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Low fuel pressure- IO360

tomn

By tomn
in General Mooney Talk

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tomn Collaborator

tomn

Posted
Posted

Started the plane up yesterday for a flight and had low fuel pressure indicated on the gauge. The engine started fine and ran normal. On the fuel pressure gauge, the needle came up to 14 - just in the red on the low side with the boost pump off. Increasing rpm made no difference, the pressure stayed at 14 (the green range is about 15-29). I let the engine run for a couple of moments and again tried various rpm settings - no change. I turned on the boost pump and no change - still at 14.

I put it back in the hanger and pulled the cowling. There were no leaks and everything looks normal. I tried running the boost pump with the mixture off and had no pressure indcated as I would expect. I then ran it with the mixture in at full rich. The pressure came up to about 20 or so indicating the gauge was working fine. Fuel did start dripping out of the tube so fuel was making it to the servo. My guess is that the mechanical pump is the cause - it does have lots of hours on it. The entire fuel delivery system downstream of the mechanical pump - servo, divider, etc. - was overhauled less than a year ago and I don't think any of those components would affect fuel pressure unless there was an obvious leak either internally or externally - no indication of either.

Anyone else ever had this occur or have any thoughts other than the mechanical pump?

Tom

Bob_Belville Grand Master

Bob_Belville

Posted
Posted

Tom, I can't be much trouble shooting help except to give you my FP for comparison (22-23 at cruise) which you no doubt know. (I happen to have a flight data from the JPI open... looks like my FP peaked at about 26 with boost on just before take off.) 

Marauder Grand Master

Marauder

Posted
Posted
Tom, I can't be much trouble shooting help except to give you my FP for comparison (22-23 at cruise) which you no doubt know. (I happen to have a flight data from the JPI open... looks like my FP peaked at about 26 with boost on just before take off.)
I'm at 23-24 psi at altitude 26-27 psi with the boost pump. Tom -- the fuel pressure is a direct measure device (i.e. fuel is sent to the gauge). It is possible that you may have some sort of partial restriction in the line.
Piloto Grand Master

Piloto

Posted
Posted

One way to check the engine pump diaphragm:

 

1. Engine off

2. Mixture and throttle open

3. Run electric pump.

4. Close mixture and throttle after 10 seconds.

5. Turn electric pump off

6. Check fuel pressure

 

If fuel pressure stays on diaphragm is good. If pressure drops after turning pump off suspect ruptured diaphragm.

 

Check also fuel tank vents.

 

José 

  • Like 1
jetdriven Grand Master

jetdriven

Posted
Posted

Mine passed that test but was still 13 PSI above 8000 feet. A new fuel pump and extra insulation on the firewall fuel line solved it.

jetdriven Grand Master

jetdriven

Posted
Posted

No. There is rumors of the firewall fuel line getting too hot and causing vapor lock at the pump inlet, and I have the 124J integral fire sleeve fuel lines so I put another piece of fire sleeve over that line to insulate it more.

N601RX Grand Master

N601RX

Posted
Posted

The reason I was asking is that the F's originally did not have a blast tube either, but the STC for relocating the Oil cooler includes a 1 or 1-1/4 scat tube for the fuel pump.

N601RX Grand Master

N601RX

Posted
Posted

I went back and looked at the STC and it says relocate the inlet for the blast tube.  My 67 did not have a blast tube, or any evidence of where one might have been, so we added it.  It's possible it was removed moved by a previous owner, but I don't remember seeing a hole in the baffle where it would have went.  I had to buy the scat flange it said to relocate.

 

I just went back and looked at the parts manual and see where it should been, but was not there when I purchased the plane.  Does the fuel pump end have some kind of shroud that attaches to the fuel pump, or just point at the pump?

Shadrach Grand Master

Shadrach

Posted
Posted

My 67 F is bone stock and has a flexible rubber cover covering the fuel pump. IIRC the bottom of the cover is capped with aluminum and is plumbed into a fresh scat tube that supplies ram air when ever the plan is moving...or running for that matter.

tomn Collaborator

tomn

Posted
  • Author
Posted

I'm at 23-24 psi at altitude 26-27 psi with the boost pump. Tom -- the fuel pressure is a direct measure device (i.e. fuel is sent to the gauge). It is possible that you may have some sort of partial restriction in the line.

Chris, that was my first thought. When I ran the boost pump with the engine not running and the mixture full forward, the pressure went up to about 20. That would indicate the gauge is OK. I may try and pump some fuel through the line just to be sure but I'm still guessing its the mechanical pump at this point.

Tom

  • 3 years later...
chrixxer Experienced

chrixxer

Posted
Posted

Hmm. Now I'm wondering about my fuel pump in my '66 E. I noticed as I was leaning in flight the other day my fuel pressure dropped just slightly below the green arc on the pressure gauge. Running the electric pump brought it back up into the green, and once I settled on a cruise power setting around my usual (~1250 EGT, ~380 CHT, can't get any more exact than that with current equipment), it was still in the green, without the electric pump. But seeing it drop below that arc at all was unsettling.

(I have only about 40 hours in the plane - with its somewhat unknown history - so far, and I'm still learning her quirks, and identifying problems.)

She also kind of "pops" at low throttle settings (idle - 1100 rpm), but runs like a sewing machine at 1700 rpm or higher (at least; I haven't really tested lower). I usually idle at 1100 rpm if I'm going to be on the surface any length of time - generator ... Wondering if the idle mixture is too lean? It's also been clearing up; it was bad when I first started flying her, it's noticeable now to me only in comparison with the "other" IO-360 in my life, a smooth running flight school Arrow.

Yetti Grand Master

Yetti

Posted
Posted

On ground

Prime engine.

Throttle 1" open

Boost pump on

Mixture open

count to 6

Mixture closed.

Pressure should hold up around 30 for 30 seconds to a minute.   If it falls off then replace mechanical pump.

 

 

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