Did I have Vapor lock?

Hi all,
Yesterday we had our RV-8 parked at the Udvar Hazy museum for the 'Pilot Family day'. Great event, we had fun. Temperatures were in the low 90's and the RV had been sitting in the sun for 7 hours. Upon departure it took a long time in a long row of airplanes before we were allowed to start a long taxi. I estimate it took 45 minutes of idling and taxiing. During the last 10 minutes my engine died when I pulled the power to idle. I switched tanks, turned fuel pump on and the engine started right up. Fuel pressure was good, even with the fuel pump off, but fuel flow with the fuel pump off sometimes went down to almost 0 with the engine idling rough. 'Fuel pump on' made everything normal again. Engine run up was fine also.
Fuel pressure was fine at all times (with and without electric fuel pump).

Once on the ground after the very short flight, the problem appeared to be gone (waiting for approx one hour with the aircraft in the hangar), however fuel flow appeared to want to go down to almost 0 on rare occasion (idle).

I strongly suspect vapor lock. Any other ideas?

Post?

Judging the fact that over 250 people read this post, but no one replied, it must have been obvious? What how obvious? I still don't have a clue....

ArjanPlomp said:

Judging the fact that over 250 people read this post, but no one replied, it must have been obvious? What how obvious? I still don't have a clue....

Click to expand...

I think it's the title of your post. Most probably don't want to be locked into doing you a favor

{ed: fixed the title! PFD}

It would be helpful if we knew what fuel set up you had, carb, FI, which engine, and what plane, but it sure sounds like vapor lock to me. The clue is high temps, long idle, and as soon as you hit the boost pump it goes away.

IMHO, the next time you are at an event where you have a 45 min. idle / taxi you might want to think about having another cold drink and letting the crowd clear out. That is a long time to idle an aircooled engine.

Sounds like vapor lock to me.
If all operations confirm normal after letting it totally cool off, that would pretty much confirm it. If it were mine I would leave the boost pump for all ground operations.
Good Luck,
Mahlon
"The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at your own risk."

ArjanPlomp said:

Judging the fact that over 250 people read this post, but no one replied, it must have been obvious? What how obvious? I still don't have a clue....

Click to expand...

IF the fuel system is fuel injection, what happened would appear to be normal. (What follows is a explanation of what MAY have occurred but there are no guarantees it is the answer.)

The flow divider area heats up during a long taxi, fuel flow to the injectors is low resulting in some fuel vaporization in the lines. This causes rough running. When flow is increased by idling at 1200-1400 rpm, fuel stays cool enough through the area to prevent vaporization. In flight, the area is cooled most adequately so it is not a problem, plus fuel flow is relatively high.

If the primary fuel delivery line before the mechanical pump is in an unprotected area and/or near an exhaust pipe, vaporization may be occurring in that area. In that case, the electric pump will help prevent it. But I don't think that is what is going on here.

The electric pump won't help much in the flow divider area. That area is under 1-3 psi fuel pressure coming from the controller no matter what. Before the controller, pressure is in the 20-30 psi range and vaporization should not occur.

Remember, fuel will vaporize when not under pressure somewhere around 110F. At 1-3 psi, that temperature is somewhat higher but not by much. That's why the engine runs rough on the ground after a lengthy delay.

I have these symptoms after a quick turn with a heat soaked engine. It is not a big deal. Opening the oil check door after shut down helps to cool the area around the flow divider and sometimes it is enough so it does not run rough before take off. Depends a lot on OAT also.

Yes indeedy.

Yes Im pretty sure you were boiling fuel. My 6a has a blast tube to the mechanical pump on my carberated 0360. So it runs fine when ideling 15 to 20 never 45 minutes. If I shut down in 90 degree wx and try restart in 10 the engine stumbles for a few minutes till cooler fuel passes through the system. A blast tube wouldnt hurt.

Thank you!

The engine is carburated.

Thank you for all replies! I see that other's experiences confirm the vapor lock probability.

Arjan
N83SE