Jump to content

Low fuel pressure at times?


Roger Lee

Recommended Posts

Hi Guys,

 

Well this is more for the CT owners with a Dynon D-120. If you have analog gauges then this won't apply.

Have you ever noticed low fuel pressure at times like right after you have flown and then go back a short time later and maybe your low fuel pressure alarm sounds or the fuel pressure is just reading low or fluctuating? Well this is from the heat under the cowl vaporizing the the fuel up in the fuel lines, but mainly in the cross over tube between carbs. This is where the fuel sender is mounted or it comes off at this point down to the firewall if it has been relocated. Then after a little run time the fuel pressure stabilizes because cooler fuel has now recirculated and the vapor is gone and the lines are all fuel again. You will notice that even when the pressure is low here the engine runs just fine. Rotax has a recirculating fuel line to help prevent vapor lock..

 

While I'm on the fuel pressure sender's relocated area. I would like to remind owners that if you use 91 octane and the sender has been relocated down by the battery then at your annual the sender should come off, just takes a few minutes, and allow some fresh fuel to drain down the fuel line. Early on I found my fuel pressure down to 1.2 psi. The engine never missed a beat. The fuel sender was clogged due to old fuel sitting in that fuel line for a year and never getting any fresh fuel. It hangs down and the 91 oct. just sits there and cooks. When I pulled the sender you could smell the old turpentine smelling fuel and it was thicker and oily feeling. Avgas doesn't do this. This is no big deal to worry about just have your mechanic pull the sender and drain some fresh fuel by opening the fuel valve. If the sender gets clogged it can be flushed out in a minute or two.

 

Hope this helps

Link to comment
Share on other sites

Yesterday after completing a 2.5 hour x-country flight, as I was descending my fuel pressure alarm went off - pressure was fluctuating 2.0-2.5. Rotax continued to run fine. After landing and taxiing back to my hanger, pressure was back to normal. This happened once before about 30 hours ago, similar situation on descent. After the first time, I flushed all of the fuel lines, checked the gascolator, pressure sensor - did not find anything. Could this be considered 'normal'? Alarms going off on downwind are rather annoying !!

Link to comment
Share on other sites

Hi Marty,

 

Is your fuel pressure sender remote mounted on the firewall just under the battery? Do you use 91 Octane? If the answer is yes then you have may have this problem. If the fuel pressure sender sits in this position for a long time (a year) with 91 octane then the fuel in the hose that goes down to the sender never gets refreshed or used. It sits and cooks and becomes very stale. Just unscrew the sender and put a little container under the hose and open the fuel valve and allow some fresh fuel, maybe a 8-10 oz. run out. The take a can of carb cleaner with the long red spray tip on it. Hold it up against the tiny little hole in the sender a give it a quick squirt into the hole and then let the sender pee the cleaner back out the sender hole. I mean only a quick squirt like a 1/2 second or so. You aren't trying to trash the sender only give it a quick cleaning flush. I do this 4-5 times and it is as good as new. I include this on my annuals for owners. I have seen this on several planes. It does not happen with Avgas use.

The other possibility is the sender may be starting to go bad. Just keep an eye on it.

 

p.s.

When you squirt the cleaner in the sender hole keep your face away in case it squirts out to the side. Don't ask me how I know.<_<

 

If a plane is flown and then sets for a short time then the heat builds up under the cowl. When you go back to start it you may get a low fuel pressure alarm intermittently for as long as 20 minutes. This is just vapor in the crossover tube up by the sender and until some cooler fuel cools the area there it may have a little vapor. Once the cooler fuel cools the area with fuel usage and the recirculation line the fuel pressure returns to normal. I just had this happen with a client that had just run his engine and it was in the hot afternoon. After he flew a ways the fuel pressure returned to normal.

Link to comment
Share on other sites

Roger - thanks for the reply.

 

I flew again yesterday - everything normal - will check / clean the sender and purge the fuel line to it, just to be sure nothing in there.

 

This is actually an RV-12 that has been flying about 6 months - 60 hours, so should not be a stale fuel issue - will keep you posted if I find anything.

Link to comment
Share on other sites

  • 1 year later...

Roger,

Still being a fairly new CT owner I wonder if you could shed some light on high temperatures and 91 Oct. Living in Northern Arizona the temps here are cooler than down in the valley and in Tucson. So far I have had no problems with 91 Octane. I wonder if you have seen any problem with 91 Octane in the 110 degrees on the ground in your area. I am flying down there next Monday the 25th and probably will experience the 110s or so. And I am running 91 Octane although it is ethanol free. Reading that the Rotax does circulate fuel I am getting the opinion that vapor lock is not a problem. I hope you can shed some light on this issue.

Thanks,

Larry Minch.... N255CT Flying Bozo

Link to comment
Share on other sites

Hi Larry,

 

91 oct. is absolutely no problem in the winter or the summer. Fly at any altitude and enjoy the flight. Don't let anyone put the 91 oct with ethanol fear in you. I will say though that if you shut down for 30 min. and start up again the fuel pressure will be low because the lines in the engine compartment get hot. By the time you taxi and fly they should be back to normal because cooler fuel is flowing and recirculating, but once in a while they are still low, but I have never seen any issue with that for a flight. The more throttle the more cooler fuel and the more cooler air.

 

I doubt it will be a 110F here. It has been running in the high 90's and just topping 100F. My advise is to fly as early in the morning as possible. After 10:30 things start to get fairly bumpy. Then it's time to head for the clouds for the air conditioning and smoother air.

Link to comment
Share on other sites

[...] at your annual the sender should come off, just takes a few minutes, and allow some fresh fuel to drain down the fuel line. [...]

 

also it's happened to me 3-4 times, especially in summer, warm start-up, on taxi - low fuel pressure [0.10-0.15psi], but normal engine running.

I'll try to "clean&fresh" as you propose.

do I need any fuel sealant when the sender is mounted back?

 

Roger, thanks for the tip :)

 

Alec

Link to comment
Share on other sites

OK thanks Roger, I can't get back in the air out of Marana til early to mid afternoon so have to put up with the bumps unless it gets too crazy and then I will just have to drive. I hate the drive since it is about 220 each way and almost 4 hours as opposed to about 1:25 in the CT. That doesn't even take into consideration the rush hour driving through Phoenix which is sometimes crazy too.

So thanks for the scoop on the temps and the 91 Octane.

Larry Minch

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

×
×
  • Create New...