Fuel Pressure issue

[Greg Ellis]

I posted this in the Vintage section but then thought it also kind of belongs here: 

I have read other postings on fuel pressure problems but none seem to apply to what has started with my 63 C model.  It has an O-360-A1D.  It has an EDM 900 as well.

I went to fly it yesterday and the fuel pressure on the EDM900 after engine start went to 7.1.  Redline is 6.  It stays there at any power setting from idle (600 RPM) to full throttle.  Plane otherwise is running fine with no issues.  The prior flight which was last week, everything was fine.  And with the engine not running and the electric fuel pump on the EDM 900 read 3.5.

My mechanic who is not at my home airport wants me to put a manual fuel pressure gauge on to see if the EDM is accurate or there is a problem with the sensor.  I am not sure where to tie one into the system and what to use.  I have a call into him to ask but thought I would seek the advice of the incredibly intelligent folks on Mooneyspace.

I read that other issues could be the fuel pump, a kinked fuel line, a partial blockage somewhere…but it seems to me that the plane would not run well if it were any of these issues.

So, I was going to tap into the fuel system with a gauge.  Where should I tap into it?  I am assuming somewhere before the carburetor?  Any other thoughts or advice would be much appreciated.

[Ragsf15e]

11 minutes ago, Greg Ellis said:

I posted this in the Vintage section but then thought it also kind of belongs here: 

 

I have read other postings on fuel pressure problems but none seem to apply to what has started with my 63 C model.  It has an O-360-A1D.  It has an EDM 900 as well.

I went to fly it yesterday and the fuel pressure on the EDM900 after engine start went to 7.1.  Redline is 6.  It stays there at any power setting from idle (600 RPM) to full throttle.  Plane otherwise is running fine with no issues.  The prior flight which was last week, everything was fine.  And with the engine not running and the electric fuel pump on the EDM 900 read 3.5.

My mechanic who is not at my home airport wants me to put a manual fuel pressure gauge on to see if the EDM is accurate or there is a problem with the sensor.  I am not sure where to tie one into the system and what to use.  I have a call into him to ask but thought I would seek the advice of the incredibly intelligent folks on Mooneyspace.

I read that other issues could be the fuel pump, a kinked fuel line, a partial blockage somewhere…but it seems to me that the plane would not run well if it were any of these issues.

So, I was going to tap into the fuel system with a gauge.  Where should I tap into it?  I am assuming somewhere before the carburetor?  Any other thoughts or advice would be much appreciated.

I’ll put my response here too. Maybe you can delete one of these two threads?  Good luck with it!

I did this on mine and tapped in right before the fuel pressure sender for my edm-930.  I put a T fitting in there and a long hose to get the mechanical one in the cockpit through the window.  My edm-930 fuel pressure still fluctuates and I have basically been ignoring it, but with the mechanical gage Td in, both were stable and read the same.

[Greg Ellis]

Thanks for the help and I deleted the one in the vintage area.

[PT20J]

If everything is running well, a single gauge reading should always be verified before taking other action, so your mechanic's suggestion is a good idea. Ideally, I would tap the mechanical gauge in as close to the transducer for the engine monitor as possible so both are measuring at the same point. If you eyeball the plumbing, it will become obvious what fittings and hoses you need.

But first, I might check for restrictions that could increase pressure. Have you checked the fuel screen at the carburetor inlet? Also, you might inspect the hose between the fuel pump and carburetor to be sure it isn't deteriorating. 

[Greg Ellis]

@PT20J I checked the hose between the fuel pump and the carburetor today.  It seemed clear.  I did not check the fuel screen though.  I will see about that tomorrow.   Thanks.

[hammdo]

Do you still have the pressure line from the top of the carb to the firewall? That is where we tapped a mechanical gauge to verify pressure when I was having high ranges (at the firewall)…

-Don

[Greg Ellis]

@hammdo I believe I do.  There is a line coming out the top that is a 90 degree elbow the is right about the large fuel line going into the carb.

[Greg Ellis]

So, an update and asking for more advice.  I removed the fuel screen on the carburetor and it was clean as a whistle. 

I was able to tie in a mechanical fuel pressure gauge right before the fuel pressure sensor on the EDM900.  It verified that the EDM 900 is correct and my fuel pressure when the engine is running at 1000 RPM is over 6 psi.  Redline max on my O-360-A1D is 6 psi.  When I run the engine up to 1500 RPM, the fuel pressure goes up to 7 psi.  At idle it stays just above 6 psi.

