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Old 06-20-2012, 11:20 AM
woods83709 woods83709 is offline
Join Date: May 2008
Location: Boise
Posts: 16
Default Vapor Lock Confirmed and Replicated

I need the forums help with this one. I have recently confirmed that that my RV-9A, O-320 E2A (Carbureted, FP Prop) is suffering from bouts of Vapor Lock. Early in the week after a prolonged taxi and hold I was cleared to depart the main runway. Elevation 2880, air temp 85 degrees F. All checks normal so I departed with the boost pump ON. About 10 feet off the ground the engine coughed and just about died. Luckily with lots of runway I was able to land with no issues. Taxied back to run-up area and engine seemed to be running rough and when power was applied about every 2nd or 3rd time the engine would cough and just about die. Something was wrong so I taxied back to the hanger.

Day 2 84 F I took the cowl off and looked for anything obvious, nothing. With the cowl off I ran the engine, hard, to try and duplicate the problem. No luck. Limited by CHT?s I must have run it for an hour ranging from static max rpm to quick idle to max. Limited by CHT?s I would have to back off after a while to prevent damage.

Day 3 85 F Replaced the cowl and repeated the ground runs. However, this time after the engine was heat soaked from prolonged running it began to cough again. Interesting note that with the boost pump on it seemed to make it worse, not better! From idle to around 2000 rpm it would sputter and want to die almost 100% of the time with the boost pump on, and only 50% of the time with it off. The fuel pressure would hover around 3-4 psi at idle. When the engine is cold it would stay there until full power, however, when hot, once power is applied it would drop to near 0 psi with the boost pump off, around 2 with the boost pump on.

Day 4 83 F Wild Card (Auto Fuel 91) I have been using 91 oct (Ethanol free, check every batch) auto fuel exclusively for the past three years with no issues, even in very hot 90-100 degree days. However after some VAF forum research I thought this might be the culprit. I drained one tank and filled it with 100LL. Went back out and did my ground runs. When operating on autofuel I could get the engine to cough and sputter like the day before. Once purged, the 100LL would not do it! Fuel pressure stayed around 3 psi at all times with no fluctuations. With the boost pump on it would go up to near 5 psi with no fluctuations and/or sputters. This is what I believe to be classic vapor lock. I also think that I probably have some of the last ?winter? blend 91 octane as I fill up a 40 gallon tank, which was done around April. Winter blend has a higher vapor pressure thus more likely to vaporize.
Gascalotor mounted on firewall copilot?s side. Fuel lines fire sleeved, fuel pump NOT shrouded.

Thought about running a blast tube to the gascalator and the fuel pump, however I am not sure there is enough airflow on the ground to make a difference. This is when my vapor lock issues are likely to occur so it probably wouldn?t do much good. Has anyone had this issue and were they able to solve it? I realize that 100LL ?fixed? the problem but I would like a better solution. Anyone have any thoughts?

If you can help it please reserve your autogas vs 100ll arguments. I have access to 91 oct and I have made the decision to use it so I would like helpful suggestions on how to solve this vapor lock issue, outside of exclusive 100ll use.

Brett in Boise
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Old 06-20-2012, 11:55 AM
MarkW's Avatar
MarkW MarkW is offline
Join Date: Sep 2011
Location: Edgewater, FL. X50
Posts: 1,184

It sounds like the problem may be before the electric pump. If the problem gets worse with the pump on it is because the electric pump is pulling a lower pressure. Vapor lock occurs when there is less pressure not on the pressure side.
RV9 - N14MW - Flying
G3X - ECI Titan I0-320
Catto three blade prop
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Old 06-20-2012, 12:13 PM
Sid Lambert Sid Lambert is offline
Join Date: Jul 2005
Location: North Atlanta
Posts: 1,120

Maybe wrap the fuel lines with heat protective sleeve. It's interesting you still get it with the boost pump on. Any chance you have developed a small leak around the boost pump (before the pump) that is making this show up now?
Sid Lambert

RV-7 Sold
RV-4 - Flying - O-320 Fixed Pitch - Red over Yellow

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Old 06-20-2012, 12:37 PM
Bob Martin's Avatar
Bob Martin Bob Martin is offline
Join Date: Jan 2005
Location: Charlottesville, Virginia
Posts: 1,283
Default mixture

Any chance it is running too rich?
You did mention the field elevation was almost 2900msl and temps at 85*.
In my plane, I would be leaning some.
I didn't see any mention of the mixture control in your write up.
Good Luck.
Bob Martin
RV-6, 0-360 Hartzell C/S, Tip up, 1200+TT
James extended cowl/plenum, induction, -8VS and Rudder. TSFlightline hoses. Oregon Aero leather seats.
D100-KMD150-660-TT ADI2- AS air/oil seperator. Vetterman exhaust with turndown tips.
Louisa, Virginia KLKU N94TB
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Old 06-20-2012, 12:59 PM
AZtailwind's Avatar
AZtailwind AZtailwind is offline
Join Date: Nov 2005
Location: Flagstaff, AZ
Posts: 447
Default Been There...

