Hot Start. Purging The Fuel Lines

[johnggreen]

After having flown my Bravo for eight years now, I have had a problem with "hot starts" only twice. Interestingly, neither time was the engine really hot. In fact, both times the airplane had been sitting for at least an hour when I tried to start it.

The first time, I had been on the ground about an hour and the OAT was at least 90. It must have taken ten tries and with starter cool downs, ten minutes. I tried every known combination and don't really know what worked.

The most recent time, last Monday, I had flown to Mobile. The OAT was only about 50 degrees and I was there for well over an hour. When I went to start, I boosted for a couple of seconds and tried a normal start. Well, anyway, it took four or five tries before I got her going.

I'm not really looking for hot start procedures as that is not my problem. Warm start seems to be a culprit though not often. The real question I have is this. I understand that the "hot start" is a result of the fuel in the lines and injector absorbing the heat from the engine and vaporizing. I recently read a thread on a Beech site and one commenter said that on a Continental, if you turn on the boost pump, it will circulate the hot fuel out of the lines and injector and back into the tanks and bring the cooler fuel from the tanks into the lines and injector solving the problem.

Does anyone know why this won't work on a Lycoming and, if so, would you have the mixture rich or full lean while running the boost pump? I certainly don't want to flood the engine and exhaust system doing this. Experiencing only one backfire on a Bravo will make you very, very cautious.

Jgreen

[carqwik]

The Bravo engine hot start procedure for me is throttle open a few twists from idle, mixture full rich, boost pump for three seconds. The go to mixture cutoff and start...when it catches, enrich the mixture quite a bit quickly but not to full rich....then adjust mixture to max lean for ground idle Some times you need more throttle too. This procedure works for warm starts....basically any time that the engine still has some warmth to it. I have no idea if this is the POH recommended method but it works like a charm so I use it.

The Continental engine set up returns fuel to the fuel tanks IIRC but the Lycoming does not. Typically running the boost pump on a warm -AF1B engine just floods it making it difficult to start.

[carusoam]

JGG,

The Continental IO550 has additional plumbing that allows fuel to circulate from the engine driven fuel pump back to the tank selector valve. There are return lines back to each tank as well.

Hot procedure is essentially...

Throttle full open

Mixture out

Low boost pump on 15 sec., then off.

Throttle idle

Mixture in

Start

Warm procedure is slightly different...

Throttle 1/2 inch open.

Mixture rich

Low boost pump "to clear fuel vapors", then off

Start

These procedures have been abbreviated slightly. Excess fuel/vapor is sent back to the tank.

Best regards,

-a-

[jetdriven]

Our 201 on a hot day hot start will not maintain adequate fuel pressure right after startup. Cycling the boost pump for 5 seconds right after it starts up stabilizes the fuel pressure and from then on it runs fine. We learned the "1000 RPM no-touch" method from David McGee at All American Aircraft. It works pretty well, it fires up in 10-20 blades.

[carqwik]

The Ovation engine is a Continental, the Bravo is a Lycoming. There is no "low boost" on the Bravo...so running it for 15 seconds on boost will result in a backfire. I suggest you try my method to see if that works for you.

[carusoam]

I'll add that for those that don't recognize the 550 as a Continental. The big difference is the extra plumbing that comes with the Ovation installation.

Best regards,

-a-

[carusoam]

I see that it says that already...

Silly me. No changes.

And the David McGee no-touch method works reliably well for the io550 also...

Best regards,

-a-

[2jetman]

I know exactly what you are going through, I had the same problem starting my bravo after a shutdown. The cold start was boost pump on for a few seconds and it fired right up. The hot start was unbelievably tough to say the least. I even had to replace the starter after owning the plane for six months. I followed all the normal hot start procedures that you could think of before I finally ran into a mechanic that recommended me changing out my left mag. At this point I was ready to try anything! One new mag and now it turns a half a crank and fires up every time, hot or not it doesn't matter. No more embarrassing moments cranking and cranking out side the FBO.

[Awful_Charlie]

I find hot-starts a bit of a git on Lycomings too. I've thought about it and seem to have settled on this sort of reasoning below - whether it works for you is your choice

After shutdown, all the fuel after the gascolator/electric pump gets hot over several minutes, so this is pipes, engine pump, metering unit, flow divider and injection pipes. As the throttle/mixture metering is in the throttle body, it is likely that all the fuel after those has vaporised out through the injectors, and being heaver that air has descended the intake pipes into the sump, where it is being kept warm by the oil and contained by the throttle plate. It is a mixture that is too rich to fire. It takes some time after shutdown for the 'heat soak' to take effect, maybe 10-20 minutes, before which you can easily restart by cranking. When the fuel has been vaporized out through the injectors however, it will sit around for several hours, gradually condensing out on the internals. It is this interim period when the start is very difficult.

What works for me (normally!) means pulling some fresh air through all this vapour, and then throw some fuel into it when it has been drawn through.

The briefest of primes - open throttle & mixture, and a 1 second hit on the fuel pump, and then ICO for the mixture. This is to get liquid fuel up to the engine pump, and you need to close one of the taps to stop it running back.

Now you need to draw some air through the sump & heavy vapour. The cold start amount of throttle is only just moving the throttle plate away from a fairly effective seal, so you need to open it a bit more than that so some air can get in, but not so much that when the engine fires it runs off to zillions of RPM when it does pick up. I would guess to wind in say two or three full turns from idle about, two or three times much as for a cold start.

Then crank with one hand, with the other hand on the mixture. Hopefully, after a few blades, the engine will fire, and then the RPM will rise before falling again. When the RPM starts to fall, you have drawn all the vapour through and it is time to smartly go to full rich before it dies. Do it too quickly, it dies due to going over rich, too slow and the inertia goes before the fuel gets back to the injectors.

If you crank and crank and crank and it doesn't fire, a couple of possibilities:

- you haven't opened the throttle far enough to get some air in (open the throttle open a touch more & try again)

- It shouldn't have been a hot start

If you have opened the throttle too far, it will fire, run up to a high RPM briefly before cutting out.

In the last two cases, now it is a hot engine without the vapour, treat it as a cold start with a minimal priming required

[AndyFromCB]

Bruce Jeager said it the best: when a continental won't start it doesn't have enough fuel, when a lycoming won't start it has too much fuel. 

 

The procedure for starting a warm/hot lycoming is pretty simple:

 

-mixture to shut off

-throttle wide open

 

When the engine catches, increase mixture to full which at the same time reduce throttle to about 1000rpm, works like a charm every time.

 

Andy

[fantom]

The procedure for starting a warm/hot lycoming is pretty simple:

 

-mixture to shut off

-throttle wide open

 

When the engine catches, increase mixture to full which at the same time reduce throttle to about 1000rpm, works like a charm every time.

 

With one hand on the ignition, one on the mixture and one on the throttle, it's simple only if you have three hands.

[AndyFromCB]

With one hand on the ignition, one on the mixture and one on the throttle, it's simple only if you have three hands.

It does look very convoluted, I agree, helps to have another in the airplane.