Engine shut down procedure

Discussion in 'Homeowner Assistance Forum' started by dh500, Jul 29, 2005.

  1. dh500

    dh500 LawnSite Member
    Messages: 58

    One common piece of advice with outdoor power equipment is to allow the engine to run at idle for a little while before shutting it down.

    Is there any significant benefit from this in practice?

    I am not talking about expensive turbo diesels here, just ordinary little one or two cylinder 4 stroke petrol engines.

  2. Smalltimer1

    Smalltimer1 LawnSite Bronze Member
    Messages: 1,223

    Idle at 30 seconds to a minute before shut down. Brings engine temps down better than a shut off with no idling.
  3. fixer67

    fixer67 LawnSite Silver Member
    Messages: 2,098

    Like has been said many time on this site, Do you shut you car off and full throttle or idle? There is many reason to shut down at idle, first off you have a lot of fuel going though the engine at full throttle and just killing it dead floods the engine and make it harder to restart. Letting it idle cools it a bit more as well before shut down. And you have a much greater chance of a backfire if you shut down at full throttle. Backfiring can do a lot of damage to a small engine. Go to the repair forum and do a search there. This question has been asked a hundred times or more here
  4. dh500

    dh500 LawnSite Member
    Messages: 58

    Thanks fixer67 and smalltimer1 for your responses and apologies for not explaining myself better.

    I am well aware that similar advice has been given numerous times on this site and elsewhere, but what I was wondering was what evidence there is as to how much difference it makes in practice.
    With modern engine designs incorporating things like fuel shut off solenoids, plus advances in metallurgy and lubricant technology (such as reduced ash), does it really matter?
    After all in the past manufacturers used to advise car owners to warm their engine up before driving off - a practice few consider necessary today.

    How much does an air cooled engine cool off in 30 seconds of idling anyway?With minimal air flow and oil circulation at low revs, I would be surprised if the internal temperature fell very much.

    To avoid any confusion, it should be said there are at least two issues here:
    (1) Throttle setting at the moment of switch off. Just killing the engine while it is revving hard is not something I would expect anybody to consider sensible, though there are some who suggest that using 1/4 or 1/2 throttle is of benefit. (In this context, we should probably be talking about "governor", rather than "throttle" settings)
    (2) How long at idle (or at least reduced revs) is a reasonable time to wait, and this may depend on how hard the engine has been working.

  5. Smalltimer1

    Smalltimer1 LawnSite Bronze Member
    Messages: 1,223

    I did an experiment for one of my engineering classes one time which involved sticking a digital thermometer up into the muffler of my GT-235. I did not let the probe touch any metal, just the exhaust exiting the muffler. It read 600 degrees at idle, and went up to about 900 at WOT. When I backed off I let it idle for a minute and took a reading, and it was down to 650.
  6. Restrorob

    Restrorob LawnSite Fanatic
    Messages: 11,029

    This is my thoughts on the subject; If you kill the ignition on any engine at full or even half throttle as the engine is winding down it is still pulling fuel into the cylinder, If there is no spark to ignite the fuel the raw fuel sits in the cylinder then you have a dry restart situation. I know you still have this at idle but much less wind down time so less fuel. Even if the carb. has a fuel shut off solenoid, Turning off at full or half throttle the engine will still pull fuel through the idle circuit.
    Best way to shut down is as smalltimer1 explained.
  7. ClippersLC

    ClippersLC LawnSite Member
    Messages: 130

    Just think about it, you dont shut your car/truck off at WOT. And you definatley dont need to do it with your mower. I always load my mowers up and let them idle while I do blower work to let them idle down and cool off. I figure if i sepend $8K on a John Deer ZTR, it doesnt hurt to take a few extra precautions with it. Same with my truck, its a2005 Ford F-250 with a 6.0 Power stroke Turbo Diesel 4-Wheel drive 4-Door cab. I let that baby idle for at least 2 minutes before I shut it down, i just unload my equipment then reach in and shut it down when I start mowing, and if I know I'm not gonna be on a job for more than 15 minutes, 20 at the most. I just let it idle.
  8. dh500

    dh500 LawnSite Member
    Messages: 58

    This experiment gives us some solid info to work on.

    That the temperature of the exhaust gases falls significantly in 60 seconds is reassuring to know, but what about the engine itself? It would be interesting to repeat the experiment measuring the oil temperature, which I suspect would show only a very small temperature drop. It would also be nice to know the temperature of engine components such as valves and piston crown or whatever items are considered most critical.

    Please do not get the impression that I am advocating killing engines under load at "wide open throttle" (or even at high revs under light load). In fact I strongly agree with all those who have stated that allowing an engine to cool down before shutting it off is "a good thing".

    What I still have doubts about is whether 30 or 60 seconds of idling is long enough to have a significant effect.

    I believe we should also acknowledge the skill of the engineers employed by engine manufacturers who manage to put out products which often continue to perform in spite of the most extreme abuse by their owners. There must be numerous engines in use which miraculously continue to run, even with stale fuel, rusty tanks, dirty sludgy oil, clogged filters etc etc...

  9. Smalltimer1

    Smalltimer1 LawnSite Bronze Member
    Messages: 1,223

    What happens when you idle an engine too long is that the fuel will eventually seep into the oil causing it to be diluted, and therefore cutting your overall engine life by reducing the oil's lubrication properties.
  10. dh500

    dh500 LawnSite Member
    Messages: 58

    Smalltimer1 raises another good point. I guess the only conclusion is that whatever we do will at best be a compromise.

    Lessons to be learned from all this must include
    (1) Not overworking engines, particularly under hot ambient conditions so that they do not become excessively hot.
    (2) Keeping cooling fins reasonably clear of grime, dust, grass clippings and other debris
    (3) Keeping carburettor mixtures properly adjusted
    (4) Keeping spark arrestors clean to avoid obstructing flow
    (5) Changing oil sufficiently often to counter the dilution effect mentioned by Smalltimer1 (which will be occurring to a certain extent even in normal use)
    (6) Selecting equipment which is suitable for the task

    and if we do all that there should be no need for a stopwatch to successfully operate a mower :)


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