M20C owners - Fuel Pump Test/Try this please...

[jrwilson]

Still working out a fuel pump issue...In climb the fuel pressure drops off, then when I level out, the pressure comes back up, usually.  Recently, the fuel pressure dropped, then the engine quit.  Twice. 

On the forum, a member gave a good suggestion on testing fuel pump health, but it didnt' work on my plane, or maybe it did and it showed the fuel pump is the problem...the dificulty is the M20C has a parallel fuel pump system, where as newer Mooneys have an inline system.  The year range is 1962 to 1964, I think.  So I'm not sure if this test will work on the parallel fuel systems.  Could you C owners in the '62 to 64' range try this test and post your results please? 

Here it is, copied from another user:

Posts: 655 Location:

Re: Fuel Pressure Question Posted May 28, 2011 9:09 AM With engine off 1. Mixture full rich, throttle open. 2. Turn electric pump on 3. After getting pressure close mixture and throttle 4. Turn electric pump off. 5. Verify fuel pressure still indicated for at least five minutes (longer is better). If pressure drops right away you have a perforated diaphragm in the pump or a bad bowl needle valve in the carburator.

My pressure drops off right away...there will be a little residual pressure, but it is gone within 2 minutes...

[rbridges]

are you sure it's only up to 64 models?  I have a 65 I would try it on if it would help.

[jrwilson]

I think it might include '65 models, but I'm not sure...Can you check your parts manual?  I would appreciate you trying if it is the same fuel system...

[Romeo-Charlie]

Hello JR. I have a 63 M20D. While flying around two flights ago, during level flight, I noticed the fuel pressure gauge dropping to just above zero and the needle started to shake. I did not wait for the engine to quit and turned on the boost pump, the pressure came up and when I turned off the boost pump, the pressure stayed up. It happened one more time during that flight. When I landed I checked the vents and sumped the drains again. On my next flight it happened again, except when I turned the boost pump off, the pressure dropped to zero right away. I have been watching your previous posts and followed your test today. The fuel pressure dropped to zero right away. The fuel pump is buried behind the engine and is difficult to access. I'm going over now to remove the top cowling to see if I can get to the pump from the top. I'll let you know what I find. Thank-you!

[moodychief]

jrwilson,

Did you order a new boost pump?  I am still trying to find my gremlin as well.  For some reason, air keeps getting in the gascolator and we can't figure where it is coming from.  We have replaced the gaskets and reseated them a couple of times.  When the boost pump is on the gascolator fills to the top. When the electric pump is turned off, all of a sudden a huge air bubble appears.

 

[jrwilson]

Hi,

Yep, new boost pump, still does it.  Pressure drops in a climb, but now it goes down to about 3, and is more steady.  They were able to get it to drop to 2lbs at a 20 degree climb.  When it levels off, pressure goes back up.  I did a lot of internet research and it seems low wing lycoming O360 engine combos do this.  There was a big thread with RV's and cherokees.  It seems the Lycoming is just marginal at pulling fuel uphill and it will be affected a bit by pressure and temperature.

I may have had something in the fuel, water, crud or something.  Air could have been trapped in that sharp bend in the fuel line causing a hiccup at climb angle.  I don't know.  They cant replicate now with multiple test flights and the pressure is better and more stable, so the things they did helped the problem. 

The overall problem of the pressure drop in a climb just seems like a normal part of 20Cs.... 

Moodychief:  The threads all talk about no change in EGTs/CHTs while the pressure drops, and .5lbs is all the carb needs, so if your EGTs are dropping, or engine roughness, then this of course is not just a problem w/ the 20C model and there is something else going on.  I still don't know the part number of the new pump.  I'll get it tomorrow.

[jrwilson]

Moodychief:  did they check your fuel tank switching handle?  Maybe a gasket there is letting in water?

[moodychief]

I think we found part of my problem.  We decided to start troubleshooting lines and look what we found.  Inside the cabin, on the left side of the nosewheel housing (behind and to the right of the pilots right pedal), this small piece of hose goes from the gascolator through the nosewheel housing through what appears to be a stiffiner and a couple inches later through the firewall where it connects to the fuel hose going to the mechanical fuel pump.  It had abrasion tape on it so it wasn't apparent when looking up in there with a flashlight and mirror.  When the mechanic pushed some air through the line he could hear the hissing.  It wasn't until some of the tape was pulled away that the extent of the damage could be seen.  There is no grommet covering the inside of the circular cutout it passed through.  Over the years the edge of the cutout must of cut into the fuel line.  Probably started as a small pinhole and proceeded into a sizable crack as it rubbed.  Since the crack was on top of the fuel line it wasn't leaking fuel...just sucking air!  This explains why air seemed to dump into the gascolator as soon as the electric pump was turned off.  Also, this is where it was sucking air when trying to operate solely on the mecanical fuel pump.  Hopefully, this is the major cause of my issue.  We are going to do more troubleshooting tonight to ensure we don't have any other anomalies in the fuel system.

It might not be a bad idea for folks having issues with fuel pressure and engine roughness/leaning to inspect this fuel line as it may be contributing to the issue. 

[jrwilson]

Hey Moodychief, hopefully that solves your problem.  Mine seems to be running fine now too, after the fuel pump change and the fuel line angle adjustment...   Here is that part number for the Facet boost pump:  476087

[DaV8or]

Quote: moodychief

I think we found part of my problem.

[moodychief]

It looks like my problem is fixed.  Woo hoo!  I was able to attend a fly-in breakfast this morning.  Also flew formation with a 6 year newer C model with different mods than mine to compare speed.  He has the 201 windshield, I have the stock, he has the stock cowling, I have the cowling speed mod, he has a fixed step, I have a retractable step.   Ended up we were dead even at 3500' with WOT,  155 KTS ground speed.