HELP with hot start in C172SP/fuel injected G1000

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David Snell

Filing Flight Plan
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DavidSnell
Hello, I own a part 91 sight seeing business in Dallas for the past 26 years called Starlight Flight and we fly C172 for our tours. Several of my CFI's and myself have had recent issues starting one plane we fly that was purchased from a guy in Alaska and we are all encountering difficulty in starting this particular C172SP with the G1000 in it. After a flight the plane may not want to easily start when going back out to fly it after we deboard one group of passengers and load the next. We have all followed the "hot start" procedures the owner who is a A&P has suggested but the plane is still hard to start if at all? the plane has gone through 4 starters past 6 months and we all know not to continue cranking on the starter but between their normal renters and us the issue continues and when we fly it is at night so in Dallas maybe mid 80s to very low 90s on any given night? I read it may be prudent to change starters to a "lighter" faster spinning starter than what I assume is stock? i do not have exact year/model but I THINK it is a 2005 airplane so relatively newer vs older. ANY suggestions of what the repeating issue may be as I have never encountered such starting issues in this model of fuel injected C172SP before. With multiple CFI's encountering same thing one might like to think it can not all be pilot error on starting it. Any suggested info here would be greatly appreciated! Thanks David Snell/CFI
 
For a quick turn I just kill it with the mags. Starts very easily.
 
This works for me usually: Prior to start, with mags off, push mixture and throttle full forward (full rich/wot), wait about 30 seconds, then follow the POH for warm start.
 
wilkersk said:
This works for me usually: Prior to start, with mags off, push mixture and throttle full forward (full rich/wot), wait about 30 seconds, then follow the POH for warm start.
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We have tried the throttle/fuel mix full rich prior doesnt seem to work. Anyone know anything about possibly changing out starters to a faster lighter starter i read about on line??
 
I've never had any issues starting hot 172s. My hot start is a cold start without any priming.
 
jordane93 said:
I've never had any issues starting hot 172s. My hot start is a cold start without any priming.
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As it should be but this one is clearly not working the same hence my post!
 
David Snell said:
We have tried the throttle/fuel mix full rich prior doesnt seem to work. Anyone know anything about possibly changing out starters to a faster lighter starter i read about on line??
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I switched out to the faster Skytec starters on the Lyc IO-540s on my Aztec. The faster starters definitely help with starting, but my starting (left) magnetos are dual point instead of mechanical impulse coupling. Nevertheless, the hot starting technique is more critical than anything else. I've had others flying the plane and they mess up and cannot get it started hot.

My method for hot starting the Lycs is to crack the throttle, props forward, start the electric boost pump, tickle the mixture open VERY briefly, shut off the boost pump, mags on, hit the starter and bring the mixture open as it catches.

I am assuming there is no boost pump on a 172 so not sure what the process is to clear vapor in the lines under the cowling.
 
Assume the engine is flooded. Pull the mixture out and while cranking advance the throttle all the way forward or until it starts.
If that doesn't work, push mixture in and while cranking advance throtle all the way forward or until it starts.
If that doesn't work, prime it, push mixture in and whle cranking advance throttle etc
If that doesn't work, prime it, pull mixture out and while cranking advance throttle etc

Keep trying different procedures and find one that works.
 
GRG55 said:
I am assuming there is no boost pump on a 172 so not sure what the process is to clear vapor in the lines under the cowling.
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All injected engines have boost pumps, including the 172SP.

The OP should try starting with the throttle well open and mixture left at idle cutoff. Hot starting is usually a flooding problem.

Some of the flow dividers on the IO-360-L2A in the 172 had the shutoff poppet machined too close and it would stick, though normally when colder.

http://www.precisionairmotive.com/Publications/SIL RS-84.pdf
 
There used to be a guy who went by "Horse" on here who fancied himself as an expert in hot-start matters...
 
Dan Thomas said:
All injected engines have boost pumps, including the 172SP.
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Good point. I'm not sure what I was thinking. :confused: That's remarkably more sophisticated than the already ancient 172s I was allowed to fly when I was younger. :)

Dan Thomas said:
The OP should try starting with the throttle well open and mixture left at idle cutoff. Hot starting is usually a flooding problem.

