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Fuel Pump Issue

rleffler

Well Known Member
As several already are aware, I had some fuel flow issues on my first flight. At the time, in the air, I wasn?t quite sure what the issue may have been. It was initially looking like a blocked fuel line. The symptom was that the fuel pressure dropped significantly, but came back up with the electric fuel pump turned on and switching the tanks. I promptly landed and started looking for the root cause.

1. The left and right tank fuel flow (measured at the firewall, forward of the filter and pump) was 58 gallons/hour. Rules out a blockage aft of the firewall.
2. The fuel vents are not obstructed

I ran the engine in a static test. Was getting about 20psi on the engine fuel pump and about 26psi with the electric fuel pump enabled. Since this is all within specifications, this is the worst of all the scenarios. No root cause and everything seems to be working as designed.

This morning I flew for about an hour on flight #2.

When I turned off the electric fuel pump, fuel pressure started to immediately drop. I let it drop to about 10psi before I re-engaged the electric fuel pump. With the electric fuel pump running, I was getting 26psi. I repeated this scenario multiple times on both fuel tanks during the flight. Results were pretty consistent.

It?s looking like I may have a bad fuel pump on this 1 SNEW YIO-540-D4A5. I plan on giving Lycoming a call in the morning.

My questions is there any other scenarios that I?m overlooking that may yield similar symptoms?

Thanks,

bob

 
Sounds like the engine driven pump or an obstruction between it and the firewall however I assume when you checked fuel flow you disconnected at the engine driven fuel pump. if so I would start with changing the engine driven pump.

My fuel pressure readings at cruise (electric pump off) stay around 23 psi. I also assume you made full power on take off and your temps are ok. If there was a flow restriction on take off you would also see it quickly in the CHT temps. If the only time the pressure falls is when you turn off the electric pump then I would again bet on the mechanical engine driven pump.

Hope you all goes well.

Pat
 
Sounds like the engine driven pump or an obstruction between it and the firewall however I assume when you checked fuel flow you disconnected at the engine driven fuel pump. if so I would start with changing the engine driven pump.

I disconnected the fuel line from the fuel pump to conduct the fuel flow tests, which is why I'm not suspecting anything aft of that connection.

My fuel pressure readings at cruise (electric pump off) stay around 23 psi. I also assume you made full power on take off and your temps are ok. If there was a flow restriction on take off you would also see it quickly in the CHT temps. If the only time the pressure falls is when you turn off the electric pump then I would again bet on the mechanical engine driven pump.

Yes, that was the excitement I had on the first flight. On the second flight, since I was over the anxiety of the first flight, I spent more time experimenting to identify the root cause. The CHTs settled down pretty quickly.

bob
 
After shipping the data logs from my two flights and ground testing to Lycoming, they are recommending replacing my fuel pump. Naturally, I am just outside the warranty period.

They recommended a part #62B26931, which I can find that it is an approved substitute for the installed part, which is a LW15473. Anyone know what the difference may be?

Bob
 
My buddy had a similar fuel starvation issue on his IO-360 Cozy MKIV after the first condition inspection. Turned out that the AFP fuel filter screen insert we replaced with a new one from Aircraft Spruce had such a fine micron rating, the mechanical pump could not pull enough fuel through it during a full power climb out. It would drop below 9 psi and the engine would sputter until the boost pump was turned on. Superior even went so far as to send him a new mechanical pump under warranty. When that did not help, we pulled the old filter out of the trash and compared it to the replacement. We never noticed the difference in screening as the old filter was pulled out and tossed one day then replaced a couple days later. Cleaned the old one up and used it with no more fuel pressure problems. The electric boost pump had no problems pulling the fuel through the filter but the mechanical pump on its own could not do it on a high OAO full power climb out with the fine mesh filter. I hope the new pump cures your problems.
 
So, your buddy has an extra one that he can send to Bob! I like that customer service from Superior.

