In Flight Fuel Pump Failure

Geico266 · #1 05-26-2008, 07:44 AM

Normal / slightly aggressive climb out from 1,500 msl, 85F day to 6,500 msl. Engine temps normal, engine starts to sputter and I notice fuel pressure is 4 PSI.

I flip on the boost pump and life is good with 27 PSI.

After landing without incident I let the plane cool down and fired it up again and the fuel pump seemed to work fine with 27 PSI.

Standard Vans set up with IO-540-D with mechanical pump fuel pump & electric boost pump back up. However, there is no blast tube to the fuel pump, and only one set of louvers on the cowel.

My plan of action is;

1. Replace the fuel pump.
2. Install a blast tube to the fuel pump.
3. Install a second set of louvers in the bottom of the cowel.

Any thoughts or other ideas?

Thanks in advance for your input, I always greatly appreciate it.

frankh · #2 05-26-2008, 07:52 AM

Either pump is somehow not keeping up with demand (blockage, hole in diaphram etc)...Or its vapour lock

Frank

Geico266 · #3 05-26-2008, 08:07 AM

Quote:

Originally Posted by frankh

Either pump is somehow not keeping up with demand (blockage, hole in diaphram etc)...Or its vapour lock

Frank

Vapour lock in the air? I was burning 12-14 GPH. Would not the cool fuel keep it from doing that? Wouldn't vapour lock stop the flow all togeather? It still had 4-8 PSI. This was the 1st run of the day also.

Frank, I'm not arguing with you, I'm just trying to figure it out. I think you are right, but everyone I've talked to locally doesn't think it was vapour lock.

flyeyes · #4 05-26-2008, 08:17 AM

I agree with frankh--this is classic vapor lock.

I doubt there is anything wrong with your fuel pump.

Most of my flying is behind six-cylinder TCM engines in store-bought airplanes, but it's not uncommon for the POH to recommend boost pump continuously in the climb, and continuously in high-temp cruise or high-altitude cruise.

My RV didn't run well at all in hot temps until I added a blast tube.

There is a cooling shroud available here for Lycoming fuel pumps under "products."

As a temporary fix, take the fresh air supply off of your heat muff and direct it towards the fuel pump,

airguy · #5 05-26-2008, 08:31 AM

The vapor lock gremlin strikes you on the suction side of the pump - high flow rates aggravate this. More nose-up attitude aggravates it as well, since the pump has to pull the fuel a little more up-hill to the suction side of the pump, further lowering the suction side pressure. The air temp may have been 85, but what was the fuel temp? Were the tanks baking in the sun before takeoff? Higher fuel temps give higher vapor pressure, leading to easier vapor lock.

I don't believe I'd pull the pump if I were you - I think I might install a suction-side pressure gauge and go out and try to duplicate those flight conditions and read that suction-side pressure - I think that will tell your story.

A blast tube onto the fuel pump is never a bad idea.

mburch · #6 05-26-2008, 08:32 AM

I flew an RV-6A that presented the same symptoms once on a hot day. The owner told me that it stopped happening once he put a blast tube on the fuel pump. That's pretty anecdotal evidence, but it might be worth something.

Since I have my fuel pump off the engine right now anyway, I think I'll take the opportunity to install one of those cooling shrouds while I have easier access. Thanks for the timely reminder and good luck finding the source of the problem.

mcb

asav8tor · #7 05-26-2008, 08:36 AM

Sometimes (depending on installation plumbing to sender) a tell-tale indication of bubbles in the line to the engine driven pump is fluctuating low pressure on the fuel pressure gauge, not just low pressure.

It's also possible to have a fuel line leak that lets air into the fuel line prior to the engine driven pump. The leak may or may not present itself when positive pressure is applied via the electric pump operation.

Was the acft sitting in the sun all day heating the fuel in the tanks? What fuel, 100LL, Mogas, Auto fuel with ethanol?

How old and how many hours on the engine driven pump?

I agree, first thought is classic vapor lock..... but many times first thoughts are wrong.

David-aviator · #8 05-26-2008, 10:09 AM

The symptoms support a vapor lock at or near the inlet to the engine driven pump. That pump will not suck vapor. You turned on the electric pump and it pushed the fuel through and the engine came back to life. Same thing would happen if the mechanical pump failed but it worked OK later on the ground. Fuel pumps don't fix themselves.

OAT and fuel temp were on the warm side, but the real culprit is heat at the firewall where the unpressurized fuel comes through. It is a hot area (you could install a simple OAT sensor in the area to see how hot it is) and it is a low fuel pressure area because the mechanical pump is sucking fuel from the tank at a good rate.

The six banger is a heat generator, more HP means more heat, I will bet more so than the six banger Subaru. I had a temp gage in the area and it convinced me to move all the fuel pumps aft of the firewall. Fuel pressure going forward at 30 psi will not lock up, not even mogas. But it will at zero pressure when it gets warmed up forward of the firewall on warm days. Your exhaust pipes are probably running at 400-500F (considering 1100-1200 EGT) not far from the fuel pump, that is a warm area for sure.

Just my 2 cents worth. I would go with the pump cooling shroud and keep my finger near the electric pump switch on warm days.

frankh · #9 05-26-2008, 11:27 AM

Quote:

Originally Posted by Geico266

Vapour lock in the air? I was burning 12-14 GPH. Would not the cool fuel keep it from doing that? Wouldn't vapour lock stop the flow all togeather? It still had 4-8 PSI. This was the 1st run of the day also.

Frank, I'm not arguing with you, I'm just trying to figure it out. I think you are right, but everyone I've talked to locally doesn't think it was vapour lock.

Hey Geico,

You can argue with me all you want...Its also called healthy debate and I have no problem in being wrong and am very happy add to the mix of ideas...A good brainstorming never hurt anyone...

Let us know what you find.

Chers

Frank

Geico266 · #10 05-26-2008, 05:34 PM

Great info guys thanks.

I have been running 91 Octane mogas & 100LL mix, it should be around 50-50. The pump has 200 hours on it and is 2 years old. The problem showed up after I leveled off and was deceasing airspeed to do some stalls & slow flight so I'm leaning towards vapour lock also.

Someone asked what the temp of the fuel was. It was 82F. I actually took the temp of it after I landed. The plane was in a hanger all day, only in the sun during start up and flight.

If I wanted to test things one at a time I would, but I just don't want to take a chance so I'm replacing the fuel pump, installing a fuel pump shroud (a hanger buddy gave me one) & blast tubes and install additional louvers. Over kill maybe, but..............