View Full Version : Engine "runs on" after switch off (car trouble!)

Andy Rylance
27th Aug 2008, 12:43
Friend has 13 year old car, VW Golf. Just started to shudder and "run on" when ignition switched off. What causes this as he is worried if it goes to a garage they will "take him for a ride" on repairs so wants to know the cause of it and whether he can carry on motoring with it or it could be really bad news....

Any help my friends out there in the world of knowledge....


27th Aug 2008, 12:46
Is it a diesel?

Andy Rylance
27th Aug 2008, 12:49
No petrol 1.6 - nothing special!

27th Aug 2008, 12:51
Usually a sign of needing a decoke - all that glowing carbon keeping it going.

27th Aug 2008, 12:52
Running too rich, I take it it's not an injection?
Sounds like the carb needs a bit of cleaning and attention...


Andy Rylance
27th Aug 2008, 12:54
Ah you may have something there, because it does tend to rev highly when stuck in traffic... tune up time does not sound too bad then... no rebuilding engines...not worth it for a car worth a few hundred quid!

27th Aug 2008, 12:57
The usual cause is a sort of overheating that occurs when carbon deposits build up in the cylinder head combustion space and this ignites the mixture instead of the sparking plug. Errors in ignition timing can contribute to this, but, even considering the age of this vehicle, I expect that this vehicle doesn't have a conventional distributor and carburettor, but probably has electronic ignition and fuel injection, so some sort of electronic sensor fault might be the cause.
Beyond me I'm afraid . . .

it does tend to rev highly when stuck in traffic... Might be worth looking at the throttle linkage return spring (or lubrication). Keeping the throttle open when it should be closed isn't going to help (quite apart from using excess fuel).

27th Aug 2008, 13:02
One of the causes is a too far advanced ignition, needs retarding a touch.

That is the first thing I would try.

Loose rivets
27th Aug 2008, 13:02
Take it out and give it an Italian Tune Up. i.e. Belt the pants off it. WAT it (Wide open throttle) time and time again. It'll love you for it.

If the engine blows up, I didn't say that.:}

Effluent Man
27th Aug 2008, 13:04
Definitely ignition timing too advanced,this has disappearedwith modern electronic ignitions.

Alloa Akbar
27th Aug 2008, 13:12
Does it do it all the time or just with the lights switched on??

Reason I ask, is i had a Ford once (Ok I admit it...:*) which used to do the same thing, only with the lights on though, and the reason was because I replaced a tail light bulb with a bulb of the wrong type and it caused a short in the electrics..

Simple, stupid but mystifying!!

27th Aug 2008, 13:15
Had a Renault and you could remove the key from the ignition switch and it would still keep on running . . .

27th Aug 2008, 13:36
I was going to suggest loose rivets method. I never heard of it called an 'Italian tune up' before. :O I wonder does he use it mainly in town, just chugging around. That does no good for an engine.

A good fast run at high revs might do the trick.

Windy Militant
27th Aug 2008, 13:44
Have a look at the Plugs*, if covered in carbon clean or replace them. Check the timing with strobe light.
Bung some Redex in the tank and go for a brisk drive on a dual carriageway or motorway. :ok:

* If they've not been removed for 13 years I suggest your mate buys a new car, rather than trying to get them out.;)

27th Aug 2008, 13:49
A new set of spark plugs is probably the simplest thing to try first.

27th Aug 2008, 14:01
As previously said, fouled plugs, sooted cylinder heads can cause run on. the Idfle stop solenoid on the carbty may be stuck in the run position (with power off, butterfly will drop a little further to stop fuel entering.).

I dont think fuel injected cars will run on as there will be no power to allow the injectors/computer to run.

Run the engine with out the aircleaner, and allow a tricle of demin / distilled water into the carby for a little while, that will de coke the engine a bit (whatever happened to water injection any way)

27th Aug 2008, 14:11
Yes, there should be a little doohickey on the carb that cuts the fuel when the ignition is switched off but the primary cause might be coking. Nowadays, with unleaded fuel, that isn't as much of a problem as it once was. Too, modern fuel injection systems cut the fuel supply when the key is off and even on over-run down to a certain r.p.m.

A pragmatic solution is simply to switch off and then gently let the clutch out with the car in gear and your right foot on the brake. It won't run on then, no matter the state of the combustion chambers!

A Haynes Manual is always a good investment, even if you do not plan on getting your hands dirty. Even better is having a good mechanic whom you trust.

27th Aug 2008, 14:21
Cheap opiton to start with; change plugs, and air filter. Then a good work out on the motorway for at least a couple of hours to blow out as much of the c**p as you can will often result in an improvement. A few hours on the motorway coming back from the west country this bank holiday weekend did wonders for the Bucketmobile. Poodling around the town does no good for any car however sophisticated.

