Warm start more difficult than hot start? What gives?

[0TreeLemur]

When on a trip and I stop to refuel and do it quickly, I have no trouble with hot start.   Just use the boost pump to pressurize the fuel system, crack the throttle, crank the engine and advance the mixture.  Viola!  Running engine.

If I go someplace for fuel and take my time re-fueling, visit with someone, enjoy a nice evening, the IO-360 doesn't want to start.   CHT's cooled down to maybe 120-160F.   The procedure above does not work.   It takes five or six tries.  With the obligatory starter cool down periods, it takes 5 minutes.   When this happens I get out and look for fuel dripping out of the exhaust pipe.  Nope.   If I waft some air from the vicinity of the exhaust and smell it, it smells like fuel.

I don't prime the engine.  When I pressurize the fuel system the mixture is at idle cut-off.  Anybody have any idea what's going on here?   

Thx.

Fred

[DCarlton]

Looking forward to this thread.   I call them jokingly cold hot starts.  I stress over them every time.  I don’t use your hot start procedure though.  Will wait for expert feedback.  

[0TreeLemur]

12 minutes ago, DCarlton said:

Looking forward to this thread.   I call them jokingly cold hot starts.  I stress over them every time.  I don’t use your hot start procedure though.  Will wait for expert feedback.  

I've started dreading them too.

[toto]

I treat a warm start like a hot start. I don’t touch the boost pump, just crack the throttle and slowly advance mixture while cranking. It usually takes about 5-6 seconds to get it started, when the mixture reaches about the halfway point. 

[EricJ]

Depending on how long it's been, I'll treat a warm start like halfway between a cold and hot start.   In other words, give it some boost pump for two or three seconds rather than the usual duration used for a cold start.    If it repeatedly catches but won't stay running, then try giving it boost pump just as it catches and see if that sustains it...which I've found I have to do at high DA.

 

[varlajo]

Oh yes, the magical time 15-45 minutes after shutdown. Voodoo territory for sure.

 

[Bartman]

If mine does not start on the first try, I do the flooded start procedure and it starts every time.  Mixture to the wall and hit the boost pump.  Mixture cutoff and throttle to the wall and crank.  She starts every time but you have to be quick with the throttle and mixture adjustments when it catches.

[Beard]

I start mine successfully, warm or hot doing the following. I bring the RPM down to 1,000, then mixture idle cutoff to stop. To start I touch nothing and as I crank the engine, I push the mixture forward slowly until it catches. There is a Youtube video on this.

[DCarlton]

10 hours ago, EricJ said:

Depending on how long it's been, I'll treat a warm start like halfway between a cold and hot start.   In other words, give it some boost pump for two or three seconds rather than the usual duration used for a cold start.    If it repeatedly catches but won't stay running, then try giving it boost pump just as it catches and see if that sustains it...which I've found I have to do at high DA.

 

That's what I do.  2-3 seconds instead of 5-6 seconds when cold.  Never tried adding the boost pump if it stumbles and won't stay running.  Will add that to the things to try list.   

[TangoTango]

33 minutes ago, Beard said:

I start mine successfully, warm or hot doing the following. I bring the RPM down to 1,000, then mixture idle cutoff to stop. To start I touch nothing and as I crank the engine, I push the mixture forward slowly until it catches. There is a Youtube video on this.

I was taught the same process by my transition instructor and have found it works well. I don't prime unless the engine has cooled to ambient. 1,000 rpm seems to be the sweet spot - on a cold start, the throttle position from the last shutdown may need to be pushed forward just a hair to hit the target 1,000 rpm.

If I do get hamfisted with the mixture control and push it too quickly on a warm start, the flooded start procedure always bails me out 

[MikeOH]

Another fan of shutdown at 1000 rpm and restart by advancing mixture with NO boost.  Works for all 'voodoo territory' situations

[varlajo]

27 minutes ago, MikeOH said:

Another fan of shutdown at 1000 rpm and restart by advancing mixture with NO boost.  Works for all 'voodoo territory' situations

Are you saying I bought that bunch of shrunken heads from LASAR for no reason?? 

[PT20J]

Let's take some of the voodoo out of it 

A couple of important points: 1) fuel and air will combust only over a narrow range of mixture, and 2) liquid fuel will not combust; it needs to be vaporized and mixed with air.

There is always plenty of air available, so the trick is to get the right amount of fuel.

Lets start with the hot start. The engine has been shut down for just a few minutes. The hot engine has heated the fuel in the lines between the flow divider and the nozzles and forced fuel into the engine but it has not had time to fully evaporate. In fact, there is fuel vapor in the heads just waiting to ignite. So you crack the throttle slightly (where about 1000 rpm would be. On mine it is just where the landing gear warning switch clicks) and you don't prime and you start with the mixture in ICO. As soon as it fires, you advance the mixture and set the throttle.

Now let it sit a little longer, say thirty minutes to an hour. The hot engine has emptied the flow divider and fuel lines. It's going to need some prime to refill them. But not too much because the warm engine will vaporize the fuel easily. So, just a second or so of prime to fill the lines. Then do a normal start with throttle cracked and mixture in ICO. Alternately, you can try not priming and starting with the mixture full rich. Sometimes this method requires a little more cranking, but it will catch.