Lycoming, on the other hand... Man, bob! that's a bummer. Plus, it's a pain to change on a mounted engine. Sorry you gotta put up with that after just getting flying. Hope you can get it going again quickly.
 
Bob,

First, congrats on flight testing. 2nd, could you share some of the pressure vs time data?

Just because the high pressure electric pump can provide the fuel, (you have 2 sources of data, 58 GPH, and consistent 26 psi) does not mean there is not a suction leak or high restriction resulting in cavitation somewhere between the mechanical pump and the tank. It seems too obvious just to blame the mech pump. Where do you have filters and screens? I think those would be the fist place to start diagnostics. A new system is the most contaminated it will ever be.

Back to data, you said the pressure decreased not dropped when switching the electric pump off. It takes a little time to build up vapor in the line if either a suction leak or just high suction and vaporization.

Just wondering - -did you do any kind of fuel system, from tank to firewall pressure test to check for leaks? I was thinking about an air pressure test with a leak down type of test for small leaks. Maybe suction?
 
I'm going to make a call to Lycoming again in the morning to debate the warranty status.

I've found that Tempest has a equivalent model for about $100 less. Does anyone have any experience with them or can comment on their quality?

Thanks,

Bob
 
Bob,
I know the quick solution is to replace the pump, but I too am thinking you have a restriction or leak in the system between the mechanical pump and the fuel selector. I know you replaced the aluminum lines with flex line. I would pull all the lines and verify that there are no restrictions or flaps inside the hose. Prior to that I would pressurize the entire system from the fuel pump inlet line to the tank, and do a leak down test. Just thinking out load......
 
Level flight?

My fuel pressures drop in climb with just the mechanical pump (same during my transition training in the company -10), so I leave the boost pump on through most of the climb. You didn't really say, and I assume the pressure drop was in level flight, but if it was in a climb, it's pretty normal.
 
My fuel pressures drop in climb with just the mechanical pump (same during my transition training in the company -10), so I leave the boost pump on through most of the climb. You didn't really say, and I assume the pressure drop was in level flight, but if it was in a climb, it's pretty normal.

Yae, same thoughts from me.

Were your fuel tanks full as well? When I fill my tanks to the top and climb out for an extended period of time I will get a fuel pressure drop that will trigger an alarm. The second I level off and burn off some fuel it goes away. My hypothesis is that the fuel breather gets fuel in it and then the mechanical pump has to pump the the fuel from the tanks with a "plugged" breather hose that is filled with fuel. It won't do it if the tanks are not full.
 
Hmm

Yae, same thoughts from me.

Were your fuel tanks full as well? When I fill my tanks to the top and climb out for an extended period of time I will get a fuel pressure drop that will trigger an alarm. The second I level off and burn off some fuel it goes away. My hypothesis is that the fuel breather gets fuel in it and then the mechanical pump has to pump the the fuel from the tanks with a "plugged" breather hose that is filled with fuel. It won't do it if the tanks are not full.

Hmm, I hadn't made that correlation. I just assumed that the nose up attitude created a greater head between the fuel pickup line and the engine, so a boost is necessary. I'll check it next time with full and not full tanks.
 
Ours runs 28-29 psi in cruise and 19-20 psi during full fuel climb without boost pump. If it went below 15 psi or ran rough/lean then I would turn it on.
 
Yae, same thoughts from me.

Were your fuel tanks full as well? When I fill my tanks to the top and climb out for an extended period of time I will get a fuel pressure drop that will trigger an alarm. The second I level off and burn off some fuel it goes away. My hypothesis is that the fuel breather gets fuel in it and then the mechanical pump has to pump the the fuel from the tanks with a "plugged" breather hose that is filled with fuel. It won't do it if the tanks are not full.

No, the tanks were about 2/3 full and this happened in both climb and level flight.
 
purge valve bypass

Bob.

