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Car running on after ignition off, causes??

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by robsalvv, May 12, 2008.

  1. Hello NR.

    The symptom: 4 cyclinder petrol car, circa '74 continues to unevenly "run on" after the igntion is turned off.

    What are the possible causes of such symptoms?

    Is this a hot carbon deposit thing on the top of the piston which will clean up with avid applications of fuel system cleaner??

    What other causes???

    Thanks NR.
  2. Petrol, not sure. One of our cars is a diesel which had a vacuum leak which caused it.
  3. Build up of carbon is the most likely though can also be caused by rough surfaces on the metal caused by wear. Wrong sparkplug can also cause it (ie anything that can create a hot spot in the engine and act like a glowplug).

    Fuel system cleaner, or even just running a few tanks of premium fuel (eg BP Ultimate) can cure the problem if it's due to carbon build up - it's worked for me in the past, along with giving the car an "italian tune" (running the engine to redline as often as possible in as many gears as possible ;)).
  4. Thanks JD. Hot carbon deposits was what I was thinking.

    I've also heard that electrical issues/dirty points can cause "diesling". I can't see why this'd be the case though...

    Wouldn't you know it, there's a wiki entry on the topic.: http://en.wikipedia.org/wiki/Dieseling and hot carbon deposits look like the main culprit. :)

  5. It is called pre-ignition and is caused by the ignition timing being wrong/in need of adjustment
  6. Taa Vic. Genuine question, how does incorrect timing cause the running on after the ignition is turned off? The wiki lists this as a possible cause too but it still doesn't make sense to me.

    If timing was off, wouldn't the car run poorly in the base case??
  7. It CAN be caused by build-up of carbon (gets red-hot, causes ignition when there is no spark), but much more commonly by the ignition timing being out of adjustment.
    Usual method of adjustment for a car of this age (what is it?) is to slacken the bolts holding the dizzy in position, and rotate it VERY SLIGHTLY to change the timing point. The trick is to get it between the positions where it runs on, and where it 'pings'.
    Ideally, you should get a timing light and do it properly.

    edit: My explanation is that the spark occurs when the exhaust valves have started to open, meaning they get very hot and ignite the mixture without the need for a spark when that is turned off (an engineer can correct me if I'm wrong).
    Similarly, very badly burnt exhaust valves (or those which don't close properly because there are no clearances) can cause the same symptom.
  8. I'm with you on this Rob. If there is no battery powered spark (the ignition is turned off), the timing should not be a factor.

    Deposits is what you'd normally expect.

    I like JD's idea. Though revving the tits off a 34yr car might be the last thing it does!
  9. Titus... I'm still confused.

    If the ignition is turned off, meaning the electrical circuits are broken, how does ignition timing cause dieseling?? :?

    BTW, I used to do what you describe for my old '75 Chrysler Galant - I'm sure I still have a full tune up kit lurking somewhere in my detritus... ahhh memories... I spent way too many weekends under the bonnet of that old clunker... incidentally, it never dieseled... though 4 head replacements meant we got to carbon free the top of the pistons regularly :roll: ... damn metal fatigue and the ports being too close...

    :LOL: Cejay, I believe the car is known as the "old gal" so it would literally be revving its tit's off! :LOL:
  10. My first thought would be the carbon deposits already mooted. From my memories of playing with older cars, anything powered by a BMC A or B Series engine was particularly prone to this, aided and abetted by a little blip of cast iron between the valves as a reult of combustion chamber shape, that tended to get incandescent. Dunno if the closely related small Datsun engines had the same problem.

    On a related note, I once had a (borrowed) Suzuki GP100 run away uncontrollably whilst revving it in neutral. The revs just kept climbing until it was screaming way beyond red-line. Shutting the throttle had no effect, nor did turning off at the key or the kill switch. Eventually I had to bang it into gear and stall it.

    Don't know what happened. A combustion chamber hot-spot might explain the lack of response to killing the ignition, but I've never worked out how it managed to scream like that on a closed throttle :? .
  11. Incorrect timing causes hot spots in the engine during normal running. When power is removed by turning the ignition off, the hot spots remain as an ignition source, so the engine runs on as long as the ignition source is hot enough. Carbon deposits act as better "Glow Plugs" than hot metal components. Hence, dieseling.

    Doesn't happen with electronic fuel injection systems, since the fuel supply is turned off with the ignition.

    EDIT: "Incorrect Timing" also includes valve timing.
  12. Possessed.
  13. Taa on the incorrect timing thing fellers. For the time being, I'll assume timing issues aren't at play.