Prior to starting the engine, when I turned on the electric fuel pump, the gauge also agreed with the EDM 900 and showed a little over 3 psi.

So, any advice on where to look next?  The fuel line going to the carburetor is clear and without restrictions just to add.

Could it be the engine driven fuel pump or a line somewhere else?

Thanks for any advice.

[47U]

On 3/18/2024 at 5:17 AM, Greg Ellis said:

Could it be the engine driven fuel pump or a line somewhere else?

I’m guessing the engine driven fuel pump.  How old is it?  I read on the RV forum that Lycoming recommends a time-change at the 10 year mark.  I’m not sure if any of us CBs are actually doing that.

I replaced my (‘63C) engine driven fuel pump three years ago when I was experiencing some fuel pressure/flow fluctuations.  That turned out to be a cracked flare on the hardline between the gascolator and the elbow in the pilot’s footwell.  The pump is sitting on the shelf in the hangar… I could send it to you for troubleshooting purposes.  IF you wanted to go through that much pain to install it, then replace it with a new/overhauled pump if the problem is with your existing pump.  It’s really not a fun job.  Might be prudent to check how old your hoses are, too, just because you’re in there.

If you’re paying shop-rate to do this work, you might be money ahead to just install a new/overhauled pump to begin with. 

Hopefully someone with more experience than me will chime in with a better idea.

[Greg Ellis]

4 minutes ago, 47U said:

I’m guessing the engine driven fuel pump.  How old is it?  I read on the RV forum that Lycoming recommends a time-change at the 10 year mark.  I’m not sure if any of us CBs are actually doing that.

I replaced my (‘63C) engine driven fuel pump three years ago when I was experiencing some fuel pressure/flow fluctuations.  That turned out to be a cracked flare on the hardline between the gascolator and the elbow in the pilot’s footwell.  The pump is sitting on the shelf in the hangar… I could send it to you for troubleshooting purposes.  IF you wanted to go through that much pain to install it, then replace it with a new/overhauled pump if the problem is with your existing pump.  It’s really not a fun job.  Might be prudent to check how old your hoses are, too, just because you’re in there.

If you’re paying shop-rate to do this work, you might be money ahead to just install a new/overhauled pump to begin with. 

Hopefully someone with more experience than me will chime in with a better idea.

@47U, Thank you for the response and the offer.  I am really not sure when or if the fuel pump was replaced but I do not recall ever seeing a logbook entry anywhere regarding replacing the fuel pump.  I am going to go back to the airport to do a little more trouble shooting this weekend.  If I cannot find any issue outside of the fuel pump then I have made arrangements to have it replaced.  I have already ordered a new one and if for some reason we find another issue, then I will either just hang onto it until needed or replace the one on the airplane if that seems to be the problem or I can return the one I ordered as well.

I read in one of the manuals that if there is a fuel pressure increase than that is likely the fuel pump.  But just want to be sure.  I am troubleshooting this myself since the only good A&P/IA at my airport stopped doing regular maintenance and just does avionics now.  Great avionics shop but leaves a pretty good size GA airport without a good general mechanic shop.  There are a couple but I have not heard any good things about them.  Oh well.... 

[Greg Ellis]

Quick update.  Removed the fuel pump and replaced with a new pump.  Fuel pressures are back to normal.  No other issues could be found so I elected to replace the fuel pump.  Can these be rebuilt so I have a spare or is it better just to buy new if I need another in the future?

[47U]

3 hours ago, Greg Ellis said:

 Can these be rebuilt so I have a spare or is it better just to buy new if I need another in the future?

I’m NOT an expert.  But I think it might be better to sell it for the core value.  I don’t know what the shelf life is on a pump sitting in box.  Five years?  Ten years?  In a hot hangar in Fort Worth (or Sacramento)?  I think I’d rather install a pump either new manufacture, or with a short time elapsed on the shelf since overhaul.

My opinion might be biased, though.  The last airframe I worked on in the Air Force, it wasn’t uncommon to go into phase inspection with over a hundred time changes scheduled.  And yes, sometimes we had mx induced failures.

[phxcobraz]

Glad replacing the pump resolved the issue. 

I occasionally see a similar spike in my fuel flow above 6psi when idling/taxing around, however it stays in normal range when I give it any amount of power above 1400rpm.