Running-on is when the
continues to turn after you've switched off
/air mixture is being ignited by a hot-spot in the
. Furthermore, because the mixture is often firing at the
wrong point in the
, the engine does not run smoothly but jerks and rocks
badly on its mountings (and sometimes
). The problem can be so bad
that the engine runs on for a minute or more if left alone.
Running-on is not good for your engine and it can cause serious damage if
you allow it to go on for too long. A stop-gap measure (if your car is manual)
is to put the car into
the engine; but this doesn't solve the
problem in the long term.
Why it happens
There are a number of reasons for the engine developing a hot-spot that
leads to running-on, some of which are easy to check and cure, while others are
The simplest causes are that the wrong grade of petrol is being used
(usually of too low
are faulty or of the wrong grade,
is badly out of tune, or the
is faulty. You
should check all these before going on to consider more serious causes such as
Spark plug checks
First take out all the
plugs and examine them carefully. Look at the
and the nose of the insulator for any signs of overheating. The
plugs should have a light brown coating all over - if they appear white or
glazed, they have been overheating.
If the existing plugs are old, it's worth replacing them with a new set to
see if that cures the problem. Make sure that they are the correct type for
If the problem initially goes away and then quickly recurs, the standard
grade of plugs may be unsuitable for your particular engine. Check with your
dealer or handbook to see whether the car manufacturer specifies different
types of spark plug for different driving conditions, for example, a cooler
running plug may be required for mainly fast driving, while for driving around
town a hotter one may be specified. Remember that if you
because most of your driving is on motorways, they will foul up more quickly
Checking the ignition and fuel systems
Take out the spark plugs and check them for signs of overheating or
to check that the ignition timing is accurately set.
to check that there are no leaks into the inlet
and adjust the fuel/air mixture to the correct setting.
Check engine tune
If the spark plugs aren't at fault, you should next check the ignition
timing. Usually, you will have had other warning of overadvanced timing, such
as pinking under hard acceleration.
It is best to check the timing stroboscopically with the engine running -
this is far more accurate than doing it statically.
If running-on still persists, your next avenue of investigation is the fuel
system. The problem may be caused by a weak fuel/air mixture because of a badly
adjusted carburettor or air leaking into the inlet manifold. A weak fuel/air
mixture can make the engine run much hotter than it should.
First make sure the
is clean and correctly fitted. Follow all the
system pipes that connect to the
or carburettor to check that
none is disconnected or split.
If all else seems well so far, start the engine and listen for a hissing or
sucking sound from any of the joints between the carburettor and inlet
manifold, or the inlet manifold and engine. Don't forget to check where any
pipes or hoses join the manifold. Try not to confuse the sound with the normal
intake roar through the carburettor.
If you think you may have found a leak, use a piece of tube as a stethoscope
to listen along the joint to make sure. As a further check, paint a little oil
on to the joint with an old paint
- if there is a leak you will see the
oil disappearing into the hole. Replace any suspect
If the manifold is leak-free, turn your attention to the carburettor. Tune
the carburettor (see the appropriate Mechanics sheet for your variety of
carburettor) until the fuel/air mixture is correct. Also check that the idling
speed is not set too high because this can cause running on. If you have
trouble getting the carburettor to tune, there may be wear in the carburettor
itself. If it is beyond saving, you need a new or reconditioned carburettor.
You may be able to save money by getting one from a scrapyard.
Checking an anti-diesellinq valve
fitted by the car manufacturer is usually the
type built into the side of the carburettor, rather than an
valve connecting to the inlet manifold.
To check the valve, disconnect the wire and carefully unscrew the valve
from the side of the carburettor using an open-ended spanner. You can
usually get it out without removing the carburettor.
Use a jumper lead to connect the live
or wire on the valve to
terminal, and touch the casing of the valve to the
negative. If the valve works, there will be a click and the rod in the
end will retract.
The other point to check on the carburettor is the anti-dieselling valve (if
your car has one). If this valve is the type built into the side of the
carburettor, which shuts off the fuel flow to the idle
when the ignition is
switched off, then it can stick in the open position, allowing fuel to flow
through and causing running-on.
To find the valve, consult your dealer or check in a workshop manual to see
if your car has one. If you can't find this information, look at the outside of
the carburettor for a barrel-shaped object with a wire coming from it.
To check if the valve is working, disconnect the wire that connects the
valve to the electrical system, then use a spanner to unbolt the valve from the
carburettor. Connect the wire or terminal on the valve to the positive battery
terminal, and touch the body of the valve to the negative. If the
the valve works, you will hear a click, and the small rod that pokes out of the
valve will move inwards.
If the valve doesn't do this, it needs replacing. But if it works, then the
electrical system may be faulty. Clip your test lamp to earth and probe the
wire on the car that feeds the valve while a friend turns the ignition on and
off. The test lamp should come on and go off with the ignition.
Fitting an air bleed valve
valves mount on the car bodywork and connect to the inlet
manifold by a short length of flexible hose. The hose can be plumbed into
any existing hose to the manifold (such as the emission control system) or
to a spare stub on the manifold. If there is only a single stub on the
manifold with a hose already on it, you may be able to get a Y-adapter
piece and connect the existing hose to one stub and the new one to the
Offer up the bleed valve to the inner wing and mark the hole positions. Use a hammer and centre punch to dent the hole position and prevent
the drill slipping. Drill out the holes, taking care not to damage anything
on the other side of the panel.
Attach the valve to the car with self-tapping screws. Push the
flexible hose over the stub on the valve, and secure it with a clip.
Run it down to your chosen take-off point on a hose or the manifold. If you
are connecting into a hose, cut the hose, fit a T-piece and fit all the
hose ends to it. Secure them all with clips.
If you are attaching to an adapter, remove the existing adapter or
blanking plug and screw in the new adapter in place. Attach the hose to the
stub and secure it with a clip.
Crimp a suitable connector to a length of wire and push it on to the
bleed valve. Run the wire from the valve to an ignition-controlled live
feed. Connect it in. Ask a friend to turn on the ignition, and listen to
the valve —you should hear a distinct click as the ignition is turned on.
Start the engine and check that it runs well and that the running-on is now
If you still haven't traced the cause of the problem, then it's most likely
to lie within the
chamber itself. Unfortunately there's not much you
can do to check where the problem lies without going to the trouble of taking
If your car has covered a high mileage or done a lot of short journeys, then
the engine is most likely to need a decoke. The best way to do this is to
head and clean out the combustion chambers.
But before you go to the bother of decoking, consult your dealer or an
engine tuning specialist to find out whether your particular car is susceptible
to running-on. If so, you may find that later models were modified with an air
bleed valve to help overcome the problem, and you can fit the parts from one of
these models to your car.
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are found on many different cars and are generally very
reliable. But after a high mileage you may find that the
quite as it should. If the rest of the
system is well tuned
but the car doesn't accelerate cleanly, the problem may lie in the distributor
The high-tension (HT) or
carries high-voltage electricity. It runs from the
to the plugs. Any of these can break down and cause