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912UL Possible Vapor Lock

rshippee

I'm New Here
Couple of weeks ago a friend and I took my 912UL powered RV12 out for breakfast at an airport about 100 miles from my hangar. We arrived and the ramp temp was in the high 90s. Went to the FBO restaurant only to find out they were closed for the week. Less than 15 minutes after shutdown we restarted the RV12 and while taxing to the run up area I noticed it appear "sluggish" for lack of better term. As I started to runup for the mag check I noticed the cylinder heads climbing into the yellow, a short taxi back to the ramp and shut down followed. There were no visible leaks, oil level was fine, and so was the coolant level. Also no noticeable fuel in the carb float pans, floats have been checked about 50 hours prior. After an hour of trying to arrange ground transportation home one of the mechanics on the field recommended a restart. We started it back up, all appeared normal and flew it back home with no problem. The local opinion is it was vapor lock. Anyone experienced this situation with the 912UL?
 
I have experienced this in a Rans S-20 I built that had a 912ULS. Over 100 degrees and shut down for 30 minutes or so then started back up. When I brought the power up for the run up the engine stumbled. Taxied back to hangar and let the aircraft cool down and then ops normal.
 
I would say more than "possible" vapor lock... I would say "probable" vapor lock. RV-12 is a very tightly-cowled engine installation. Rotax 912ULS has fuel lines routed on top side of engine, and as such, they get “cooked” from rising heat when the engine is shut down and air flow in the cowling stagnates...

See following posts:

https://vansairforce.net/community/showpost.php?p=1519444&postcount=86

https://vansairforce.net/community/showpost.php?p=1520013&postcount=100

https://vansairforce.net/community/showpost.php?p=1520306&postcount=109

https://vansairforce.net/community/showpost.php?p=1522183&postcount=135
 
Last edited:
Winter blend fuel

I ran into this twice in the last month. On climb out, the engine shuddered for 1-3 seconds. Got my attention and a quick turn back to the field. I talked with Joe at Lockwood Aviation. He suggested a vapor lock possibly due to winter blend avgas. I called the station and they confirmed that the date range when I purchased it still had winter blend fuel in the tanks. I drained all the fuel from the tank and replaced it with fresh fuel. 3.5 hours later, no problems...
 
Vaper Lock

This happens on my Rans S7 912S short tail with a tight cowling, flying, landing on a hot day. I Found out when taxi up to stop I shut off the fuel for a short time and let the engine idle and then shut down the engine. On start up prime the carburetors with primer and start engine. It will start and run just fine, probably because of cold gas from the system, filling up the bowls.
George S7 flyer and RV7 builder
 
I drain any left over gas in May and use it in my truck. Start fresh with summer Premium or 100LL as needed. Winter I definitely lean to 100LL mix.
 
Yes. Using MO-Gas in Florida during winter, I had a couple of these situations on takeoff. Very scary!

We tested afterwards on the ground and were able to consistently re-produce the vaporization problem:

https://photos.app.goo.gl/gBimcq5KF1x4py9H9

We tested @ 87 degree outside temperature, increased the engine temp (slightly below CHT yellow), then full power (see video).

Results:
1) 100% 93 octane MO-Gas winter blend -> Vaporization issue
2) 25% AV-Gas + 75% 93 octane MO-Gas winter blend -> Vaporization issue
3) 50% AV-Gas + 50% 93 octane MO-Gas winter blend -> NO Vaporization issue

Lesson Learned: I always mix 50/50 AV-Gas and MO-Gas to be on the safe side.
 
open up

I made a habit of opening the oil-door on the top-cowling to let the heat out. No matter how long/ short I intended to park the plane. Never experienced vapor lock - so far.
 
I made a habit of opening the oil-door on the top-cowling to let the heat out. No matter how long/ short I intended to park the plane. Never experienced vapor lock - so far.

Yes, I did that also... until I experienced Vapor Lock on take-off and almost didn't return to airport runway. Vapor Lock is very serious and life-threatening...

See my post above for what I consider best practice for tightly-cowled Rotax 912....
 
Jim,

Did you experience vapor lock with the electric fuel pump running? The only time I have had it is on climbout in PHX summers with the boost pump off. The minute I turned on my electric pump it cleared.

Rich
 
Jim,

Did you experience vapor lock with the electric fuel pump running? The only time I have had it is on climbout in PHX summers with the boost pump off. The minute I turned on my electric pump it cleared.

Rich

Excuse me for saying... but this is stupid, or ill-informed, question. Van's Aircraft said that the electric fuel pump is to run continuously as-designed. It is not a boast pump. Van’s Aircraft has, on several occasions, explained why it is necessary to operate the electric fuel pump to maintain head pressure and prevent cavitation. If you like, I can go into details once again, but it is covered very well in several threads in this forum…

So, to answer your question… yes, my electric fuel pump runs continuously. There is no provision to shut it off short of removing fuse.
 
Winter blend fuel is what we believed to be the cause of this. We mixed with avgas during the transition time, and didn’t have the problem again. I also put a layer of white vinyl wrap on top of the fuel tank so it would stay cooler in the sun.

The quick turn-around are part of the problem too.
 
