Symptoms of a dying fuel pump?

[Roger Lee]

I have been using ethanol for 9-10 years without any issues and I use it in the CT as the other 7 CT's at my field do.

[pilot1950]

Would vapor or air in the line be the same thing. Maybe I have a bad blend of gas. But seems strange I've never had the problem in my 4 years of ownership.  Is there not a way to stop the vapor from forming.  Does a different sensor work better?

[Roger Lee]

Air or vapor in the pressure sensor line will cause this. It doesn't necessarily mean it's everywhere. Once the engine is shut down the fuel lines have no more cool fuel running through the lines so they just sit there and soak up the heat from the engine. No way to stop it and it really isn't something to loose sleep over. After running the engine while taxing out to the runway it usually goes away. I see it on hose changes from other people where air gets trapped in the line and it can fluctuate with altitude. Another sensor won't help. Relocating the sensor on some planes can help depending on the original location.

 

It really won't be a problem for you. If you didn't have a fuel pressure sensor like many don't you would never know.

[pilot1950]

I will give it a cold startup tomorrow and if pressures are good the vapor issue would seem to be the problem. If so maybe it is the current fuel blend as this was the first time for this to happen.

thanks again

[Roger Lee]

It can still be air or vapor that has worked its way up to the sender.

[Anticept]

I think it's the blend right now. We've been having the same problems with both of our CTs. This was an unusually cold winter, and the refineries might have made the winter blends a little stronger.

[pilot1950]

My sensor on my CTLS in mounted on the firewall about 2-3 inches from the bottom. (aside and below the airbox.) The hose runs up the firewall then up to top of the engine.

[pilot1950]

OK. After shutting down with low pressure on a hot engine and the next day chanking up a cold engine the pressures were fine. Im convinced its the fuel blend. Flew for 30 minutes no problems. Funny, talked to a rotax guy in FL and he has never heard of it happening there.

[Roger Lee]

A low pressure signal doesn't always translate to a low fuel flow to the carbs. One way to test this is put a mechanical gauge in line and run it after it's hot. My guess is your fuel feed is okay because the pump and system are pumping and moving fuel. It has to go somewhere unless your pump has nothing, but vapor and if it did the engine would quit.

[Brucegoose]

My guess is the fuel is vaporising under the closed cowl, this happends at 55c.

 

During summer I open the inspection flap after shut down to release any build up of heat under the closed cowl, and the vaporising problem went away.

 

Try it, it may work for you. I also agree with what Roger says about this.

 

Regards to all

 

Bruce

[FastEddieB]

In the Cirrus community, it is suggested that after a flight, the oil door be opened and the oil cap removed while tying down and securing the aircraft.

 

This allegedly allows the engine compartment to cool down more quickly, and to allow moisture to escape from the crankcase.

 

Can't think of a reason this would not be advantageous in a ROTAX as well.

[Anticept]

It wouldn't be, FastEddieB, because cirrus uses continental engines, which are wet sump. The crankcase breather and oil port are open air between them. Rotax uses a dry sump, which means the oil reservoir is separate from the sump, and therefore no air passage between the crankcase breather and the oil reservoir cap exists. The oil reservoir has it's own breather line as well.

 

I don't really understand the logic here though. Why would opening the oil cap have any significance vs just letting the engine nautrally aspirate through the breather lines? On top of that, do you really want airborn particles working their way into the oil sump?

 

Finally, having an engine cool down SLOWLY is much better than cooling it as quickly as you can. Again, the significance is miniscule, but having a slow uniform cooling through natural conductive and convective means in the cowl would be better than opening doors and caps and letting air blow in.

[Brucegoose]

I did not say the oil cap. I said the inspection "flap" where you check oil level. Where does the engine temp of plus minus 90 deg escape. It is held there under the closed cowl and can only flow out of the gap in the front where the prop is. Open the flap and feel the hot air flow! Heat rises!

I was not talking about Cirrus I'm talking about CT and Rotax. The engine will not cool quickly, it will cool down naturally, conductive and connective, thus SLOWLY! and naturally drawing in air from the bottom and allowing it to flow out the inspection flap/ door.

All fuel lines including the fuel pump under a closed cowl would be subjected to temps above 55deg c, thus vaporisation, especially if the OAT IS SITTING AT 30/35deg c. Apart from a difficult hot start, I'm sure one wouldn't like a coughing motor after take off.

If the hot air is not allowed to escape then the ambient under the cowl will rise and cause vaporisation in the fuel lines.

As I said, try it, it works!

Bruce

[Anticept]

Hi Bruce!

I was referring to FastEddie's post.

Anyways, if you are having a coughing engine, then there might be bigger problems. Carbs use float bowls which act as head pressure and as a way to purge air from fuel. The moment you start the engine, the float level will drop, which will lift the inlet needles off of the seats and open the lines. If there is air in the line, it will immediately float to the top of the float bowl and out of the carb breather line. The needles will not close until the fuel float level rises back up. If the float level drops significantly, then the inlet needle will open significantly, allowing a LOT of fuel in until the float level rises back up.

Unless you have a serious problem, air in the lines are not a problem as the engine fuel pump and head pressure from the wings will push air out of the system before you even get close to the runway. Have a look at page 6-2 of the CTLS MM. You can see they installed a fuel backflow line which leads back to the gascolator. This further helps any vapor pressures by keeping the fuel moving and under pressure. If it's a really really hot day and you are at high altitude, then you will show engine problems and difficulty starting from fuel vaporization before you even take off because it would be vaporizing in the carburetor and discharge nozzle, making fuel delivery difficult.