Then look at iginition. This means the use of a mechanic so costs will start to rise. Full tune up perhaps.

How old is the car? Mileage etc? Cars wear out, sooner or later.

Best of luck. Hope you fix it.


Alloa Akbar
27th Aug 2008, 14:32
**** it.. buy a new car man!!:ok:

27th Aug 2008, 14:50
Timing, timing, timing. I'd bet my mortgage on it. For a garage to adjust the timing, you should pay a fiver. Tenner tops. Takes two minutes to do.

Or you could get a timing strobe & do it yourself.

Andy Rylance
27th Aug 2008, 14:54
I always knew the car crew out there would solve the problems.

The car is used for long straight motorway journeys nothing in town.

My main issue would be advising them on additives - thinks like redex and carbon reducing liquids - are you sure they cannot screw up the engine in anyway?!!

27th Aug 2008, 15:05
The thing about car problems is you should deal with the problem you know about first, not the ones you suspect you have. The overwhelmingly likely cause of running on is timing. It's also the cheapest to fix.

So, get the timing done first, then if the car runs fine after that, leave well alone - if it ain't bust, don't fix it.

However, if after that, there is still noticeable loss of performance, then go through the likely causes one by one. Filters, plugs & ignition compnents first, then go on to stuff like the fuel system & how clean the engine may be.

Logically, if a car runs poorly, then it can only be one of three things: fuel, ignition or mechanical - I'm generalising a little of course & disregarding modern ecu controlled cars, but first principles is always a good place to start.

Loose rivets
27th Aug 2008, 17:44
mechanic whom you trust.

Shouldn't that be, 'In whom you have trust.' ? :}

27th Aug 2008, 18:42
This is caused by advanced timing or weak mixture or a combination of both, causing excess cylinder temperatures. As mentioned previously, the problem is exacerbated if the engine is coked, or the wrong grade of plugs are used. A cooler grade may help.

With older cars, even with settings correct, they often struggle a little with the p*ss now referred to as fuel. If this is the case, avail yourself of something called an anti-dieselling valve, you'll have to search!

This opens the inlet system and vents in air when you switch off, stopping the run-on. Easy to fit, usually in the brake servo vacuum line. Would probably be around 20 at a guess.

27th Aug 2008, 20:32
Weak mixture and running-on can be caused by an air leak on the inlet side, either carb to manifold, or manifold to cylinder head.

I would also check the idle cut-off solenoid for proper operation on a car this old.

I don't see how the ignition timing would advance itself. Unless someone's been fiddling under the bonnet I would be surprised if this is the cause.

27th Aug 2008, 22:07
I've usually heard this referred to as "dieseling", i.e. ignition via compression only, sans electrical spark.

Which leads me to a funny story... :}

simon brown
27th Aug 2008, 22:21
I remember in the old days when i was a Lad our chap had a Triumph 2.5PI that he used to drive a couple of miles to work and back every day. By the end of a month or so it would start running rough without fail , so he used to take it for a high speed run on a Sunday to "sort out the cob webs" as he used to say...used to work a treat

27th Aug 2008, 22:39
In the days when engines were fitted with contact-breakers in a distributor there was usually a vacuum advance device that 'sucked' a diaphragm that moved the baseplate of the distributor and thus 'advanced' the spark timing. I believe that there was also (at least on some engines) a sort of 'gimbals' whereby weights acted centrifugally to adjust the engine ignition timing. Over time (and mileage) these mechanisms could become worn and alter the basic ignition timing, so rectification of incorrect timing might involve a new distributor.
Modern engines don't have these problems, but I'm not cognisant with the degree of modernity of the 13 year old Golf 1.6 litre engine and I can't find specifications on-line.

27th Aug 2008, 22:52
Running on is caused by coke build-up, as others have said. The coke glows red hot like a glow-plug in a diesel, and this provides the residual source of heat to ignite the fuel-air mixture even after the ignition is switched off.

The coke build-up is caused by running with too rich a fuel mixture. Most common cause of this is a faulty automatic choke or a blocked air cleaner, or both. It is made worse by lots of idling in traffic or short cold runs. Your observation that it revs highly when stuck in traffic sounds more like choke than anything else. If the car has a carburettor, which it probably doesn't, you can check the mixture and idle setting screws. If it's injected, you're probably back to air cleaner and choke again.

Either way, the engine can't run-on if the fuel shuts off with the ignition key, which in this day and age, it should. Even quite old cars more often than not have a fuel cut-off solenoid, expressly for preventing run-on. Find this solenoid and get it checked. It's probably knackered (or maybe a lead has come loose).

