the problem is likely to be found. and air together in the correct ratios dependent

Although the examples given relate to a standard on engine loads and temperature. Fuel flow is

fixed choke carburettor, the stages are still rele- caused by the low air pressure around a spray

vant to all other types. The six stages along with outlet and atmospheric pressure acting on the

problems and possible solutions are listed fuel in the float chamber.

below.

Whilst the engine’s requirement for air fuel mix- ture is infinitely variable, there are six discernible

Possible solution or symptoms Cold starting when a richer

Stage

Query or problem

Difficult starting due to a weak mixture mixture is required to reduce the

What would be the result of the choke not

closing fully when starting a cold engine?

effects of condensation within the

To decrease the depression below the inlet manifold and to aid combustion

When starting a cold engine with full choke

why must the choke valve partially open

choke valve thus preventing the mixture

from becoming too rich Idle when the minimum amount of

when the engine is running?

Idling fuel jet restricted fuel should be provided to ensure

A customer complains of erratic idling and

Mixture strength too weak at idle complete combustion and efficiency

hesitation when moving off from rest?

What would be the result of exchanging the

Weak idling air to fuel mixture

idle fuel jet for one of a smaller size?

Erratic idle and flat spots when accelerating from stationary

Ensures an even changeover to the main sequence of change is necessary

Progression when a smooth

What is the purpose of the progression

drilling?

system

from a range of drillings. This is

Lack of response during initial acceleration normally from idle until the main

What would be the result of the progression

with warm engine jets come into play

drillings becoming restricted?

Enriches the mixture for hard acceleration increase in the supply of fuel is

Acceleration when a measured

What function does the accelerator pump

perform?

required to sustain an initial burst

Faulty non-return valve in the pump circuit of speed

The customer complains of a flat spot during

Accelerator pump diaphragm holed Cruising where the need is for

hard acceleration when engine is hot?

Poor mid throttle range performance metered fuel to maintain speed at

What complaint would the customer have if

combined with high fuel consumption the most efficient setting

the main air correction jet was restricted?

If mixture was too weak on cruise, what

Possible misfiring and poor performance

Flat spots on acceleration High speeds where a slightly richer

would be the effect on operation?

Reduced top speed mixture is required to maintain

What operating symptoms could be caused

by blocked full load enrichment tube?

efficient combustion and to avoid

Possible poor fuel consumption and damage to the engine

What symptoms would be observed if the

mixture was too rich at high speed?

reduced engine performance

7.4.4 Electronic control of

Choke

A rotary choke or some other form of valve or flap operates the choke mechanism

carburation

depending on engine and ambient temperature

Electronic control of a carburettor is made in the conditions

following areas. A small solenoid operated valve or similar

Overrun

fuel cut off

cuts off the fuel under particular conditions. These conditions are often that the engine

Idle speed Controlled by a stepper motor to prevent temperature must be above a set level, engine stalling but still allow a very low idle speed to

speed above a set level and the accelerator improve economy and reduce emissions. Idle

pedal is in the off position speed may also be changed in response to a

signal from an automatic gearbox to prevent either the engine from stalling or the car from trying to creep

The air fuel ratio is set by the mechanical design of the carburettor, so it is very difficult to control

Fast idle The same stepper motor as above controls fast idle in response to a signal from the

by electrical means. Some systems have used

engine temperature sensor during the warm

electronic control of, say, a needle and jet but this

up period

has not proved to be very popular.

106 Advanced automotive fault diagnosis Figure 7.8 shows the main components of the

on switching off the ignition. When the ignition is system used on some vehicles. As with any con-

next turned on the stepper will drive the idle and trol system it can be represented as a series of

choke mechanism by a certain number of steps inputs, a form of control and a number of out-

determined as at the end of the last paragraph. puts. The inputs to this system are as follows.

The other main output is the overrun fuel cut off solenoid. This controls the air pressure in the

Engine speed From a signal wire to the negative side of the

float chamber and when operated causes pres-

ignition coil as is common with many systems

sure in the float chamber and pressure in the ven-

Engine This is taken from a thermistor located in the

turi at the jet outlet to equalise. This prevents any

coolant cylinder head waterways. The same sensor is

fuel from being ‘drawn’ into the air stream.

temperature used for the programmed ignition system if fitted Ambient

A thermistor sensor is placed such as to temperature register the air temperature. A typical position is at the rear of a headlight

7.5 Diagnostics – fuel

Throttle This switch is placed under the actual pedal

system

switch and only operates when the pedal is fully off, that is when the butterfly valve in the carburettor is closed

7.5.1 Systematic testing example

If the reported fault is excessive fuel consump- The main controlling actuator of this system is

tion proceed as follows.

the stepper motor. This motor controls by reduc- tion gears a rotary choke valve for cold starting

1. Check that the consumption is excessive for the conditions. The same stepper motor controls idle

particular vehicle. Test it yourself if necessary. and fast idle with a rod that works on a snail type

2. Are there any other problems with the vehicle, cam. The system can operate this way because the

misfiring for example or difficult starting? first part of the movement of the stepper motor does

3. For example if the vehicle is misfiring as well not affect the choke valve; it only affects the idle

this may indicate that an ignition fault is the speed by opening the throttle butterfly slightly.

cause of the problem.

Further rotation then puts on the choke. The extent,

4. Remove and examine spark plugs, check HT to which the choke is on is determined from engine

lead resistance and ignition timing. Check CO temperature and ambient temperature.

emissions.

5. Renew plugs and set fuel mixture. motor before setting the choke position by a

The ECU ‘knows’ the position of the stepper

6. Road test vehicle for correct engine operation. process known as indexing. This involves the

stepper motor being driven to say its least setting

7.5.2 Test equipment

Note: You should always refer to the manufac- turer’s instructions appropriate to the equip- ment you are using.