Some of the flow dividers on the IO-360-L2A in the 172 had the shutoff poppet machined too close and it would stick, though normally when colder.

http://www.precisionairmotive.com/Publications/SIL RS-84.pdf
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I'm not aware of how an engine that has been shut down and secured correctly would flood just sitting there prior to a hot start attempt? Flooding while trying to hot start is not uncommon, but it is invariably the result of someone trying to do the hot start incorrectly, thus flooding it.

Most hot start problems are because the fuel lines forward of the firewall are heat soaked and contain vapor.
Most often the engine will start momentarily on any liquid left in the distribution lines to the cylinders and then quit. Continentals have a return line for excess fuel so the lines can be purged with cold fuel from the tank by running the boost pump, with the mixture at idle cut-off, for 20 or 30 seconds. In my experience that doesn't work on a Lycoming, and if the mixture is opened for any length of time with the boost pump on it will flood it.

I find I need to open the mixture for only one or two seconds maximum with the boost pump on to pressure up the lines and eliminate the vapor. 9 times out of 10 the Lyc engines will start within two rotations on the first try.
 
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GRG55 said:
I'm not aware of how an engine that has been shut down and secured correctly would flood just sitting there prior to a hot start attempt? Flooding while trying to hot start is not uncommon, but it is invariably the result of someone trying to do the hot start incorrectly, thus flooding it.

Most hot start problems are because the fuel lines forward of the firewall are heat soaked and contain vapor.
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And why is there vapor in the lines? Because the engine's heat has boiled the fuel out of them and into the manifold. Check valves in the fuel pump prevent backflow, and any fuel in metal lines or rubber hoses in the engine compartment gets hot and expands, some of it hot enough to boil. The flow divider poppet opens under the pressure and lets the injectors dribble the fuel into the manifold, bringing the air/fuel ratio to incombustible levels. Using prime, or trying to start with the mixture rich, just maintains that ratio.
 
Dan Thomas said:
And why is there vapor in the lines? Because the engine's heat has boiled the fuel out of them and into the manifold. Check valves in the fuel pump prevent backflow, and any fuel in metal lines or rubber hoses in the engine compartment gets hot and expands, some of it hot enough to boil. The flow divider poppet opens under the pressure and lets the injectors dribble the fuel into the manifold, bringing the air/fuel ratio to incombustible levels. Using prime, or trying to start with the mixture rich, just maintains that ratio.
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We agree on the reason for the vapor in the lines. :D

But if the mixture was left at idle cut-off after the engine was shut down it would seem only a very limited amount of fuel can enter the engine through the flow divider. The mixture control on most systems is on the fuel injector servo, downstream of the engine driven fuel pump. Only the small volume of fuel downstream of the mixture control idle cutoff can make its way into the manifold as vapor if the flow divider poppet lifts. These are predominantly stainless steel lines so the cumulative internal volume is limited. Unlikely enough to "flood" the engine.

On my IO-540s everything upstream of the engine driven fuel pump gets pushed back to the fuel tank as the fuel supply lines inside the cowls heat soak. There is a bypass around the electric boost pump including its check valve as the engine driven pump draws fuel around, not through, the electric boost pump when it is off. The only way that fuel will not get pushed back towards the tank is if I put the fuel selectors to "off", which is not my habit.

The engine driven fuel pump doesn't work well when it's being fed vapor. How many times have you seen a hot fuel injected engine start on the fumes in the manifold/fuel divider and then immediately quit. Followed by endless cranking of the starter while the liquid starved mechanical fuel pump is unable to supply fuel to the injector servo (we've all watched that performance on the ramp a time or two, non?). I've found judicious use of the electric boost pump and the mixture control during a hot start eliminates that problem consistently. YMMV.
 
Assuming your mags are good and timed well. Mooney Lycomings are hard to hot start. Try this:
1. Shutdown Mixture cut off at 1100 RPMs
2. To start: Don't turn on pump, Don't move any knobs.
3. Hit the starter and crank till it pops then add mixture.
It's flooded when it pops and you add mixture and dies. Then do throttle wide open. Crank till pops
Get a B&C starter. It will last longer than a Skytec. Bearings vs. bushings.
 
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