Moving on to a cold engine, we can see that the lines will be empty from the previous shutdown. And, since the fuel and engine are cold, the fuel isn't going to vaporize as well. So it takes more prime to get enough fuel into the engine so that some of it will vaporize to combust. I've found that four seconds is usually about right. Open the throttle a bit more than for a hot start and set the mixture to ICO. It also helps if you wait a ten or 15 seconds between priming and cranking to allow some of the fuel to evaporate and form a combustible mixture.

A point about priming: The RSA fuel injector doesn't meter fuel according to airflow accurately below about 1200-1500 rpm and so there is a linkage connected to the throttle that adjusts the mixture in the idle range. When priming, you should set the throttle at least 1/4 open so that it isn't reducing the fuel flow. Otherwise, the amount of prime will be dependent on the throttle setting as well as the length of time the mixture control is not in ICO and this just adds another variable to the mix.

Of course, all this assumes that you ignition system is putting out a hot spark an the mags are timed correctly.

 

[A64Pilot]

I put mine full rich, throttle to idle and just start the thing. it does take a few seconds but not many. I do this every Sun after we fly to breakfast, often in the Summer when I go to lunch she is still hot and needs the hot start procedure.

Done correctly the “hot start” procedure will work at any temp, because it’s simply intentionally flood the thing, crank with zero fuel flow then push mixture in.

It will work because turning a flooded engine over at any temp will clear the flooding if you spin it long enough, hot or cold.

95% or more of hard starting problems assuming a good battery are weak ignition, can be as simple as the plugs need cleaning.

[Hank]

I took the mystery out of cold and hot and warm starts by flying a C model. The O-360 is stupid simple to crank, the difference between cold and hot is running the fuel pump before or not. The important part is to turn the key and push!  

[Fly Boomer]

1 hour ago, PT20J said:

Of course, all this assumes that you ignition system is putting out a hot spark an the mags are timed correctly.

So you always crank with mixture at ICO -- cold, warm, or hot?

[varlajo]

Edited 17 hours ago by varlajo

[toto]

5 hours ago, Beard said:

I start mine successfully, warm or hot doing the following. I bring the RPM down to 1,000, then mixture idle cutoff to stop. To start I touch nothing and as I crank the engine, I push the mixture forward slowly until it catches. There is a Youtube video on this.

The standard Don Maxwell hot start video?

 

[PT20J]

1 hour ago, Fly Boomer said:

So you always crank with mixture at ICO -- cold, warm, or hot?

Normally. But, like I said on a warm engine you can usually just start it by putting the mixture to rich before cranking.

[MikeOH]

2 hours ago, Fly Boomer said:

So you always crank with mixture at ICO -- cold, warm, or hot?

Yes

[0TreeLemur]

5 hours ago, PT20J said:

Now let it sit a little longer, say thirty minutes to an hour. The hot engine has emptied the flow divider and fuel lines. It's going to need some prime to refill them. But not too much because the warm engine will vaporize the fuel easily. So, just a second or so of prime to fill the lines. Then do a normal start with throttle cracked and mixture in ICO.

When you run the boost to fill the lines, what throttle position do you set?   Half?

[PT20J]

7 minutes ago, 0TreeLemur said:

When you run the boost to fill the lines, what throttle position do you set?   Half?

That would be good. Anything past what would be about 1500 rpm is OK.

[AndreiC]

Hot starts in my IO360 work pretty well with the procedure of leaving the throttle at 1000 rpm and cranking with mixture at ICO. (This is, say, 15 minutes after shutting down on a hot day.)

But what I don't understand is why, after the engine starts, it will stumble continuously at low power settings (say, under 1400 rpm). Moreover, if I keep trying to run it at 1000 rpm while it does this, I will almost certainly hear backfires from the muffler, which can't be good.

My current procedure is to run it at a high idle (1500 rpm) for a few minutes, while also running the boost pump. That seems to work, but I don't know why it does that. My guess is that there is vapor lock in some of the injector lines, and somehow fuel gets in the cylinders that does not burn and explodes in the muffler. Why? I don't know.

[EricJ]

1 minute ago, AndreiC said:

My current procedure is to run it at a high idle (1500 rpm) for a few minutes, while also running the boost pump. That seems to work, but I don't know why it does that. My guess is that there is vapor lock in some of the injector lines, and somehow fuel gets in the cylinders that does not burn and explodes in the muffler. Why? I don't know.

It's more likely vapor lock/cavitation at the input to the mechanical pump, which creates low pressure/suction at its input during pumping.   The low pressure at the input to the pump may approach the fuel vapor pressure when the system is hot, so turning on the boost pump increases the fuel pressure at the input to the mechanical pump, reducing the likelihood of boiling/cavitating.   As more ambient temperature fuel flows through the system it may cool the mechanical fuel pump enough to increase the fuel vapor pressure at that location and decrease the likelihood of cavitation.   The fuel downstream from the mechanical pump will be at higher pressure due to the pump, and so also less likely to boil.   The fuel in the distribution lines can boil after shutdown once the supply pressure is removed at engine shutdown, but generally won't before that.

[0TreeLemur]

32 minutes ago, EricJ said:

turning on the boost pump increases the fuel pressure

 According to the video above, the normal state for a Lycoming IO-360 before hot or warm start is flooded.    I did not realize that.   Don said that the flow divider acts like a coffee "percolator" and it drains fuel through the injectors after shutdown.   Based on that I believe that by switchiing on the boost pump I didn't prevent cavitation, I caused the flooded condition to persist longer than I needed to.

Note to self:  boost pump not used for hot or warm starts.   I was over thinking it.