The design setup in CTs for the fuel PRESSURE system is that they have that single isolated line leading up to the sensor. There's only one entrance, and it also acts as the exit, therefore there's no real easy way for vaporized fuel to be purged. However, the rest of the system is fine. That's why momentary alarms from the fuel pressure system following opening the throttle isn't really an issue, it should stabilize soon. The best way to really fix that problem would be to shorten the sensor line as small as possible.

That's the one thing carbeuration has over fuel injection, is it's very high tolerance to fuel vaporization, as long as there is head pressure to deliver it to the carb. Old cars had a vapor lock issue because they used to be vacuum fed to the fuel pump, which meant it was very easy for fuel to hit the vapor pressure point, and it was all but eliminated when they started putting fuel pumps in the fuel tanks.

EDIT: there are 22ish gasoline refineries in the US. Each state EPA agency sets limits on vapor pressures of gasoline to keep VOx emissions down. If you use a winter gasoline produced for Minnesota in Florida on a hot day, there is a very good chance the gas will boil off once you get to altitude. I mention this because I am not sure that Lockwood is a good resource for inquiring about vapor issues, since it's almost always a warm state.

[Brucegoose]

Never had a cough, but others have when it's hot and high and vaporisation has been the cause! A back track and immediate take off with the lines flowing, everything has run smoothly. All I'm saying is to be aware of this, as it has been experienced often and it's something to be considered.

Regards

Bruce

[S3flyer]

Pretty sure my pump is about dead.  Roger is this what I need: http://www.cps-parts.com/catalog/rtxpages/fuelpumpassy.php?

[Tom Baker]

I think this is what you want. 881-360

[S3flyer]

Looks like this would it then: https://www.cps-parts.com/catalog/rtxpages/cpsfuelpumpkit15-02530.php?

 

Interesting that this part number includes the nipples and clamps and is less expensive.  I think I can spare $100 or, as Eddie says, 1/10 of an Aviation Unit.

[Tom Baker]

This is supposed to be the replacement for the AC pump.

[Roger Lee]

That's the pump, but they have a special for $98 for the replacement. I'm up in the mountains right now so hopefully someone else can give you the promo Rotaxpart number.

[S3flyer]

I'm not going to worry $3.44 (including the lock washers) which is the net difference 

[S3flyer]

Thought I'd update the thread and keep in mind I have a low wing. We put the new fuel pump on and it seemed fine for a few short flights. I also have cycled through the MoGas in the tank and have run 100ll for a bit over a tankful. I still occasional get a slow decrease in fuel pressure on climbs typically starting at 4.5 psi and dropping to as low as 2.4. If I turn on the Aux pump, I'm never below 3.5. Once in level flight the fuel pressure slowly decreased but always stayed in the green and eventually worked its way back to the mid 3s. I'm beginning to think this is just how the new pump works for my installation. From the SB:

 

"Low fuel pressure indications are possible and allowed. But the pressure must stabilize to the operating limit within 5 seconds. If not, the cause should be determined and rectified.

Due to the technical design and installation conditions (construction of the return line, etc.) pres- sure fluctuations, at the fuel pump part no. 893110 are possible. These pressure fluctuations within the specified operating limits are not considered to a problem."

[Anticept]

Thought I'd update the thread and keep in mind I have a low wing. We put the new fuel pump on and it seemed fine for a few short flights. I also have cycled through the MoGas in the tank and have run 100ll for a bit over a tankful. I still occasional get a slow decrease in fuel pressure on climbs typically starting at 4.5 psi and dropping to as low as 2.4. If I turn on the Aux pump, I'm never below 3.5. Once in level flight the fuel pressure slowly decreased but always stayed in the green and eventually worked its way back to the mid 3s. I'm beginning to think this is just how the new pump works for my installation. From the SB:

 

"Low fuel pressure indications are possible and allowed. But the pressure must stabilize to the operating limit within 5 seconds. If not, the cause should be determined and rectified.

Due to the technical design and installation conditions (construction of the return line, etc.) pres- sure fluctuations, at the fuel pump part no. 893110 are possible. These pressure fluctuations within the specified operating limits are not considered to a problem."

 

My god, you have just laid to rest a problem I've been chasing for months now and I've been asking a whole bunch of people everywhere. I think I love you.

 

The relevant SB is here: http://legacy.rotaxowner.com/si_tb_info/serviceb/sb-912-063ul.pdf, and it's paragraph 3.6.1 on the last page.

 

It should also be advised, that some avionics suites such as the Dynon D-120 EMS average out the readings over a few seconds for fuel pressure and fuel flow, to help prevent electrical noise interference caused by the horrible ducati regulator-rectifier. So, if it takes longer than 5 seconds to stabilize, keep in mind it might be the software on the D-120 giving you a slow indication.

[FastEddieB]

I did get the new style pump, and so far no problems.

 

Which figures, since it's still in its box in my luggage compartment!

 

I simply have not been motivated to replace the original pump which is still soldiering on after 7 years and 400 hours.

 

Maybe one day I will, but if it ain't broke...

[S3flyer]

Corey - glad I could help   I've been chasing this behavior off and on for 11 months and am now confident the fuel system is in good shape and fuel pump works fine.  I may extrapolate from Roger's information overload comments and remove fuel pressure from the visible fields but keep the high/low alarms so I'm alerted if there's a real problem.  

 

Eddie - the shop I used for my annual last year has a general position to adhere to applicable Rotax SBs   I was not asked and my prior pump was 6 years old so they just replaced it.  If asked I probably would've agreed, though.