    Pat B, I think Cejay's onto something... that sounds supernatural. :eek:hno:
  14. My main worry about posession was the bike's ownership by a serious martial arts expert with an anger management problem. I didn't particularly look forward to his reaction if I had to give him his engine back in a box :grin: .
  15. Carbon deposits form a heat barrier and can be a contributing factor to preignition. Other causes include: An overheated spark plug (too hot a heat range for the application). Glowing carbon deposits on a hot exhaust valve (which may mean the valve is running too hot because of poor seating, a weak valve spring or insufficient valve lash).

    A sharp edge in the combustion chamber can cause it as well as sharp edges on valves that were reground improperly.

    All of these factors can cause run-on, depending on the age and condition of the motor.

    I would check the compression pressure to see if the compression ratio has gone up due to carbon build-up, hence causing heat soak and hot spots within the combustion chamber.

    I remember yrs ago doing a little work on a Ford Cortina that persistantly ran-on, I removed the inlet manifold to reveal the complete port was full of carbon deposits from leaking valve stem seals and caused the motor to run extremely lean and causing the exhaust valves to glow, this was the cause of ignition once the motor ignition was turned off

    Best advice would be to check compression pressures, valve clearances and then determine where to go from there!!!!!!

    Hope it helps
  16. I can't give you any technical or metalurgical reason, but any time my old Triumphs used to 'run on' it was always the timing, because re-setting it, and changing nothing else, always fixed it.....
  17. idle rpm set too high can and will cause run on ,Does your car have an idle solenoid?? and also a fuel cut off solenoid??. Also is it running hot all of these things cause the symtoms you speak of. If it is carbon build up there is an old mechanics trick to remove carbon. run the engine at reasonably high revs whilst slowly trickling automatic transmission fluid down the throat of the carby( this crystalizes and in effect blasts off the carbon in the combustion chamber etc )

  18. exactly what I was thinking about idle soleniod and fuels cut off soleniod..

    Thats where I'd start.

    If its carbon just take it for a good freeway run, get it good and warm for an hour or so. Shoudl blow most of the crap out, it used to work on the old Jag XJ6

  19. I had a 180B that used to run on, and as the timing guys have said yep it was the timing being too high meaning run on.

    check your timing Rob.
  20. Many factors cause dieseling or run on.
    Ignition timing is a BIG factor, if your engine has carbon buld up bad enough, short of pulling the head off, you will have to retard ignition timing 5 degrees or so, that should cure it.
    Fuel could also be an issue. I would try some mid grade fuel in it(rather than regular unleaded) before I changed anything else. You could also try a lead substitute for a giggle, may help, may not.
    Engine temperature could be another factor. Higher engine temps mean hotter combustion chambers, is teh cooling system up to par?
    I had a Ford Galaxie with a 400M engine I modified. It was a low compression engine (8 to 1). It originally had an idle solenoid, which would activate when the ignition was on, setting a correct idle speed of 750 rpm by moving the throttle. When deactivated, the idle solenoid produced an idle throttle setting too low for it to run.
    When I modified it, I changed carburettors and lost teh idle solenoid. It would run on, but I used to just shut it down in gear. Problem solved, and I was able to keep my higher performance ignition timing (low compression means you can raise ignition timing by a fair amount, and gain a fair amount of benefit from starting teh burn eariler, creating higher combustion pressures).
    I would doubt spark plugs, unless teh car has been recently serviced with new plugs, then they could be too high a heat range.
    Vacuum leaks generally don't cause run on in a petrol car.
    So, the main problem after my rambling will probably be carbon deposits. There are a few ways to get rid of them. Seafoam is the bomb. It is available here, but you have to hunt it out.
    Another way is to run the engine at around 1500 rpm, and mist water spray from a trigger spray bottle down teh carburettor throat (take air cleaner off). Get about a litre of water down there, it steams teh carbon out of teh combustion chamber. The engine wil stumble a fair bit, just keep the revs up to it and keep spraying as much as you can in volume, whilst keeping teh engine running.
    Follow this treatment up with an Italian tune up. I will leave you to look this term up, if it exists on the www!
    Jaguar used to advocate Italian tune ups in their owner's manuals for their V12 engines, when used sedately around town, they suffered from carbon build ups. The book read something like 'Take teh vehicle out onto teh open road, and make several heavy acceleration runs in second gear up to 100 mph. Ignore the heavy pinging and smoke from teh exhaust during this procedure :shock: , it will disappear after a few runs".

    Regards, Andrew.