Normal vs. abnormal boost pump sounds

[Vance Harral]

Aircraft is a Mooney M20F, engine is a Lycoming IO-360-AIA.  It's equipped with a Weldon 8163A electric boost pump (certified replacement for the original Dukes).

POH start procedure specifies mixture to idle cut off, boost pump on to pressurize the lines, then advance mixture to prime.  When the boost pump is turned on with the mixture at idle cutoff, pressure builds to about 25 PSI and is rock solid.  The pump makes a fairly loud hum while it's on, of course, and the frequency of the humming noise (i.e. the speed of the pump) decreases a bit as the fuel line pressurizes.

We don't prime for hot starts, but sometimes we prime when the airplane is "sorta" hot, e.g. sat on the ramp for an hour or two during a $100 hamburger run.  Sometimes there is still vapor in the lines in these cases.  If so, the pump seems to "strain" a little as pressure builds, and the frequency of the humming noise from the pump warbles a little.  I've always considered this normal, during the 10+ years this particular pump has been in service.

Now, after a recent annual, my airplane partner said he heard the hum from the pump warble a bit even on a cold start.  This morning I turned on the pump for a cold start and let it run a full 30 seconds or so before priming.  Sure enough, the speed of the pump varied a little after the lines were pressurized.  Fuel pressure needle never wiggled, it was rock solid at about 25 PSI as expected.

We're now trying to decide if the warbling frequency from the pump is "normal" or not.  I confess I've never paid much attention to steady vs. warbling sounds from it, since I know some variation is normal as the lines pressurize (especially if vapor is present).  The pump has about 10 years of service on it, and probably somewhere in the neighborhood of 2000 on/off cycles, so I suppose it's possible the brushes on the motor are going.  It's also possible the system is sucking air from somewhere.  There are no blue stains around the pump or gascolator upstream from it, that would indicate an obvious leak.  But the bowl of the gascolator was R&R'd at the recent annual, to inspect and clean the screens.  It was reassembled with new gaskets and safety wired, but that doesn't guarantee a good seal of course.

What says the collective wisdom of the board?  Leave it alone?  Inspect the system?  Replace the pump now even though it's not obviously defective?  We're not necessarily trying to avoid the cost of a new pump.  But we are reluctant to replace what may be a perfectly good pump, due to the risk of a maintenence-induced problem in the swap.  Any/all opinions appreciated.

[orionflt]

run it until it dies, but also check and make sure that it is not failing internally and dumping fuel out the drain when it is running.

 

Brian

[FloridaMan]

Mine does that sometimes too. Sometimes it sounds like it struggles a little, but it's always done that. 

[Jim Peace]

I have a Dukes that was put on as an overhauled unit 4 years and 120 hours ago.  I just noticed this today....is this from the pump?

 

1964 M20C # 2799

 

 

[Jim Peace]

What is the latest on using this pump from Weldon?

 

http://www.qaa.com/products/aircraft-fuel-pumps-1/weldon-fuel-pumps/8164-A-weldon-fuel-pump

 

Can we just replace the dukes and add this with some bracket work?  Do we need some sort of kit from mooney for something?

[Browncbr1]

Hi Vance,

I have the same plane and Weldon pump as you. I just had mine rebuilt and currently has about 12 hours on it. It warbles the same as you describe. Before the rebuild, it warbled and that's why I had it rebuilt (10years since last overhaul). One caveat is that I don't really trust that the company that rebuilt it did too good of a job. They sent it back with all the pressure and leak test paperwork, but there was no evidence of fuel having been in it and upon installation, it leaked bad and had to send it back to them to correct. So, no telling what they actually did inside. Needless to say, I won't be having them do any of my stuff anymore. The certainly fake declaration of testing makes me wonder how many customers they may have killed.

Craig

[N201MKTurbo]

Once the pump pressurizes the system it is just pumping fuel through the internal pressure relief valve. If a bubble gets stuck in this loop it will make the pump warble. As soon as you let some fuel flow in the system the bubble will pass and everything will be OK.