Timing does not cause this problem, but given your other issues, it may need checked as well. If your ignition is electronic, which it should be, there won't be anything wrong with the timing. Electronic ignition either works or it doesn't. If you're worried about the timing being too far advanced, put the car in fourth at about 30mph on a flat road and floor it. If the engine "pinks" (a rattly tinking noise), then it's too far advanced. If it's TOO too far advanced, it will behave as if the battery is flat when you try to start it. If it's too far retarded you'll get hard starting, overheating, low power, rough running, and clouds of blue smoke.

So...check your choke and fuel cut-off solenoid, give it a new air cleaner and a fresh set of plugs, and take it for a long blast down the motorway. That'll burn the carbon off the pots, and besides, it's fun.

Good luck.

Andy Rylance
27th Aug 2008, 23:28
take it for a long blast down the motorway. That'll burn the carbon off the pots, and besides, it's fun.

So it is not necessary to buy all this fancy stuff additive to decoke it - a firm run high speed will sort it?!

27th Aug 2008, 23:36
Yep, so long as you fix whatever is causing the mixture problem in the first place, so the coke doesn't keep building up, hot running will burn it off. And you'll still need to make sure your solenoid is working.

Fancy additive stuff might burn it off quicker, I don't know, we never 'ad fancy additive stuff in't olden days, we just had to DRIVE REALLY FAST :ok:

27th Aug 2008, 23:43
The condition you describe used to be called 'Pre-ignition'
The engine in fact is running backwards, I refer to a petrol engine, a simple test is to put it in a forward gear, apply the brakes let in the clutch and stall it, the motor will jerk backwards.
The crankshaft is in danger of being damaged, my advice is to retard the timing and go from there.
To do that, one releases the distributor locking screw and turn the distributor very slightly in the same direction that the rotor arm travels.


28th Aug 2008, 00:00
If it needs decoking, one cheat's method is to gently dribble water into the intake (no, not through the air filter, take that off first). The steam softens the deposits so they are burned off. Others advocate using ATF as the fluid to use to loosen it all up in there.

I was always taught that "pre-ignition" is actually where the mixture ignites before the spark, not when there is actually no spark / ignition turned off. That's auto-ignition in my book.

P.S. Is this a five minute argument, or the full half hour? :E

28th Aug 2008, 00:11
I would find it hard (maybe impossible) to give my current car a 'blow-out'. It is a diesel, but that's not the problem. The gearing means that the engine only runs at 3000rpm at 90mph, so achieving a fast run (as far as the engine is concerned) would involve exceeding the speed limit by 'quite a lot' (although I suppose I could run it in 4th gear).
Gone are the days (for various reasons) when one could blow out the cobwebs with a quick blast along the by-pass . . .

As far as 'burning off the carbon', if the fuelling is set rich (perhaps if the choke mechanism is faulty) or if the air-filter is clogged or the ignition timing is faulty, then you might be wasting your time.

Removing the spark plugs after a 'high-speed' run (yes, they'll be hot . . .) will give an indication of how accurately the mixture setting is (as will examining the exhaust tailpipe - assuming that the exhaust system is gas-tight).
A sooty deposit will suggest that the engine is running too rich (and the fuel consumption wil probably endorse this) whilst bleached white deposits indicate that the mixture is lean or weak. A mid-brown is more desirable (for petrol-engined cars).

The suggestion that you should get a Haynes Manual for the particular model is a good idea, as this will confirm the specification and give guidance about the adjustments and changes that affect the proper running of the engine. For example, some automatic chokes (such as that fitted to the Hillman Imp) wear to the extent that they are constantly operating, even when they shouldn't be.
A general service of the engine (plugs and air cleaner element) should be within the capability of many people (IMO) and should eliminate some of the potential faults that could contribute to the running-on. It's possible that this vehicle has been neglected, and an air cleaner that is clogged isn't efficient and can effect good running.

Once the running-on is solved, it's time to consider an oil filter change (with fresh oil of course), especially if it's been 'a long time' since this was done. You don't need to use the most expensive oil (or filter) - go to a motor factor and take their advice on cheap oil and 'pattern' filter rather than the pukka VW original parts. Spark plugs likewise (and the air filter element, though this might be one that you rinse out in petrol and soak in oil rather than a renewable paper one).
RTFM . . .

28th Aug 2008, 00:20
I was always taught that "pre-ignition" is actually where the mixture ignites before the spark, not when there is actually no spark / ignition turned off. That's auto-ignition in my book.

P.S. Is this a five minute argument, or the full half hour?

Yeah, pre-ignition is pinking, and running-on is dieseling.

5 minutes is fine by me, I have to go and pretend to commit work just now. :ok:

28th Aug 2008, 03:54
Timing does not cause this problem.

It's not the primary cause once the ignition is switched off, but if it raises running temperature, it has an effect.

If you're worried about the timing being too far advanced, put the car in fourth at about 30mph on a flat road and floor it. If the engine "pinks" (a rattly tinking noise), then it's too far advanced.

