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.