Originally Posted by woods83709 View Post

If you can help it please reserve your autogas vs 100ll arguments. I have access to 91 oct and I have made the decision to use it so I would like helpful suggestions on how to solve this vapor lock issue, outside of exclusive 100ll use.

Brett in Boise
Hi Brett.
I had to drain most of my Auto Gas (91 Oct non ethenal) from one wing on conditions that you describe. I have seen the exact symptoms that you describe. Therefore In summer months (i'm in AZ so we see some heat!) I will run on one tank 100LL for departure and climb and the other tank is 91 Oct auto for cruse. There are a few of us doing exactly this type of operations in the summer. I believe that FankH has the solution where the engine driven fuel pump is removed from the system and all fuel pumps are installed away from heat- in the wings. Please do a search with this in mind and you will find much info on altering the fuel system install for exclusive Autogas use. For myself (and others) it is a wing to wing mix in the summer.

Brad Ransom
RV-6A 180HP-acquired
RV-4 180HP-acquired
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Old 06-20-2012, 01:05 PM
woods83709 woods83709 is offline
Join Date: May 2008
Location: Boise
Posts: 16

I dont think there is a leak, I will double check tonight. I also have been running full rich. I will lean it out and see if I can duplicate tonight. Just about everything that can be insulated is insulated, except the gascalator.

I am considering going the route of a return line from the carb inlet. Andair makes a fuel selector that has a return port built in so what fuel is not used by the engine will be returned. Not sure if this will help as it seems to me this would not actually cycle new fuel but just recirculate hot fuel.

Maybe the return line needs to be vavle operated and ported to the opposite tank, similar to the purge systems recommended by others on this forum.

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Old 06-20-2012, 01:49 PM
Beancounter Beancounter is offline
Join Date: Oct 2007
Location: Canada
Posts: 100
Default Purge line

In one of the recent discussions about fuel purge lines it was suggested to attach the purge line to the carb float drain. This should completely flush the system.
RV9a Slow(ly) built
Flying as of July 2012
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Old 06-20-2012, 03:24 PM
scsmith scsmith is offline
Join Date: Jan 2008
Location: Ashland, OR
Posts: 3,011
Default may be upstream of your boost pump

From your description, this sounds like it may be vapor locking upstream of your boost pump.
Factors to consider:
1) having winter gas may be a significant contributor.
See if you can duplicate with summer gas.
2) was airplane parked in the sun so the tanks would be warmer?
3) is there much height change between the tank and the boost pump? Asking the boost pump to pull fuel uphill will aggravate the situation.

These three factors could easily transition you from a marginal situation to an actual vapor lock situation. When everything is normal, you never know how close to a vapor lock you might be. A few small changes might explain why it is suddenly happening to you now, and not before.

In summer, you could consider keeping 100LL in one tank, as others suggested, or try a blend of 50/50 or so, and see if that solves it. You have a head start on these solutions for the time being, since you already filled a tank with 100LL. So when it is half gone, top it up with your 91 auto gas and see how that works.
Steve Smith
Aeronautical Engineer
RV-8 N825RV
IO-360 A1A
WW 200RV
"The Magic Carpet" Flying since Sept. 2009
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1/4 share in 1959 C-182B (tow plane)
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VAF donation Dec 2021
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Old 06-20-2012, 03:44 PM
rocketbob's Avatar
rocketbob rocketbob is offline
Join Date: Jun 2006
Location: 8I3
Posts: 3,800

I recently had vapor lock running 100LL. The RV6 I am now flying has some goofy fuel line routing which is prone to vapor lock.

The most vapor-lock-proof fuel system is going to be one that has a vapor return line, is all-electric, with minimal fuel line routing firewall forward.

That said a return line via an orfice fitting (around .040" or so) at the carb will go a long way in preventing vapor lock since it is constantly flushing hot fuel/vapor with cool fuel. A shroud and blast tube on the mechanical fuel pump will also help quite a bit.

Please don't PM me! Email only!

Bob Japundza CFII A&PIA
N9187P PA-24-260B Comanche, flying
N678X F1 Rocket, under const.
N244BJ RV-6 "victim of SNF tornado" 1200+ hrs, rebuilding
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Not a thing I own is stock.

Last edited by rocketbob : 06-20-2012 at 03:49 PM.
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Old 06-20-2012, 04:02 PM
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nomocom nomocom is offline
Join Date: Jun 2009
Location: Caldwell ID
Posts: 268

When you ran 100LL the bad behavior went away. Would fresh mogas do the same as the suspected winter blend?

Some of the Peterson mogas STC documents warn against the winter fuel in warm conditions- so what you are seeing isn't really a surprise. The easiest solution might be to use up the winter gas in your car, and resupply the aviation fuel tank with the summer blend.

I've got a Hodges Tester over here in Caldwell. You are welcome to try the different fuels and see if the suspected winter fuel does vaporize easier. The Hodges kit lets you reduce pressure on the fuel to the point it begins to vaporize. When it boils, you read the pressure on the gauge. The winter blends begin to vaporize at a higher absolute pressure.
1990 RV-3 (now apart, upgrades in the works)
1959 C172 O-360
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