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Vapor lock comes in varying degrees, so a single technique to purge the fuel lines of “gas air” won’t necessarily work. I break it down by short heat soak (about 10 min or less) and long soak (about 30-60min) and yes, there is that charlie foxtrot area in the middle ~15-30 minutes where anything can happen. This discussion is for a fuel injected engine. During a short heat soak the likely culprit will be the upper fuel lines to the distribution device and the injector lines to the cylinders. Note, these fuel lines often sit atop the hot engine, and since heat rises, the relatively small volume of liquid fuel in these lines atomizes quickly and becomes a vapor (gas air). To fire a mixture off in the combustion chamber, there must be an atomized fuel mist suspended in the surrounding air. If the fuel is too atomized as in a vapor, the fuel density won’t be sufficient for firing, hence a hard start, actually due to a overly lean mixture. To clear a short soak, you need to pressurize the upper fuel lines with just enough fuel to push out the vapor, and not too much that you flood the intake ports. This is where most pilots get in trouble with hot starts; a hot engine needs far less fuel to start than a cold engine. The theory is to begin the starting sequence for a short heat soak with NO fuel flow and then ADD fuel slowly until it fires off. Most often, we do it backwards and that makes things exponentially worse. During a long heat soak, the entire fuel system comes in to play from the fuel tank feed lines to the fuel pumps, to the pump chamber, to the feeder lines, on up. It generally takes longer for these components to heat up and begin to vaporize after sitting than do the upper fuel lines. The clearing technique here is different than a short soak, as you want to (have to) push the vapor out of the pump circuit and that takes time; sometimes a lot of time. To clear a long soak, you need to purge the entire fuel circuit of vapor, not just the lines going to the injectors. In a long soak, there is a lack of liquid fuel in the pump circuit (because it got hot and vaporized away) ... and a fuel pump, while good at pumping a liquid, is very inefficient at pumping vapor. So you crank and crank and crank and nothing happens because nothing is happening - no fuel is flowing - because the fuel pump is essentially pumping air. In cases of a long soak, using just techniques that will effectively clear a short heat soak will do little to clear the pump circuit, causing a hard start. Knowing a little about why it’s hard to start a hot engine often makes it easier to find a solution. If this works for anyone send money; I have airplane payments to make and my wife and mistresses want jewelry. Short Soak: Throttle cracked to the point where it would need to be to have about 1000-1200RPM if the engine were running Boost pump OFF - (Do not use the boost pump or primer at all) Mixture Full Rich for about 5 seconds then Idle Cut Off Begin cranking the engine, wait a 2 seconds then slowly (over 5-10 seconds) move the mixture toward rich. Don’t exceed recommended cranking time. As soon as the engine starts, keep the mixture at about that point, adjust it and throttle for smooth operation. You should ALWAYS run the engine as lean as possible on the ground. Rationale: The throttle is cracked open so that when the engine fires, there is proper air flow for the fuel that is being slowly added by the mixture control. The Mixture is open fully for a few seconds first to allow any built up vapor pressure to purge out, then its closed; this gives liquid fuel a clear path down the lines. The mixture is kept closed until the cranking so that you have complete control of how much fuel to add to get the engine lit off - which likely will be different every time - this technique also significantly reduces the changes that you will flood the engine. There is NO boost pump used because the mechanical pump should be able to provide fuel at a rate that keeps excess fuel low, unless you have a hot soak condition... You will know if you have a hot soak event because the above technique will not work after two tries. Summary: There is liquid fuel available at the fuel pump but there is vapor in the injector lines. The vapor does not ignite easily and it blocks liquid fuel from flowing past the vapor area (vapor-lock) causing a hard to start condition. You have to relieve the pressure of the vapor and then slowly add liquid fuel to the lines and the injectors so that the mixture of fuel and air is correct for ignition in a hot cylinder. Long Soak: Throttle Closed Mixture Full Rich for about 5 seconds then Idle Cut Off (Be very sure it is fully at idle cut off) Boost pump on (or on low if two speed) for 30-60 seconds. (yes, a full half a minute to a minute) Boost pump OFF Throttle cracked to the point where it would need to be to have about 1000-1200RPM if the engine were running Begin cranking the engine, wait a 2 seconds then slowly (over 5-10 seconds) move the mixture toward rich. Don’t exceed recommended cranking time. As soon as the engine starts, keep the mixture at about that point, adjust it and throttle for smooth operation. You should ALWAYS run the engine as lean as possible on the ground. Rationale: The Mixture is open fully for a few seconds first to allow any built up vapor pressure to purge out, then its closed; this gives liquid fuel a clear path down the lines. You want to be very sure that the Mixture is fully at idle cutoff because we do not want any fuel to get past the metering circuit. Running the boost pump with the mixture closed will pressurize the fuel circuit and circulate some liquid fuel which will help cool things down and reduce additional vaporization. Excess vapor will be expelled through a vent port and the mechanical pump and the lines leading to the metering circuit will become fresh with cooler fuel. This takes time, and you have no worry of flooding the engine because the mixture is at idle cutoff. You finish by following the same procedure as a short soak. Summary: The mechanical fuel pump has to have the engine cranking to do its job. It would take too long and be too hard on the starter to use this pump to purge vapor. (Hence why I’ve seen pilots cranking the engine for absurdly and dangerous amounts of time). The electric pump is parallel* with the mechanical pump and has the ability to run quickly and more efficiently to do the job of purging the vapor, but its not an easy job - it takes a lot of time therefore you need to run the pump for at least 30-60 seconds. Once the vapor is purged, liquid fuel can flow past the metering circuit, and now you have the situation of a short soak to deal with, see above. So why not just use a Hot Soak procedure every time? You could, but after you understand why the damn thing won’t start and you think about it, you can use the technique that works the best. If I land the plane, shut down hot, and then go for a restart in less than 10 minutes or so (all thing considered equal), I know that my problem is not at the pump, its at the top lines, so why waste time. Disclaimer: Follow your POH unless you fully understand the pro’s and con’s of using other methods and other’s advice. DVA *As Don Kaye correctly pointed out, the electric fuel pump (on the M20M) is physically in “series" with the mechanical pump as shown on the schematic. The point is that neither of the two are dependent on one another and both can participate alone or together, in parallel, to provide fuel flow and pressure.

I have an 1986 M20K 252 and pretty proud of it. But I do have a problem with the engine that I can by now almost predict on hot days. After the plane has sat all day in the sun it will start up very nicely but when applying take-off power the engine will quit. If I put on the low boost pump it will not quit and continue running. Mostly in the flight levels above 100 and some cooling it will operate with the low boost pump off but I have had one flight lately where it would start to run rough and stop if not re-engaging the low boost pump (maybe not enough cooling time). Next morning and with cooler temperatures normal operation. This happens with outside temperatures above 90°F (33°C). This is clearly temperature related. First I suspected vapor lock but now I am not so sure about this since vapor locking should terminate once it has been pushed out of the system once. The fuel pump has recently been revised but I still suspect some problem with fuel pressure or maybe an air leak somewhere in the system (there is no fuel leak noticeable though). The maintenance shop believes it might have something to do with the fuel selector valve. Any ideas on how to proceed with troubleshooting?