Fuel Pressure drop/Engine stumble in flight

[jrwilson]

So, I've asked on the forum before, had a problem with the fuel pressure dropping during climb (M20C).  When I would level out, the fuel pressure would come back up.  The old mechanic went through the fuel lines, looking for leaks, replaced the engine driven and boost pump.  Problem still persisted.  The mechanic said some mumbo jumbo about fuel getting hot, vaporizing under pressure during climb....didn't make sense.  Asked a Mooney service center and they said since the engine isn't running rough, its probably the gauge (it is old).

Well today, was flying to that Mooney service center for an oil change (the first time this place has seen this plane) and during climb, at about 3000', the fuel pressure dropped (normal), down to red line (not as normal), then the engine stumbled.  I leveled off, and the pressure came right back up.  Had plenty of runways around, so climbed again and it stumbled again with a pressure drop.  Leveled off and it came right back up.  Switched tanks, pressure held, landed because that was enough for me. 

The new shop, which seems really good so far, thought immediatly something was plugging the fuel sender, but it was clear.  They did a pressure test on the boost pump and it isnt' holding pressure, so maybe a check valve.  What ever it is, it is annoying. 

Assuming it is the boost pump, which is new, why would this happen on a new pump?  The same thing the old pump was doing.  Could something be screwing up the check valve?

[Skybrd]

I had similar experiences in my Experimental Tailwind and it was from a dirty fuel filter. I also changed my fuel pump and checked the fuel vent line but it was the filter. Have you checked the fuel vent system and filter ? 

[Piloto]

Check the fuel tank vents. You should be able to blow air by mouth into the vent freely.

José

 

[Shadrach]

If your boost pump is not holding pressure, it can create "slack" in between the engine driven and electric pumps. If it is hot enough, the fuel will bubble (read boil) during the pressure drop, causing your engine to burp. Do you climb with the aux pump on?

If the unit is a Dukes pump and you're thinking of sending it out, I must recommend Russell Romney/D&G Supply. I had my pump IRANd by D&G last year and they were fast, professional and communicative with competitive pricing.

http://www.dgsupply.com/

[drapo]

I would try to climb with the Electric fuel pump ON then OFF (around the airport that is) to see if I could pinpoint the problem to the boost pump. Then, I would try to climb with the fuel selector on LEFT then RIGHT. With those results, I would talk to a few MX I know to troubleshoot the problem.

I had this happened to me with my Cherokee, ended up being a fuel vent that was clogged only in the climb attitude.

[carusoam]

Some things to consider...

 

When pressurized with the aux fuel pump, my M20C would hold it's pressure for a long time.  ( no pressure loss while waiting for a few minutes.)

 

It seems that if the system is not holding pressure, there is a lot of cleaning and checking that can be done prior to committing to flight again.

 

Your A & P can probably help you disconnect the fuel system from the carburetor and test the fuel flow and vent system all the way back to both tanks.  Full flow from both sides should be expected without any air bubbles.

 

While at the carb,  have the A&P check the fuel nozzles for cleanliness.  It is possible to get foreign objects that far into the jet.

 

While this is on the menu, consider pulling out the sump valves and flushing the tanks.  The pick-up filters should be considered for inspection and replacement at this time.  While you are at it, check the health of your gascolater.  It would not be completely unusual that an old gascolator with improper seals is allowing air to be sucked into the system.  If the gascolator does not seal after draining, this could be a problem.  Do you have rusty sediment in the gascolator?

 

As Jose' pointed out, check your vents.  Blocked vents have a way of tearing up good pumps.  I recently found a blocked vent using  Jose's guidance.

 

Take a look at the fuel system drains.  One is for the gascolator, the other is in case the pump has a leak.  Neither of these should be wet or stained.  Unless the are used intentionally....

 

Always, remember working with fuel is incredibly dangerous.  Be extremely cautious it is easy to spill, spray and accidentally light on fire.