Just thought...but if you had purge valve bypassing slightly... when you turn on electric pump on is it possible it increased flow to raise pressure even with purge valve bypassing?
Again just thought I had.
 
Bob, I looked at the data (nice data service, BTW) and it did not appear that the low pressure occurred on the last ground run where you were testing various combinations. It looked like it was never lower than 23 when the boost pump was off and the rpm was even up to near 2700. If this is correct, then it indicates (to me) that the mechanical pump is functioning properly even at high fuel flows. And, that the issue of dropping pressure is due to higher suction from angle of attack and lower absolute ambient pressure from altitude resulting in fuel vaporizing due to the low pressure to the point that the mech pump can not keep up.

So, unless the "community" as some absolute suction pressure target for a clean system to check against. The only option seems to be to clean and check screens and filters, and verify there is no tiny leak that is allowing air into the system. It is still possible a tiny leak could be there but has not reduced the pumping effectiveness enough on the ground to cause pressure drop.

Clearly this situation needs to be addressed as it will only occur more easily during hot fuel conditions. I will add more if my brain lets me.

Curiosity, where did you place your red cube in the system?
 
just a thought--

years ago in automotive, we would get low, or very little fuel pressure from some 350 chevrolet engines that had a return line at the pump. Blocking off the return, solved all of those issues.
Granted, we are dealing with a different situation, it's similar to what Bob is experiencing. I know that he has inspected all the hoses from the firewall aft to the tanks, and found no issues. I know that he has looked at every component, its location, and how it all interacts with each other. This one is puzzling.
 
I just received this from Lycoming. At the moment, they are following the legal technicality of the warranty term. Which means that the fuel pump, while it may be DOA, the repair is on me. I'm still attempting to convince them that it should be a warranty repair.

---------
Bob,

Thank you for the information. It appears that the engine driven fuel pump needs replaced.

The part number of the fuel pump is 62B26931. I downloaded this issue in our system, the reference number is 2567. Please order the part through an authorized Lycoming Distributor. http://www.lycoming.com/Lycoming/PRODUCTS/DistributorLocator.aspx

Thank you,
 
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Same here

Your numbers look just like mine!

After all this it's probably too late to convince you that nothing is wrong with your fuel pump.
 
We'll see what the real root cause is over the next few days. I'll be checking the filter screens as well as replacing the fuel pump.

Lycoming has authorized a warranty repair.

bob
 
We suffered from fluctuating fuel pressure on our RV 10. After replacing the engine driven pump twice, I installed cooling shroud and that was the last of the problems. Rock steady the past 300 hours.
 
All is finally well!

I ended up replacing the fuel pump and added a cooling shroud and blast tube to keep the pump cool. The pump never dropped bellow 22psi during the hour long test flight. Now I can start resuming normal Phase I flight testing.

Thanks to Steve Beaver for assisting with the pump installation

I also inspected the fuel filter and it was perfectly clean.
 
Bob,
Congrats on finding the problem! Did you take any photos of the shroud installation? It sounds like a good idea
 
Bob,
Congrats on finding the problem! Did you take any photos of the shroud installation? It sounds like a good idea

There is one on my build log before I did the safety wiring.

I had to enlarge several of the openings to fit the lw-15473 pump. Apparently one size doesn't fit all.
 
In deed good news.

I had to replace my FP and add a blast tube also. Since I did both at the same time I'll never know which fix really worked. My guess is the blast tube.

I did not add the best shield, just a blast tube on the suction side of the engine mounted FP.
 
Thanks for the update, and congratulations on your diagnostics and success in getting the problem solved.

I am now convinced that we, the AB community, need an engine installation guide book with some definitive specifications to allow every builder to test an installation for acceptable operation. This can provide data to understand if it is an assembly or design problem. We, are a huge group of people and have the resources to gather data one piece at a time. This can help us to generate an engine installation audit of temperatures and pressures that will ensure proper operation over a wide range of operating conditions. Like an EAA "standard" . Maybe these already exist, if so someone please let me know what and where they can be accessed. We should be able to completely eliminate installation short comings for all builds regardless of their technical knowledge.
 
