hot start engine surging (and sizzling)

[stevecampbell]

The other day after getting gas in our '78 J, I restarted (hot start) and had to give it a little prime to start (  I used 3 seconds).  It started right up, but the rpm was surging from 1000-1500.  After a taxi back and a 2000 RPM "run-up" with the boost on, the surging progressively calmed down.  

I sent an e-mail to the other owners and they thought that my priming may have caused the surging, until it happed to another one of the guys (he did not prime at all) a few days ago.   My best guess it that is may be some fuel vaporization in the line and that it's not all cleared out unless you run the boost pump for a while before starting?  

I could also hear what sounded like some very quiet and intermittant sizzeling as I was pushing the plane back by hand...fuel re-vaporizing in the lines?

Does anyone have any ideas as to the cause of this or has seen it before?   

 

Steve

[Bolter]

I don't have absolute answers, but can give you some reasssurance that you are not alone, so it may all be normal.  

On a hot day re-start (we are +100°F, often at this time of year) I will get some engine idling issues which could be described as surging, though I think it is more like unsteady idling, and is much less than 500 rpm in my case.  I sometimes prime, sometimes don't, depending on the circumstances.  This lasts about 30 seconds after starting.  Certainly goes away after a run up.  I tend to leave the throttle alone, and it clears out.  I assume it is hot fuel lines that take a little bit of fuel flow to cool down.  

The sizzling sound has comfounded me, but I have learned to accept it.  I hear it when I get my ears close to the cowling when pushing the plane back...regardless of touching the prop or not.  I cannot pinpoint the source.

So I am curious to see what theories show up, but I think there is nothing unusual in what you are observing.

-dan

[sleepingsquirrel]

The sizzeling is probably fuel which is spraying into the cylinder due to absorbing heat after shutdown. It may sometimes be heard depending on the position of the valves. ?????

[ionel]

Quote: stevecampbell

The other day after getting gas in our '78 J, I restarted (hot start) and had to give it a little prime to start (  I used 3 seconds).  It started right up, but the rpm was surging from 1000-1500.  After a taxi back and a 2000 RPM "run-up" with the boost on, the surging progressively calmed down.  

I sent an e-mail to the other owners and they thought that my priming may have caused the surging, until it happed to another one of the guys (he did not prime at all) a few days ago.   My best guess it that is may be some fuel vaporization in the line and that it's not all cleared out unless you run the boost pump for a while before starting?  

I could also hear what sounded like some very quiet and intermittant sizzeling as I was pushing the plane back by hand...fuel re-vaporizing in the lines?

Does anyone have any ideas as to the cause of this or has seen it before?   

 

Steve

[bd32322]

for my first ever hot start in my mooney (30-minute fuel stop- 90F ambient) I primed for 1 second because I heard that helps to get rid of vapors in the lines. Then I had a bear of a time starting the damng thing. Ever since I have had much better luck with not priming at all, mixture back all the way, crank and feed the mixture in

 

[ionel]

Quote: bd32322

for my first ever hot start in my mooney (30-minute fuel stop- 90F ambient) I primed for 1 second because I heard that helps to get rid of vapors in the lines. Then I had a bear of a time starting the damng thing. Ever since I have had much better luck with not priming at all, mixture back all the way, crank and feed the mixture in

 

[KSMooniac]

I concur with ionel's techniques posted above.  I'll only add that for the anticipated hot start, I'll idle the plane at 1100 RPM for a few seconds prior to shut down, and LEAVE THE THROTTLE ALONE.  To restart, simply crank and feed the mixture in slowly as it starts to catch... that usually works great.  

[larryb]

 

There is one question has been bothering me for a while with the hot start technique: Where does the fuel come from to start the engine? After all, the engine was stopped with the mixture being pulled to ICO. And the mixture is still at ICO. So once again, where did the fuel come from?

