O-360 Engine Quit After Extended Idling on Hot Day

[nickw9815]

My friend’s O-360 engine on an RV-6 quit while idling after sitting on the taxiway for 25 minutes. They were given hold short instructions on a hot day and had to idle at 900 RPM for the entire wait. The engine began to sputter, so they added a little power, which reduced the sputter but caused CHTs to climb to 420°F. They then reduced power, and the sputter returned. When cleared to taxi, the engine ran fine with more power, but when they pulled power to hold short again, it quit.

They suspect the carb was heat soaked after extended idling. Any other ideas on what may have caused this?

 

[lr172]

My friend’s O-360 engine on an RV-6 quit while idling after sitting on the taxiway for 25 minutes. They were given hold short instructions on a hot day and had to idle at 900 RPM for the entire wait. The engine began to sputter, so they added a little power, which reduced the sputter but caused CHTs to climb to 420°F. They then reduced power, and the sputter returned. When cleared to taxi, the engine ran fine with more power, but when they pulled power to hold short again, it quit.

They suspect the carb was heat soaked after extended idling. Any other ideas on what may have caused this?

If it was boiling fuel in the carb bowl, it should be rough off idle as well. Have you ever set the idle mixture. An overly rich idle mixture can do this when things get really hot. Setting this is an important step, so if unsure it has never been done, suggest doing it.

 

[nickw9815]

@lr172 If it was fuel boiling, wouldn't adding power start to cool the carb and fix it? Not sure if idle mixture was set, but it was not leaned for idle during this.

 

[lr172]

@lr172 If it was fuel boiling, wouldn't adding power start to cool the carb and fix it? Not sure if idle mixture was set, but it was not leaned for idle during this.

The only issue related to fuel being to hot is when it reaches its critical temp where it starts converting to vapor. This is how you get vapor lock in the fuel pump or start getting bubbles in the lines, which can create all sorts of issues. When this happens in a carb, the vapor production/boiling of fuel in the bowl causes the float to drop (bouyancy issues, I believe) and allows too much fuel. The fuel level in the bowl is what sets the mixture level in a carb, so it goes rich. If the fuel in the carb is at critical temp it is likely that ALL the fuel in the system FF is there as well. So, initial power application does nothing to immediately address that issue. Probably takes 10-15 seconds at full power to do that. This phenomenon has caused a handfull of take off accidents, where the hot fuel hits the carb after initial power application and engine stumbles right at lift off from excessive richness.

Idle mixture is not set by the red knob. it is via the idle mixture screw on the carb. There is a procedure to do this. Your symptoms sound more like an excessively rich idle mixture than fuel vapor to me (idle mixture too rich, but as soon as you get off the idle circuit all is fine). Not a conclusive diagnosis by any means. Most carbs come from the mfg filthy rich on idle. This needs to be adjusted on every new engine, yet many never do it. Suggest you start with that.

 

[georgemohr]

If heat is determined to be the enemy here (and I like others strongly suspect so), consider adding louvers or other expansion of the exit air from the cowl.

Edit: BTW, those idle CHTs seem high. I'd also check the baffles for good integrity.