In deed good news.

I had to replace my FP and add a blast tube also. Since I did both at the same time I'll never know which fix really worked. My guess is the blast tube.

I did not add the best shield, just a blast tube on the suction side of the engine mounted FP.

There are thousands of RV's (and way more certified aircraft) flying without blast tubes or cooling shrouds on the fuel pump without a problem so I have my doubts about this being a "fix" for a fuel pressure problem :confused:
 
IIRC, somewhere in the FWF plans from Vans they recommended a shroud for the engine driven fuel pump on the RV-10 due to heat so I went ahead and put one on mine before hanging the engine. Since I plan on running mogas in one of my tanks I wanted to keep the fuel delivery as cool as possible anyway.
 
So am I reading this that there was a vapor lock problem which is resolved by a cooling shroud on the fuel pump?

If it is that would be somewhat of a common problem with tightly cowled airplanes.
 
There are thousands of RV's (and way more certified aircraft) flying without blast tubes or cooling shrouds on the fuel pump without a problem so I have my doubts about this being a "fix" for a fuel pressure problem :confused:

Me too.

Has anyone actually measured the temperature of a fuel pump (or to be more precise, the fuel inlet and outlet temperatures) before and after a blast tube and shroud installation?

Personally I suspect a reflective heat shield below the pump would do more. You have very hot exhaust pipes radiating just inches away from the pump chamber.
 
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Thanks for the update, and congratulations on your diagnostics and success in getting the problem solved.

I am now convinced that we, the AB community, need an engine installation guide book with some definitive specifications to allow every builder to test an installation for acceptable operation. This can provide data to understand if it is an assembly or design problem. We, are a huge group of people and have the resources to gather data one piece at a time. This can help us to generate an engine installation audit of temperatures and pressures that will ensure proper operation over a wide range of operating conditions. Like an EAA "standard" . Maybe these already exist, if so someone please let me know what and where they can be accessed. We should be able to completely eliminate installation short comings for all builds regardless of their technical knowledge.


I concur. This is an area that I'm still very much in the learning mode.

I was expecting Lycoming to ask me to perform additional testing before authorizing the warranty repair as oppose to let's start replacing parts until fixed approach. I have yet to find a more scientific approach troubleshooting guidance to use in my situation. It's much needed for many of us that are more novices in regards to the engine and fuel operations.
 
[/quote]Has anyone actually measured the temperature of a fuel pump (or to be more precise, the fuel inlet and outlet temperatures) before and after a blast tube and shroud installation?[/quote]

Dan,
I thought I read a thread a while ago on this, where some one did measure with not-so-good results
 
So am I reading this that there was a vapor lock problem which is resolved by a cooling shroud on the fuel pump?

If it is that would be somewhat of a common problem with tightly cowled airplanes.


That was one perceived scenario that was neither proven or disproven. There were a few RV-10s have antedotal information that the builder's believe that the cooling shroud fixed their issues.

Per Dan's concern, I was looking for a more scientific approach to determine the root cause, but two issues prevented me from accomplishing my goals. The first was getting the aircraft back to flying status asap. I didn't want it to be AOG for weeks while I conducted testing.

The other reason, was the lack of a process to formally test. As mentioned previously, I asked Lycoming, but didn't get much assistance. I even solicited assistance from several local A&Ps. They all seemed stumped.

Thanks to Don Rivera and Allen Barrett, they did give me some items to look for which did rule out other issues, but not vapor lock. Additionally, most of us don't have the instrumentation to effectively measure it on hand.

I agree with Bill, this is an area if there are knowledgable subject experts, there is area in there are plenty of educational opportunities to improve the knowledge base.

bob
 
Bob,
Would you be willing to test fly with your new blast air tube temporarily blocked off? That might suggest if the issue was due to temperature, or the pump itself.
 
