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8 Answers

Hot start 172S

Asked by: 31407 views Aircraft Systems

Recently purchased a 2002 172S. Days are getting much hotter now and I am having trouble on the hot start. Seems I am flooding the engine because the plane does not start as quick when it is real hot and when it does not start as quick I advance the mixture thinking it needs more fuel. Any suggestions?

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8 Answers



  1. Wes Beard on Jun 19, 2011

    I am assuming your 172S is a fuel injected airplane.  First place is to read the POH and see what the procedure is for a hot start.  The issue most likely causing you trouble is the fuel vaporizing in the fuel lines.  You will need to reprime just a little bit and use a different technique.
     
    From my 172S POH it states. Page 4-25 of the normal procedures expanded checklist.
    In warmer weather, engine compartment temperatures may increase rapidly following engine shutdown, and fuel in the lines will vaprozie and escape into the intake manifold.  Hot weather starting procedures depend considerably on how soon the next engine start is attempted.  Within the first 20 to 30 minutes after shutdown; the fuel manifold is adequately primed and the empty injector nozzle lines will fill before the engine dies.  However, after approximately 30 minutes, the vaporized fuel in the manifold will have nearly dissipated and some slight “priming” could be required to refill the nozzle lines and keep the engine running after the initial start.  Starting a hot engine is facilitated by advancing the mixture control promptly to 1/3 open when the engine starts, and then smoothly to full rich as power develps.
     
    If the engine does not continue to run, set the fuel pump switch to the ON position temprarily and adjust the throlttle and/or mixture as necessary to keep the engine running.  In the event of over priming or flooding, set the fuel pump switch to OFF, open the throttle from 1/2 to full open, and continue cranking with the mixture in the idle cutoff position.  When the engine fires, smoothly advance the mixture contrl to full rich and retard the throttle to desired idle speed.

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  2. John D. Collins on Jun 19, 2011

    I haven’t flown a 172S, but it has the Lycoming IO360L2A 180 horsepower engine, which is fuel injected.  Often, the reason a fuel injected engine won’t start has to do with vapor in the system which prevents fuel from getting to the cylinders.  The engine driven fuel pump has difficulty purging vapor in the pump and lines.  When the engine is hot and shut down, it heat soaks the fuel lines including the engine driven pump and fuel vapor forms.  The technique used in the TCM big bore engines may work in the Lycoming engine, so it is worth a try.

     

    Here is the hot start method used with the big bore TCM engines.

    1. Set the mixture to idle cutoff so that fuel does not get to the injector lines and cylinders.
    2. Set the throttle to wide open.
    3. Turn on the electric boost pump and run it for up to two minutes.  The fuel will flow from the selected tank thru the fuel lines to the engine driven pump and up to the mixture control mechanism.  Since the mixture is set to idle cutoff, fuel flows back thru the return lines into the selected tank.  This cools down the lines and the engine driven fuel pump using the fuel as the coolant.
    4. Turn off the boost pump.
    5. Set the throttle and mixture to their normal positions for priming the engine and prime it, most likely with the boost pump.
    6. Do a normal start.

     

    The other common problem is that the engine is over primed and flooded when it is hot.  It is best to wait a while if this happens, rather than attempting a start as a backfire is possible. This can be particularly dangerous if the ground is wet and fuel puddles near the exhaust stack, as a backfire can start a ground fuel fire. 

     

    Normal procedure for a flooded start is to start with the mixture at idle cutoff and the throttle at wide open. Cranking the engine will purge some of the excess fuel and eventually, the right mixture for combustion will occur and the engine will roar to life.  The pilot has to be fast on retarding the throttle to a normal RPM and advancing the mixture to sustain the engine running.  There is also the danger of sufficient fuel in the cylinder to get hydraulic lock.  For this and other reasons, if you have flooded the engine, it is best to wait sufficient time to allow the fuel to drain out of the engine and any remaining fuel on the ground or in the engine to vaporize before starting the engine again. 

     

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  3. James MacGregor CFI on Jun 19, 2011

    With the FI 172S and 200 series cessnas I have had some vapor lock issues, mainly caused by DA and some caused by our nice 110 degree days out here. 
     When the POH method doesnt work thisis what has worked fore me. I will go mixture ICO and full throttle and crank the engine, once she kicks, push the mixture in as you pull the throttle back.

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  4. Tommy D Eldridge on Jul 15, 2011

    I have a friend who owns a 1967 Piper Arrow.( You can see the airplane at http://www.privatepilotinsider.com/flying-piper-arrow/)  Last Wednesday we went for a 45 minute flight. After landing he taxied to refuel. After refueling he had the same problem. He explained that “Hot Starts” have always been an issue. John, would the same method work for the Arrow? 

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  5. John D. Collins on Jul 16, 2011

    Tommy,

     

    The procedure I outlined assumes the aircraft is fuel injected.  Your friend’s aircraft has a carburetor and are normally easier to start than a fuel injected engine when  they are hot .  You can try purging the system of fuel vapors before attempting the start by setting the mixture to idle cut off and running the boost pump for a minute or so.  Often a hot engine will start without a prime. To try this, set the mixture to idle cut off, open the throttle a half inch or more, turn the boost pump on, and crank the engine. When it starts to fire, advance the mixture to keep it running. 

     

    I owned a 1968 Arrow many years ago, but it was fuel injected.  I don’t recall if the carbureted engine has a separate push to prime system.  If you do prime the engine, do it after you have run the boost pump for a minute with the mixture at idle cut off, and don’t over prime.  I also don’t worry about the engine over speeding on startup when it is hot because it is still well lubricated, so sometimes the engine will fire by slowly advancing the throttle until the right mixture is achieved for combustion and then to keep it running advance the mixture and retard the throttle to a normal RPM.

     

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  6. Kent Shook on Jul 16, 2011

    John, two minor corrections: 
     
    1) All Arrows are fuel-injected, even the 180hp. I’ve flown all four normally-aspirated models, and I checked the TCDS as well to be sure.
     
    2) Lycoming and Continental use a completely different type of fuel injection system. Whereas the Continentals have the return line that returns excess fuel from the spider to the tank, the Lycomings do not have that return line. So, using the cooling technique you described for the Bonanza doesn’t do anything for a Lycoming-powered airplane except wear out the fuel pump. Here are a couple of links for more info:
     
    http://www.kellyaerospace.com/articles/ContinuousFlow.pdf (mostly about the Continental style)
    http://www.littleflyers.com/engserv/lycfuel.htm (mostly about Lycoming style)
     

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  7. John D. Collins on Jul 16, 2011

    Kent,

    Thanks for the correction on the 180 HP.  The TCM injected engines I am familiar with have a fuel return from the metering valve on the throttle body and from the engine driven fuel pump, and there isn’t a return line from the fuel manifold spider. When the mixture is at idle cut off, fuel can get no further than the return at the metering valve, so it never gets to the fuel manifold spider.  So this technique only cools the engine driven pump and purges vapor from the line feeding the fuel to the metering valve on the throttle body.

     

    I am not as familiar with the Lycoming fuel injected engine, so I would defer to your superior knowledge.

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  8. Tommy D Eldridge on Jul 16, 2011

    John and Kent,

    I thank you both for sharing your knowledge on this “Hot start” question. I did not mean to take away from Wes’s original question however. I guess this is the great thing about blogs. It triggers other questions. 
    I look forward to reading more of what you guys have to say.
    I will forward this on to my friend.
     
    Thanks again,
    Tommy Eldridge
    http://www.PrivatePilotInsider.com

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