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Running-on is when the engine continues to turn after you've switched offthe ignition because the fuel/air mixture is being ignited by a hot-spot in thecombustion chamber. Furthermore, because the mixture is often firing at thewrong point in the cycle, the engine does not run smoothly but jerks and rocksbadly on its mountings (and sometimes backfires). The problem can be so badthat the engine runs on for a minute or more if left alone.
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Running-on is not good for your engine and it can cause serious damage ifyou allow it to go on for too long. A stop-gap measure (if your car is manual)is to put the car into gear and stall the engine; but this doesn't solve theproblem in the long term.
There are a number of reasons for the engine developing a hot-spot thatleads to running-on, some of which are easy to check and cure, while others aremore difficult.
The simplest causes are that the wrong grade of petrol is being used(usually of too low octane), the spark plugs are faulty or of the wrong grade,the carburettor is badly out of tune, or the ignition timing is faulty. Youshould check all these before going on to consider more serious causes such asexcessive carbon build-up.
First take out all the spark plugs and examine them carefully. Look at theelectrodes and the nose of the insulator for any signs of overheating. Theplugs should have a light brown coating all over - if they appear white orglazed, they have been overheating.
If the existing plugs are old, it's worth replacing them with a new set tosee if that cures the problem. Make sure that they are the correct type foryour car.
If the problem initially goes away and then quickly recurs, the standardgrade of plugs may be unsuitable for your particular engine. Check with yourdealer or handbook to see whether the car manufacturer specifies differenttypes of spark plug for different driving conditions, for example, a coolerrunning plug may be required for mainly fast driving, while for driving aroundtown a hotter one may be specified. Remember that if you fit cooler plugsbecause most of your driving is on motorways, they will foul up more quicklyaround town.
Take out the spark plugs and check them for signs of overheating or weak mixture.
Use a timing light to check that the ignition timing is accurately set.
Use a hose to check that there are no leaks into the inlet manifold, and adjust the fuel/air mixture to the correct setting. Adobe illustrator cs4 mac download.
If the spark plugs aren't at fault, you should next check the ignitiontiming. Usually, you will have had other warning of overadvanced timing, suchas 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 fuelsystem. The problem may be caused by a weak fuel/air mixture because of a badlyadjusted carburettor or air leaking into the inlet manifold. A weak fuel/airmixture can make the engine run much hotter than it should.
First make sure the air filter is clean and correctly fitted. Follow all theemission system pipes that connect to the filter or carburettor to check thatnone is disconnected or split.
If all else seems well so far, start the engine and listen for a hissing orsucking sound from any of the joints between the carburettor and inletmanifold, or the inlet manifold and engine. Don't forget to check where anypipes or hoses join the manifold. Try not to confuse the sound with the normalintake roar through the carburettor.
If you think you may have found a leak, use a piece of tube as a stethoscopeto listen along the joint to make sure. As a further check, paint a little oilon to the joint with an old paint brush - if there is a leak you will see theoil disappearing into the hole. Replace any suspect gaskets.
If the manifold is leak-free, turn your attention to the carburettor. Tunethe carburettor (see the appropriate Mechanics sheet for your variety ofcarburettor) until the fuel/air mixture is correct. Also check that the idlingspeed is not set too high because this can cause running on. If you havetrouble getting the carburettor to tune, there may be wear in the carburettoritself. 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.
Compare your spark plugs to the ones shown here to see whether they are the cause of the problem. A badly glazed or eroded plug has been overheating and must be replaced. White deposits on the end of the plug indicate that the fuel/air mixture is too weak.
The anti-dieselling valve fitted by the car manufacturer is usually the type built into the side of the carburettor, rather than an air bleed 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 terminal or wire on the valve to the batterypositive 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.
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The other point to check on the carburettor is the anti-dieselling valve (ifyour car has one). If this valve is the type built into the side of thecarburettor, which shuts off the fuel flow to the idle jet when the ignition isswitched off, then it can stick in the open position, allowing fuel to flowthrough and causing running-on.
To find the valve, consult your dealer or check in a workshop manual to seeif your car has one. If you can't find this information, look at the outside ofthe carburettor for a barrel-shaped object with a wire coming from it.
To check if the valve is working, disconnect the wire that connects thevalve to the electrical system, then use a spanner to unbolt the valve from thecarburettor. Connect the wire or terminal on the valve to the positive batteryterminal, and touch the body of the valve to the negative. If the solenoid inthe valve works, you will hear a click, and the small rod that pokes out of thevalve will move inwards.
If the valve doesn't do this, it needs replacing. But if it works, then theelectrical system may be faulty. Clip your test lamp to earth and probe thewire on the car that feeds the valve while a friend turns the ignition on andoff. The test lamp should come on and go off with the ignition.
Most air bleed 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 other.
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 cured.
If you still haven't traced the cause of the problem, then it's most likelyto lie within the combustion chamber itself. Unfortunately there's not much youcan do to check where the problem lies without going to the trouble of takingoff the cylinder head first.
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If your car has covered a high mileage or done a lot of short journeys, thenthe engine is most likely to need a decoke. The best way to do this is toremove the cylinder head and clean out the combustion chambers.
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But before you go to the bother of decoking, consult your dealer or anengine tuning specialist to find out whether your particular car is susceptibleto running-on. If so, you may find that later models were modified with an airbleed valve to help overcome the problem, and you can fit the parts from one ofthese models to your car.