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Bob,
Would you be willing to test fly with your new blast air tube temporarily blocked off? That might suggest if the issue was due to temperature, or the pump itself.

At some point in time, probably, but not in the near time frame. I need to get through Phase I before the weather starts to get nasty. I still have plenty of items on my punch list to complete. But I would be interested in knowing as well.
 
My Skyview system EMS has two points that can be used as thermocouples. I got two plate type thermocouples on 6' of wire. One goes to the engine compartment and the other in the cabin. First flight one will be on the fuel pump, the other in the tunnel. They can be put anywhere the cables reach and info is desired.

Unfortunately it will be next spring (?) when the test happens.

For those who want to try it, they are type J leaf type thermocouples with fiberglass insulation from TC Direct (tcdirect.com) and cost about $21 each. They are not stocked and took about a month to get. Accurate to 1degree from minus something to about 800 degrees F.
 
Fuel pressure issue in climb only.

I have been reading this thread with interest since I have recently been experiencing some fuel pressure issues.
A trip in early Dec. from Florida to Puerto Rico was cut short due to fuel pressure drop during the climb out of Exuma in the Bahamas. I had had this problem just a few times in the past but this time it happen numerous times during the climb as soon as I turned off the boost pump.

The fuel flow was not interrupted nor did the engine stumble during any of my reduced psi events. I have had my warning settings set to Yellow@20psi and Red@18psi. Yellow gives me a audible warning "Check Fuel" and at 18psi the VP-200 automatically turns on the boost pump and the VP-200 display annunciator lights up with the word "Turbo Boost"

Since the FP was stable in cruise flight we returned to Florida and immediately installed a new fuel pressure sender, that didn't help. A new fuel pump was then installed which didn't cure the problem either although somewhat improved.

I my research I found that the normal psi range of a mechanical fuel pump is 18-28psi. Since then I have lowered my warning settings by 2psi from 20-18 to 18-16, since having done that I have only had a warning a couple of times when the fuel pressure momentarily dropped into this caution range but not low enough to turn the pump on. Obviously since this has been happening I have been keeping a keen eye on the fuel pressure gauge. Since lowering the threshold of my warning settings the boost pump has not been triggered so this has given me a chance to observe the fluctuations as they happen.

It appears that these events in fuel pressure fluctuations are momentary in nature and isolated to the climb phase and only after the boost pump is turned off. I have observed the fuel pressure drop just below 18psi, which will trigger a warning, but almost immediately return to normal of 23/24psi. After cruise flight has been reached and cruise power settings set I have never had a problem.

It bothers me that this is happening after 550+ hours of operation without any trouble, Why now? Scott Schmidt made an interesting comment on the VAF Forum. His hypothesis is that the fuel breather gets fuel in it and then the mechanical pump has to pump the fuel from the tanks with a "plugged" breather hose that is filled with fuel., sounds like a crazy idea but who knows.

I am now of the opinion that it was not the mechanical fuel pump, at least not in my case. I have just added a cooling shroud and blast tube so we will see how that works out although I am not convince that it will solve this mystery. I think the problem is more likely to be found in a vapour lock issue or a small leak somewhere between the mech. pump and the tank where small amounts of air is sucked into the fuel line and causing the pump to momentarily cavitate. Possibly even an obstruction somewhere but that is unlikely since the screen in the Andair gascolator has always been clean as a whistle.

I understand that other RV-10 drivers are, or have had, similar fuel pressure issues, if you have come up with a fix for it I would surely like to know what that is. Thx
 
Temperature measurements in cowl

A British poster did some rather exhaustive measurements while working on a Rotec Throttle Body installation. My (challenged) memory indicates that he found temps between 35 and 40 Cent. in there before he started adding blast tubes and cowls around the TBI as well as the fuel pump.

I have run Lycomings with and without pump shrouds, no issues for me, but different engine installations than you are working with
 
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