 

Best regards

 

-a-

[jrwilson]

Looks like vents are clean, fuel senders are clean and the filters are good.  The boost pump looks fine.  They found a fuel hose that was possibly a problem, but they ran it and had a pressure gauge hooked up and there was 7-8 lbs of pressure variation which they tracked to the carburator.  So they got a gasket and are going to poke around in the carburator becuase maybe it is the carb float or something in the carb.  Hopefully they find a smoking gun and fix it. 

[jrwilson]

The mechanics thought it was the carb float rubbing/sticking on the side of the bowl, so they soldered it and moved one of the floats.  I did the test flight and the engine quit at three thousand feet, luckily I was close to the airport, so no problems but did need the tug to get back.  Seems when they soldered the float it opened up a hole, which they didn't find when they soaked it in gas (pressure may have opened it up), which caused the float to sink.  They've ordered a new float (an overhaul kit actually) and that should fix that.  No idea if the original problem is solved or not.  They rerouted a fuel line which had a sharp angle, which could have been a problem too.  They're saying they've never seen this type of problem before, which isn't very comforting...

[moodychief]

I am going through a similar situation.  I have a 63C Mooney.  I discovered my fuel gauge wasn't operating properly so I can't attest to the fuel pressure drop.  When climbing to altitude my EGT would start going up as though it was leaning out.  I caught it before the engine quit, applied electric fuel pump and the EGT came down to normal.  I had to keep electric pump on during the climb.  Once at altitude and level flight, I could lean and fly as normal.

All screens were cleaned and the troubleshooting began.  I had a new fuel pump put on and thought that had fixed the problem.  The issue seemed to be gone until I hit 7,000 feet climbing to 11,000 and it started again.  All of my fuel lines are less than 2 years old and I too have a fuel line with a sharp angle.  My mechanic pulled all of the intake tubes, inspected, cleaned and reinstalled with new gaskets and rubber fittings to the carb.  I also decided to put a new carburetor on since there were a couple of SBs and ADs since it was installed in 2005.  We put new gaskets in the gascolator.

After spending a lot of money we may have found the culprit. The gascolator didn't look like it was sitting square.  Upon looking at the top side with a mirror we found that the bailing wire was fitting into the bracket and top of the gascolator only on one side.  The gascolator was cockeyed and the other side was only through the bracket but not into the gascolator. 

I haven't had a chance to flight test it yet but will probably do so Friday evening.  I will let you know if we found my gremlin.

[jrwilson]

Did your problem start about 2 years ago after your fuel lines were changed?  I think mine started when I got new lines installed too.  I think the intitial problem might have been that fuel line, becuase air getting trapped there could be angle of climb/pressure altitude dependant.  I'm pretty confident my gaskolator is ok because the mechanic was under the plane looking in the gaskolator while it ran, looking for bubbles, which there weren't any.  You have the parralel boost pump/engine pump configuration too, does your boost pump allow back flow?  Will it hold pressure once the engine is off? 

Mine should be test flown tomorrow maybe, so I'll let you know.

[jetdriven]

JR I think you need to find a new mechanic before they get you killed.  Seriously, they shouldnt touch that carb without the proper manuals etc.  Thats the wildest story I have ever heard.

Quote: jrwilson

The mechanics thought it was the carb float rubbing/sticking on the side of the bowl, so they soldered it and moved one of the floats.  I did the test flight and the engine quit at three thousand feet, luckily I was close to the airport, so no problems but did need the tug to get back.  Seems when they soldered the float it opened up a hole, which they didn't find when they soaked it in gas (pressure may have opened it up), which caused the float to sink.  They've ordered a new float (an overhaul kit actually) and that should fix that.  No idea if the original problem is solved or not.  They rerouted a fuel line which had a sharp angle, which could have been a problem too.  They're saying they've never seen this type of problem before, which isn't very comforting...