 

Larry

 

[Bolter]

Quote: KSMooniac

I concur with ionel's techniques posted above.  I'll only add that for the anticipated hot start, I'll idle the plane at 1100 RPM for a few seconds prior to shut down, and LEAVE THE THROTTLE ALONE.  To restart, simply crank and feed the mixture in slowly as it starts to catch... that usually works great.  

[KSMooniac]

Quote: larryb

 

There is one question has been bothering me for a while with the hot start technique: Where does the fuel come from to start the engine? After all, the engine was stopped with the mixture being pulled to ICO. And the mixture is still at ICO. So once again, where did the fuel come from?

 

Larry

 

[jetdriven]

The fuuel you heard boiling in the lines after shutdown went into the cylinders. If you leave the throttle set to 1000 rpm and touch nothing, and crank it will start in 15-20 blades. It often does take the boost pump to maintain stable fuel pressure so it will run smooth. Briedly running up to 1500 rpm helps to flush the heat out of the cowl.

[ionel]

Quote: jetdriven

The fuuel you heard boiling in the lines after shutdown went into the cylinders. If you leave the throttle set to 1000 rpm and touch nothing, and crank it will start in 15-20 blades. It often does take the boost pump to maintain stable fuel pressure so it will run smooth. Briedly running up to 1500 rpm helps to flush the heat out of the cowl.

[jetdriven]

I'll try that.  But I have levers, so I'm not sure how many turns.

[bd32322]

Quote: larryb

 

There is one question has been bothering me for a while with the hot start technique: Where does the fuel come from to start the engine? After all, the engine was stopped with the mixture being pulled to ICO. And the mixture is still at ICO. So once again, where did the fuel come from?

 

Larry

 

[ionel]

from reading on the web it looks like ... 

The web articles say ... 

So I wonder what will happen if ... 

Worth a try...

 

Bodi

[KSMooniac]

I think you'll be surprised how long it takes for the engine to quit after you turn the fuel selector to OFF.  I'm not that patient, especially in the summer.  

[jetdriven]

Ours takes exactly 5 seconds.

[jetdriven]

Also, if you run that boost pump without it 100% completely full of fuel its life is measured in single digit minutes.  And it is, at last count, a 1200$ overhaul.   I run tanks dry, but will only us the pump if it will not relight.  Which has not happened yet.

[bd32322]

Quote: jetdriven

Also, if you run that boost pump without it 100% completely full of fuel its life is measured in single digit minutes.  And it is, at last count, a 1200$ overhaul.   I run tanks dry, but will only us the pump if it will not relight.  Which has not happened yet.

[m20j]

Almost ...

Pulling the mixture stops the fuel flow, but does not empty the lines.

Fuel lines sitting on top of the hot cylinders boils out through the injectors and into the head / manifold (hence the "sizzle".)

Resulting condition leaves an slightly too-rich-to-start condition that takes a few blades to pull through with the mixture OFF until it is lean enough to fire.

Touching the boost pump at all only floods it worse.

There is no blocked flow with the Lycoming fuel injected engines.  It's not starting because there is too much fuel, not too little.

As said earlier, try shutting down with the mixture while idling at 1000 rpm if you are going to be hot starting in the next 30 minutes or so.  Then don't touch anything but the starter switch.  Kicks somewhere around 2-4 blades on my J nearly every time.  After it fires, then push the mixture back in.

Quote: bd32322

from reading on the web it looks like extra fuel is not the problem. The fuel exists in the fuel lines despite pulling the mixture out. This fuel turns into vapor because of the heat of the engine which then blocks fuel flow to the cylinders when you later try to start.

The web articles say this is usually not a problem with injected engines because the fuel is under high pressure in the injectors and so trying to prime it with the boost pump ends up dumping extra fuel.

 

So I wonder what will happen if I just cut off the fuel from the fuel cutoff valve and keep the mixture in the full rich position - the next time I anticipate a hot start. The engine should use up all the fuel in the lines on the engine side of the cutoff valve I think - and then when attempting a restart I really will need to use the boost pump to provide fuel to the cylinders.

 

Worth a try...

 

Bodi