I think most modern-ish engines tested in this way would pink due to the water-like fuel now sold, were it not for timing retardation via the knock sensor. Some systems don't retard as far as others, and pinking will be experienced even when all settings are correct if loading the engine like this.

28th Aug 2008, 13:55
I'm no mechanic, but the timing has no effect when the ignition is off. Try new spark plugs as mentioned as they can trap glowing carbon deposits. While you have the bonnet up change the distributor cap if it has one, and check the timing anyway.

28th Aug 2008, 14:16
All pollocks. The sole cause is that the turbot charger is clammed up and needs a tuna.

OK OK I'll skate off.

28th Aug 2008, 20:01
I haddock clue someone might mussel in like this with a fish pun. :suspect:

Of course, I would never ........ :O

Andy Rylance
28th Aug 2008, 21:04
Thank cod salmon out there still helped me!

The Flying Pram
28th Aug 2008, 21:19
Have you checked in the handbook to see what grade of fuel it was designed for? Chances are it would have been at least 97 octane, not the 95 octane that "ordinary unleaded" is these days. Run it low on fuel and put some of the higher quality stuff in just to see what happens.

green granite
28th Aug 2008, 21:31
Arrrrr the Italian tune up, in the old days used to do that by putting in the top octane rating fuel available (5star) and belt it like hell in 3rd gear for an hour or so, preferably at night so you could watch the firework display coming out of the exaust pipe as the carbon burnt away. :E

I had a sierra that suddenly wouldn't switch off with the ignition, I found the cause was a sticky relay in the engine management system.

The Flying Pram
28th Aug 2008, 21:40
Of course in the "Old Days" we would fit 2 head gaskets to reduce the compression ratio........

Loose rivets
28th Aug 2008, 23:05
Pah!! In the really old days, we would fit a skinny stainless head gasket to increase the compression ratio. One does not grow tired of reminiscing about an 850 mini with a 13:1 ratio. It would only run on 5 star. 50 mpg at 50 mph. mind you, that had so much meat taken off that the oil duct had to have some brake pipe brazed in cos it was exposed. Now, what were you youngsters saying?

Running on...Ah! Now there's a thing. One purchased, nay stole, a Chevvy Blazer from One's sunny Jim. He told me the thingie valve was so prone to coaking up that he had put a metal plate over the duct. But, every so often, it would suddenly run rich...really rich. Blub blub blub...just like a choke in when hot. When the engine felt rough at tickover, it was time to reset the computer. He used to dissconnect the battery, but I, being lazy, learned which fuse to pull. Wait ten secs and replace it. Fire it up, just like new.

So, a computer reset can be done by powering down.


29th Aug 2008, 00:42
I'm sure AndyRylance is thoroughly confused and ruddy wels can't work out what to mackeral this advice. Perhaps it's time to buy a different motor carp (but not a Roe-ver).

Dan D'air
29th Aug 2008, 05:57

Do you think perchance that he should try a Tarbut Sunbream??

29th Aug 2008, 09:04
Or a Ford Carp-ri.

29th Aug 2008, 10:10
Salmon should put a stop to all this baiting.

Beatriz Fontana
29th Aug 2008, 11:48
It's a load of pollocks...

29th Aug 2008, 12:54
Some valid and good suggestions amongst the three pages of posts but equally some of the theories, misconceptions and plain rubbish also posted is very amusing.

I wouldn't let 90% of you near my car!

Andy Rylance
29th Aug 2008, 20:05
I think we may have solved the problem. The car went for the "Italian tune up" option and was something like out of Star Trek - as the passenger cried out "Captain she canna take nae more" as the car started wheel shaking slightly on the steering and the driver was getting funny tunnel vision. I think this was at about 76 mph or so. Anyway as it came down from warp speed the running seems to be a whole lot better, so far... just trying to find the bits the dropped off during this hyperspace moment...:)

Fair plaice to everyone out there, but I have advised the owner to get a pike instead as two fins are better than four.....

Paul Wilson
29th Aug 2008, 21:59
Me old dad used to call it the M1 tune up, 3rd gear 70mph see what happened, I still remember the day when he said "I didn't know this car had a rev limiter", "It doesn't dad, that's valve bounce"
Best way to get yoor engine to last is, vary your driving style, thrash it sometimes (but never from cold) pootle around others, and ignore the service schedule and change the oil and filter every 6000 miles. 12000 mile service intervals are only for the convienience of Sales departments and fleet managers, not for the private owners who want their cars to last.

30th Aug 2008, 02:01
weak mixture on vw bottom of carb an air screw turn in abit till the engine hunts screw out a turn or till the engine runs smooth a weak mixture causes the valves seats to glow r/hot because its a ventura asperated carburretor it keeps pulling in fuel and ignites