Good Info

Thanks to all, good to know that Im not alone on this. Have run the engine for two hours since the suspected vapor lock incident, and all appears normal. My local Rotax mechanic said given I only spent a few minutes at idle with the cylinders in the yellow that no significant damage was likely done. From now on I will be more careful on very hot days, give the engine an hour or so to cool and also if it happens again I will catch it early now that I have had the experience. To those that have not seen this yet, if you crank up on a hot day and the airplane appears "sluggish" then shut down immediately on the ramp.
Thanks for the replies.
Ron
 
Jim,

Not excused. This has been rehashed many times on the forum. There are those that subscribe to the continuous pump run and those who like to keep it in reserve for emergency use after climb out. I respect your decision and don’t question it. Grant me the same courtesy.

Rich
 
Jim,

Not excused. This has been rehashed many times on the forum. There are those that subscribe to the continuous pump run and those who like to keep it in reserve for emergency use after climb out. I respect your decision and don’t question it. Grant me the same courtesy.

Rich

Courtesy is good... I wouldn't take passenger with you if you shut off electric fuel pump defying Van's Aircraft directive.

OK to risk one's own life, but not that of passenger who unknowingly is involved...
 
My experience

I want to relate an incident I had. I am not taking sides, just sharing. On a very hot summer day last year I landed at Thermal airport by Salton Sea. I was there for a quick 15 minute visit. When I went to leave I started up without the electric pump because of the hammering sound and people were watching. I had no idea that the fuel lines in the engine compartment had heat soaked and vapor locked. As I taxied out the engine stumbled, I knew immediately what happened and switched the electric pump on The engine was using the remainder of the fuel in the bowl and the electric pump could not push enough volume past the vapor lock. The engine died, I cranked to restart but no fire. I allowed the electric pump to run for 30 seconds and retried the starter and it stumbled to life and finally smoothed out after 5-10 seconds. I wonder to this day if I had gotten airborne would I have had enough time to restart before the ground had gotten in the way. I do have the higher pressure electric pump. My procedure now is to let the electric pump run several minutes before cranking to circulate the hot fuel under the cowl back into the tank.
 
These debates about continuous running of the Facet pump vs. shutting its down are more passionate than abortion and Jan 6th discussions... but I digress. I've added a switch on the panel to shut off the electric pump when I'm doing database updates or other panel work in the hanger to mitigate the pump noise. Minor irritation I confess. I've only used it in flight as curiosity to quickly gauge how much pressure the electric pump is actually contributing in cruise. When I do, I try not to remove my finger from the switch as I frankly don't trust myself to remember to turn it back on. Another use of PPE....preventative pilot error. :D
 
My procedure now is to let the electric pump run several minutes before cranking to circulate the hot fuel under the cowl back into the tank.

I think the more recent revisions of the POH specify doing so as well, in hot conditions. Along with other procedures to mitigate excessively elevated fuel temps such as not leaving the canopy fully closed in hot sun, etc.
 
The builder of my RV-12 supplied a lovely little 12V fan which can be placed on the oil access door and connected to the cigarette lighter in order to assist in pulling hot air out of the cowling while getting lunch.

If you don't already have one, I'd recommend searching Amazon for "120mm USB fan" and spending $10 picking up one you can plug into a portable battery pack, so that if you leave it on for too long there's no risk in draining your starter battery.

If there's demand, I can make a design for a 3D printable bracket that'll keep everything in place.
 
When I first installed a switch my procedure was to use the electric pump for starting, takeoff and landing. The only time I experienced vapor lock was on a hot PHX summer day when I secured the electric pump after initial climb out to about 2500 AGL. I immediately turned the electric pump back on and the vapor lock disappeared. To be clear the engine never stumbled, but the fuel pressure and fuel flow indications became very erratic until I turned it back on. I now leave it on in summer until I’m about 5000 AGL in cruise, and carefully monitor fuel flow and fuel pressure when I turn it off. I turn it on when I begin my descent for landing. Seems to work without issue.
 
I have a switch on mine as well which I find very handy, but I'm just curious why you turn your pump off in flight. Not criticizing, just wondering if there's a particular reason you turn it off.
 
My reasoning is two fold. First, I see no reason to put operating hours on the electric pump when it isn’t needed. Second, if my mechanical pump fails I want to know right away. The pressure will drop slightly if only one pump is running, but a mechanical pump failure might go unnoticed until the electric fails too if the electric pump runs constantly. In that scenario I would be VSF! I like the idea of at least having the initial option of turning on the electric pump if the engine quit.
 
Fair enough, although I'll leave mine on. It gets checked on every startup, and if the mechanical pump does fail at a critical moment, I want the electric pump already running.
 
In addition to everything else mentioned, I try to throw something over the top of the fuel tank to shade it while parked on the ramp. Keeps the ramp guy from only filling to the Full line and helps to keep the fuel from being heated by the sun.
 
Vapor Lock

The following was sent to me from Van's Tech Support on November 3, 2017 in response to my inquiry on RV-12 vapor lock. Note the statement involving "winter blend fuels during hot conditions." This was nearly five years ago.

The current POH has this p.4-2

WARNING
During high ambient temperature conditions,
run the fuel pump for 5 mins to flush the fuel
lines and minimize the potential for vapor lock

4-15
NOTE
To prevent vapor building in the carburetor after shutdown in hot
days, the oil door should be left open to let heat out of the cowl.
Leaving the canopy in the open position latched with the F-
1231G Canopy Catch, will reduce the risk of vapor-lock

8-3
CAUTION
Use of poor quality fuel or winter blend fuels in hot
conditions may result in vapor lock.

Van's Aircraft, Inc.
14401 Keil Rd NE
Aurora, Oregon 97002
503.678.6545
 
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