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I just recently started flying a fuel injected cessna after flying carb heated M and P models, and have a question about the starting procedure- particularly about the “warm start”.

  • when we start the engine for a “warm start”- POH tells us to skip priming with the AUX fuel pump, and have the mixture ICO until the engine starts, at which point we advance the mixture.

My question is- where does the fuel come for the initial start? Shutting down on the previous flight we shut down with cutting the mixture, so there should be no fuel left. And now we are starting with the mixture in ICO position and not priming, so where are the cylinders getting the initial fuel to fire?

Thanks for your answers.

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  • $\begingroup$ Small nitpick, and I assume you know this: it's not "carb heated" vs. fuel injected, it's carbureted. The carb heat function is tangential to the fuel supply function served by a carburetor or by fuel injectors. $\endgroup$
    – TypeIA
    Commented Jun 4, 2023 at 12:38

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I'm pretty sure what is happening is when you select idle cutoff to shut down, the drop in fuel pressure closes a diaphragm valve in the fuel distribution spider on top of the engine and the immediate fuel flow stops, shutting down the engine.

However, over time, some of the fuel still in the individual lines between the spider and each cylinder drains out the injectors by gravity, the path being down hill. So a small priming charge of fuel, seeping out the injection lines going to the cylinders, is present in the intake ports for some time after shutdown.

Eventually it all finishes seeping and evaporates away, and you're back to a cold engine. But while the engine's still warm, there's just enough fuel there to provide sufficient prime to get the thing to run for a second or two, giving enough time to get the normal system running by pushing the mixture in.

It's a very inexact process, because you don't know just how good a residual prime there is, and when it's time to do additional priming, which if too early will probably flood the engine.

Besides the usual problem of fuel vapour locking in the distribution lines from post shutdown heat, it's one of the things that makes injected engines a challenge to start in hot weather.

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  • $\begingroup$ just curious where the throttle setting is while all this is going on. Sort of reminds me of tuning the air mixture screw and the idle screw over and over again on a carburetor. $\endgroup$ Commented Jun 3, 2023 at 15:29
  • $\begingroup$ Unless you think it's flooded, throttle cracked. WOT is for flooded engines and with just a slight prime charge will be too lean and it won't start. Generally, when in doubt, you flood it on purpose (not too much), and crank with WOT waiting for the air/fuel mixture to get to the sweet spot to light off. $\endgroup$
    – John K
    Commented Jun 3, 2023 at 17:54
  • $\begingroup$ Throttle is 1/4” advanced so just very slightly in. $\endgroup$
    – leha007
    Commented Jun 4, 2023 at 1:07

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