Cherokee Six, 32-300 1969. Lycoming engine is fuel injected with 450 SMOH. Cold starts are easy: 1/2" throttle, full rich, fuel pump on and hit starter after fuel flow starts. Runs great once running. Hot starts are hit or miss. If it sits over an hour it may start but if I miss it it has to sit for at least another hour till I try again or forget it. I've tried 1/2" throttle, full lean with and without fuel pump; full throttle, full lean with and without fuel pump and all combinations I can think of. Most of the time it spins without any cough or catch. Due for 50 hour oil change and will ask that plugs and magnetos are checked. I'm afraid to go anywhere for fear of being stranded for hours.

  • $\begingroup$ Have you asked your A&P about this? $\endgroup$ – Fred Larson Jun 24 '14 at 16:23
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    $\begingroup$ You might try reading this article: flighttraining.aopa.org/students/presolo/skills/hotengine.html $\endgroup$ – Fred Larson Jun 24 '14 at 16:44
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    $\begingroup$ Troubleshooting a no-start situation like this is probably too difficult for a simple Q&A site - about the best advice we can offer is that you need fuel, air, and spark. You likely have no problem with spark (because you can start the engine when cold), so you'll need to work with your A&P to troubleshoot fuel (vapor lock?) and air - try to get the plane to exhibit the problem for your mechanic. Also make sure you're trying to start the engine in accordance with your POH or the Lycoming operator's manual - the published techniques generally work unless something is wrong. $\endgroup$ – voretaq7 Jun 24 '14 at 16:47
  • $\begingroup$ For what it's worth I also had very difficult hot starts with a Lycoming IO-540 (I assume that's the engine you're talking about). Had to be very careful not to flood it and I often wondered whether vapor lock was an issue. Never solved that mystery. $\endgroup$ – TypeIA Jun 24 '14 at 18:13
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    $\begingroup$ @dvnrrs Vapor Lock is absolutely the issue. The fuel injection plumbing is on the top of the engine which is the cold side of the engine when you are flying but when parked, heat rises! The accepted answer below cures this by running cooler fuel through the plumbing while flooding the engine, then the flooded start procedure is necessary. $\endgroup$ – Skip Miller Jun 24 '14 at 23:31

The issue is undoubtedly vapor lock. In the fuel injected Lyc 540, the fuel injection plumbing is on the top of the engine which is the cold side of the engine when you are flying but when parked, heat rises! The following procedure runs cooler fuel through the plumbing while flooding the engine, then the flooded start procedure is necessary.

First, check your POH for the hot start procedure for your plane. If not there, try this as it is for a very similar airplane:

Hot Starts on the Saratoga PA-32-300:

  1. Intentionally flood the engine. This runs cool fuel through the plumbing and "breaks" the vapor lock.

    • a) Full rich
    • b) Wide open throttle
    • c) Fuel pump on, watch fuel flow gauge
    • d) When gauge registers flow, wait 3-5 seconds then red knob to ICO and pump off.
  2. Crank engine with red knob still at ICO and with throttle wide open at first. Gradually close throttle (at a rate that will take you about 10 seconds to close the throttle) and it should start up at some point.

  3. When engine catches, do several things more or less simultaneously:

    • a) release ignition key
    • b) throttle to idle to avoid over-revving
    • c) mixture full rich.
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  • $\begingroup$ Tried flooding it and no excessive fuel spilling on ground going along with vapor lock and fuel starvation. 2 lower plugs with high resistance too were replaced but mechanic didn't think that was the culprit. Haven't had a chance to try a hot start since all the suggestions. Also no fuel return line in Lycoming per mechanic so just running electric pump with mixture ICO for 1 minute unlikely to flush cold fuel in the lines but will try anyway next unsuccessful hit start. $\endgroup$ – Marshall Jun 29 '14 at 21:22
  • $\begingroup$ Right, no fuel return line. It pushes the fuel/fuel vapor into the intake manifold flooding it. The procedure I posted is not the flooded start method in the POH but it was taught to me by my CFI and it worked for me. Still a bit of an art, though. Keep us posted. $\endgroup$ – Skip Miller Jun 30 '14 at 0:48
  • $\begingroup$ @Marshall: I just re-read your comment. Note that the procedure calls for full rich so you are not running the pump with mixture at ICO. And please note that one minute is waaay too long to run the pump in this case. The fuel flow gauge doesn't move for a while as the pump is pushing mostly vapor through the system. When the gauge starts to indicate some flow, then 5 seconds, maybe 7 max and then back to ICO and pump off. $\endgroup$ – Skip Miller Jul 7 '14 at 21:35

No need to run the fuel pump as @Skip Miller says. (though it may say that in the Saratoga manual)

I have done the following with success for hot starts on a IO-320, IO-360, and IO-540.

  1. Throttle Full Open. (Push in or forward)
  2. Mixture Full Close. (Pull out or back)
  3. Crank engine.
  4. Once it starts firing, put the mixture back in and pull the throttle back to idle.

Happy engine starts!

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  • $\begingroup$ Tried that but it doesn't work unless I flood it first with full throttle, full rich mixture and pump on for at least 5-10 seconds. Seems as if the hot engine sits for more than 10 minutes it vaporizes all the fuel and too lean to start. I've had more success at that point with 1/4 inch throttle 1/2 mixture rich and pump on. $\endgroup$ – Marshall Oct 12 '14 at 15:15
  • $\begingroup$ This is exactly how I do it in my IO-540-powered M20M. Works perfectly. $\endgroup$ – rbp Oct 12 '14 at 23:03

Also have the injectors checked. The symptoms you describe can occur when one of the injectors is not closing completely, causing flooding or an overly rich mixture.

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  • $\begingroup$ @Breveleri, the injection system on the Lycoming 540 engine is constant flow. They don't close. $\endgroup$ – Skip Miller Jun 26 '14 at 16:58

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