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How do you reliably start fuel injected engines?

How do you reliably start fuel injected engines?

As a prelude, the POH is supposed to be the “bible” for proper operation of the aircraft and its systems. If you see a conflict between the POH and this article, the POH is supposed to rule. Note that the POH information is often not as detailed as might be desired, so this article may still be of some help.

Several people have recommended opening the oil door as soon as you get out of the plane. I usually have the first person out open it, if it isn’t me. This helps reduce the heat soak on all the rubber parts under the cowl, on any plane, as well as cooling off the fuel system faster, on the fuel injected planes.

The Lycoming and Continental fuel injection systems, and in most cases the engine-driven fuel pumps, are different. As a result, whether hot or cold, they require somewhat different starting techniques. The Continental FI system recirculates fuel through the servo assembly and back to the tank, metering part of it to the fuel distributor and cylinders. The Lycoming system is one-way; the fuel system feeds/pressurizes the servo, which feeds metered fuel to the distributor and cylinders. There is no recirculation. The Continental positive-displacement engine-driven fuel pump, and its pressure/flow-setting system, often does not flow enough fuel at cranking speeds, especially for cold starts. The engine-driven AC-type diaphragm fuel pump most often seen on Lycomings (with its internal self-regulation) usually does move enough fuel for cranking and cold starts. This engine-driven fuel pump difference makes the electric boost pump a much larger player in reliable Continental starts (cold or hot).

A key factor in fast successful starts, whether carbureted or fuel injected, is to have everything else done before you prime or boost; you should even have the “Clear” out of the way. You want to be able to turn the key to “start” at the very instant that you complete the priming procedure. The idea is to crank the plane while there is still some vaporized fuel available for ignition. Especially in a cold engine, any fuel vapor from priming will rapidly condense on the cold metal surfaces. Liquid and large-droplet fuel will not burn. If you don’t crank “right now”, you have to wait for manifold vacuum and luck to produce some vapor and a combustible mixture, in one or more cylinders, before the engine will fire. Just a couple of seconds delay in cranking can mean the difference between a cold start in two or three revolutions, versus several minutes of aggravation (and wear and tear on the starter and dry cylinders).

FI Lycoming cold starts usually work best if you get everything ready, then set the throttle at about the 1,500 RPM position. Full rich mixture, boost pump until the flow meter first “jumps”, then an immediate start (throttle back to 1,000 RPM). For hot starts, no boost pump is needed (and it should not be used). Set the same throttle position, start cranking, and at the first sign of ignition you briskly move the mixture full rich. The engine may falter momentarily and seem to die, but just be patient; it will almost always recover and smooth out nicely with no further action. If it does die, just immediately crank again without changing anything, and it will start again. A hot start is defined as anything after the first start of the day, unless you are in relatively cold weather (maybe 45 degrees and below). It doesn’t matter whether it has been ten seconds or ten hours since the engine was shut down; you still treat it as a hot start.
NOTE: Priming the fuel injected Lycoming this way does not present the fire risk that exists with over-priming the carbureted engines; and especially not the risk created by pumping the throttle on carbureted engines. The injected engine risk is low because only a small fuel shot occurs, if you stop the pump when you see the flow needle ‘jump’; and all the fuel is injected at the back of the intake valve. Unless you seriously over-prime, there just isn’t enough fuel to travel down the runners into the air box. Even if that occurs, it is sitting in the airbox where it does no harm; or it drains harmlessly out of a sniffer valve. It won’t be dripping out the end of a carburetor, where a lean-backfire can ignite it.

FI Continental starts tend to be less consistent. They require maybe ten seconds (sometimes more) of boost pump operation with a closed throttle and closed mixture, to purge all air from the servo metering body and valve (and to cool it off when hot). The Lycoming system tends to remain somewhat pressurized between the pump and servo, due to the pump discharge check valve and one-way fuel path that “traps” the liquid fuel. The Continental fuel system often develops air bubbles between the pump and servo after shutdown, as pressure rapidly goes away through the fuel return path. Pressurized fuel resists bubble formation (“vapor lock”), while unpressurized fuel is more susceptible, when ambient and engine compartment temperatures are relatively high. Once the pump-to-servo fuel path has been purged of air (and filled with cooler fuel when hot), the start process can proceed. On a cold start, the throttle is set to high idle, the mixture is set full rich, the boost pump is run while watching the flow gauge for movement, and the engine is then immediately cranked. You have to be ready to keep flicking on the boost pump if the engine fires but seems to fade; the engine driven fuel pump usually won’t provide enough fuel pressure for continuous running on a cold Continental engine until the RPM is up to or past the normal idle speed. A hot start is similar to the Lycoming; high-idle throttle position, lean mixture, immediate cranking when the throttle is opened, and (briskly) full mixture when the engine fires. The key difference is that (again) you have to be ready to flick the boost pump on if the Continental engine fades after starting, to assure adequate fuel flow. It takes longer for most folks to master the Continental FI starts, due to the need to use the boost pump so much more.

In my experience, most people tend to over-prime engines. This is partly because they don’t start cranking fast enough after priming. The engine fails to fire, so they add even more fuel. I have seen people priming cold FI engines, with liquid fuel running onto the ground under the plane from the snifter valve drain line. When that happens, I shuffle a bit closer to the nearest fire extinguisher, when I can. The best tool in refining your starting technique is to understand how your system works. Then you can recognize what may be needed under different circumstances. You can further refine your starting technique with the help of willing observers; perhaps a flying friend with a hand-held radio. Whether cold or hot, there should be only the briefest puff of dark gray or black smoke when the engine fires. If there is none at all, and you get backfiring or crackling/spitting noises, you are starting too lean. When there is a billow of black smoke, as you so often see, the start is too rich.

Engine mixture settings can affect fuel requirements for starting. An engine set for a lean idle, and/or operated at sea level, will require more starting fuel. One that has a rich idle setting, or is being operated at higher density altitudes, will require less starting fuel. An engine tuned for a lean idle will have little or no RPM rise during mixture-off shutdown. It may stumble a bit as the throttle is advanced, and the idle may be a bit erratic; sometimes with a crackling sound. An engine with a rich idle will usually have a pretty consistent RPM, but will have too high an RPM rise during shutdown. It may often have a pronounced “lope” at idle. It sounds like an old hot rod cam engine, but it’s really just an over-rich mixture. You often hear this on some twins, especially those with turbos, and on most radial engines.

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