[jrwilson]

Jetdriven, you know, I think that sometimes too, but in this case, I think it was just human error.  It is a mooney service center with a good reputation, and their diagnostic procedure has been pretty good so far.  I'll let them test fly next time:)  My last mechanic said there was not a problem at all, then the engine stumbled.  I don't think this failure was the result of the original problem.  After the soldering of the carb, they did test it, and based on their test, they should have found the hole, but errors do occur.  They took responsibility, found the error, are correcting it and I don't think another mechanic would be any better.  After maintenance is when most problems happen and it is a reminder to let them do the test flight.

[jetdriven]

Here's a question.  Are they charging you shop labor rates to disassemble and reassemble the carb again after they messed it up and then caused you to land dead stick?

[jrwilson]

Sure hope not.  They haven't charged me yet.  The owner said this is a situation (the whole problem) where they would discuss it, and come up with a reasonable price where they wouldn't lose a customer and wouldn't lose money on the job, factoring in mechanic time.  He said they wouldn't be charging for the actual time it took to do the job, but would be taking off lots of hours because some of the time wasn't productive.  As for the carb, I don't know, I doubt it.  I was wondering should I pay for the carb float kit ($100)?  Mine  was good, supposedly, until they soldered it...but then again, it was bent and sticking, so they should have gotten a new one in the first place...

[jetdriven]

Mike Busch refers to this as a MIF, a maintenance induced failure.

[jrwilson]

Sounds about right.  When it happened, I referred to it as "WTF!!!" 

[moodychief]

jrwilson,

I test flew my airplane today and I still have issues.  As mentioned before, my problem was only happening during climb.  Now after installing all of the new parts it is doing it in level flight as well.  In less than 15 seconds of turning off the electric fuel pump, my EGT starting rising rapidly as though it was leaned out too much.  Since I knew what to watch for, I immediately turned the electric pump back on when EGT starting rising and prevented a power out situation.

Have you had your fuel line (with the bend) replaced yet?  That may be my next attempt to fix even though it is only two years old.  It almost looks like that line is made too long.  Supposedly it was custom made to Mooney specs for my make/model.

To me it either is a fuel flow issue or it is sucking air somewhere.  

Please keep me informed if you find the source of your issue and I will do likewise.

 

[DaV8or]

How about fuel venting issue? Tested the vents on the tanks? Maybe your engine driven fuel pump is dying.

[jrwilson]

Moodychief:

Mine is still having a problem too.  In a climb, the pressure drops.  The engine isn't stumbling now, but the orignal problem of pressure drop is still there.  Yes the fuel line was changed, and they also changed a fitting to decrease the angle of a bend.   Mine were done 2 yrs ago also and they used new fire shrouded lines, so maybe that caused the difference because they used my original lines as a measure.  The mechanic found the wrong gasket in the gascolator, but he doesn't think that was the issue. 

It wasn't the carburator, it now has a new float and the problem persists.  They're going back to the fuel boost pump, thinking it is the wrong pump and is allowing backflow.  This is a new pump, and the problem happened before the new boost pump was put on, but it looks like the old mechanic put on a boost pump for a later model mooney.  Maybe the old pump was going bad and the check valve was loose, letting back flow?  I don't know.  My egt (on the JPI engine monitor) doesn't show any change when the pressure drops (except for the time the engine stumbled) but I'm still getting a pound of fuel pressure and the carb only needs .5 pounds...  I and the test pilot can initiate the problem by putting it in a steep climb, or bank and watch the pressure drop.  Level off, pressure right back up.

They found the original fuel boost pump is being made, so they ordered one (<$100 compared to the $300 one old mechanic put on).  Hopefully that will solve it. 

Let me know what you find. 

[jrwilson]

Quote: DaV8or

How about fuel venting issue? Tested the vents on the tanks? Maybe your engine driven fuel pump is dying.

[moodychief]

jrwilson,

Can you let me know what the part number or make/model of original boost pump?  I am going to try that next as that is the only part of the fuel system not changed yet.

Thanks!

[jrwilson]

Won't know until Tuesday, but yes, when I find out I'll let you know.