In many ways a
is less complicated than a petrol
greater reliability and a longer working life. There is little difference
between servicing a diesel and a petrol engine except that the diesel has no
system to worry about. However, the
system needs very careful
attention, as part of the regular service, to prevent dirt and water in the
fuel damaging it.
The very small working
in diesel engine
injectors means it is vital that the fuel passing through the system is always
There is always at least one
fitted in the fuel supply line, usually
in the engine bay. It may be mounted on the bodywork, or sometimes on a bracket
attached to the
. The filter
is disposable and should be
replaced at the recommended service intervals or sooner.
The fuel filter also traps any water in the fuel and you should drain it at
regular service intervals, or sooner if the engine is suffering from
The fuel filter is usually fitted with a drain tap or plug to drain off the
water. First, open the vent
on the top of the filter housing to allow the
water to drain easily. You will find it usually has a screwdriver slot in the
If you can't find a vent valve, there may not be one fitted, in which case
you can create a vent by carefully undoing the uppermost fuel pipe union that
is connected to the filter casing. Hold an old jar under the drain plug or tap,
then open the plug or tap and let the fuel flow out.
Watch the fluid that emerges to see how much water is in it. Close the drain
tap or plug only when the flow has changed to pure diesel fuel. Retighten the
vent valve (or reconnect the fuel pipe). If the engine still runs badly, the
next job is to renew the fuel filter.
Draining the filter
To drain water from the filter, release the vent yalve positioned at the
top of the filter housing. If there isn't a valve, loosen the uppermost
fuel pipe union from the filter housing.
Hold a jar under the drain plug or tap, and loosen the plug or tap.
Let the filter drain until clean diesel fuel flows out, then close the
plug or tap and then the vent valve.
Servicing a canister filter
Undo the vent plug or pipe union in the top of the filter casing and
drain the filter. Spin off the filter and throw it away. If the filter
is very tight, use a strap wrench to remove it.
Smear a little clean diesel fuel on the filter sealing ring then
the filter to the housing and screw it on hand-tight. Make sure the drain
plug or tap is closed.
With all types of filter, start by cleaning off all traces of dirt from
around the filter casing and housing - absolute cleanliness is essential when
working on the fuel system. Then drain off all the water and fuel from inside
the filter in the way described above.
Some cars have a canister filter much the same as an oil filter. The filter
element is replaced complete with its casing.
A second filter design has a semi-canister filter that is held between two
. Find the securing nut or screw that holds the two plates together
(usually in the centre of the top endplate). Hold the bottom endplate and the
filter element, undo the nut or screw and lower the endplate and element away,
together with the 0-rings that
the joins to the endplates.
Fit new 0-rings to the filter element, then reassemble it to the endplates -
do up the securing nut or screw tightly to prevent leaks.
A third design of filter has a replaceable filter element contained in a
housing, much like the older design of oil filter. Find the bolt or screw that
secures the filter bowl to the filter mounting. Hold the bowl, and undo the
fixing. Remove the bowl, making a note of how any sealing washers fit.
Pull the old filter element from the bowl. Wipe out any dirt in the bowl,
then fit the new element. Fit a new sealing 0-ring into the filter mounting,
and replace the bowl, doing up the fixing bolt or screw tightly to hold it
Where the filter sits inside a metal bowl, you first have to get the
bowl off. Drain off the fuel, then find the bolt or screw that holds the
bowl to the filter. Hold on to the bowl and release the fixing. Release
the bowl, then lift out the element from inside it.
Clean out the bowl with a non-fluffy rag then fit the new element
and reassemble the bowl. Don't forget to shut the drain tap.
Replacing a semi-canister filter
If the filter consists of a metal cased element sandwiched between two
metal endplates, you only need to renew the element.
Drain off the fuel via the drain plug or tap, then undo the nut or screw
holding the assembly together. Release the bottom endplate and filter
When refitting, fit new sealing 0-rings to seal the joins between
the filter element and the endplates.
Bleed fuel system
Fitting a new fuel filter allows air to get into the fuel system, which will
prevent the engine from running properly - the same thing can also happen if
you run out of fuel. Most engines have an automatic
system which gets rid
of the trapped air but others have to be bled manually. However, it is often a
good idea to bleed the system manually, even if it is supposed to be
Some systems have a hand-operated bleed pump built into the injector pump,
but otherwise you have to turn the engine over on the
to make the main
Undo the vent valve on the fuel filter housing. If there is no plug, slacken
off the top pipe union on the housing. Operate the hand bleed pump or turn the
engine over on the starter (a helper may be useful) and watch the vent plug or
union. You will see a mixture of fuel and air bubbles coming out.
Continue pumping until the emerging diesel fuel is free of bubbles, then
close the vent valve (or tighten the pipe union). Sometimes there is another
valve on the injector pump that you have to bleed in the same way - ask your
dealer if your car has one.
Oil and air
Renewing the oil filter is identical to that on a petrol engine. But when filling with new oil, make sure you use the
correct grade for your car. Many diesels use a different grade of engine oil to
petrol engines - check in your handbook to see the exact specification
is usually the same as on a petrol engine although some newer diesels use large air filters, housed in a
two-piece plastic casing held together by spring clips.
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have always been seen as noisy, smelly and
of little use other than in trucks, taxis and vans. But as
diesel engines and their injection system controls have become more refined,
the 1980s have seen that situation change. In the UK in 1985 there were almost
65,000 diesel cars sold (about 3.5 per cent of the total number of cars sold),
compared with only 5380 in 1980.