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High fuel pressure?


Guest Spike Kavalench

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Guest Spike Kavalench

My “new to me” ‘61 M20B indicates about 4.5 psi fuel pressure with the electric boost pump on and about 8.5 when the mechanical pump kicks in after start. 

Is that out of limits and if so, why would that be? I believe the mechanical fuel pump was replaced with a new one a while back. 

 

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I get just over 6 psi on the mechanical during the run up at times and on final with no electric pump on.... I never noticed it on my 64C until I got the JPI 900.

It was explained to me by JPI and various other credible sources that my plane probably has been doing that for the last 50 + years and its only now with precise measurements and master warnings that I notice.  All is well and it operates like it did for the past several years that I owned her.....

 

There is a chance that your fuel pressure gauge may not be calibrated correctly.  I get near 6 psi with the electric on and about 4.5 during cruise with no boost pump down low....

There will be times in a climb or high power up high where you will see .5 psi , yes I typed point 5 psi....it is still in the green arc though.....

8.5 psi seems high

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8.5 is above the redline in my 1970C.

Running electric pump before starting, i generally see right at 6psi. Pumping the throttle makes it fall, and I generally run ~2psi in cruise, with occasional excursions below 1.0 psi without noticable effect.

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Hmmmm....

Red lines are usually looked up directly on the instrument.

If it isn’t there... look in the limitations section of the POH....   Stay with me for a moment...

M20B didn’t get a real POH... But...

The M20C power plant and instruments are very, exactly, similar....?

The M20C got a more real POH... in the later years, there is one posted in the download area around here...

6 psi is what my M20C would get with its electric FP, prior to start... it is not very much pressure at all...

the height of a column of fuel would cause the fuel pressure to change... my C’s fuel pressure gauge would change based on the attitude of the plane... that’s why I thought the electric pump was being used during the initial climb to 1k’agl...

My European engineering friends like to use a zero when writing 0.5 psi...  it keeps the decimal point from getting lost.  It’s an excellent skill to adopt.

8.5 psi is really high on a mathematical comparison to 6... threads around here have discussed the 6psi red line and what is acceptable when a digital gauge with a decimal point is used...

I think 6.4 psi was acceptable... because it rounds down to 6, on an analog scale.  Where 6.5 would be considered 7 on the same analog scale.... and 7 was out of bounds...?

8.5 would be considered a 9 on the analog scale... which is about 50% more than the 6psi redline...

Still, not very much pressure to begin with... unless it is being supplied to a large surface area... then it becomes a lot of force!

Sounds like something that needs to be looked into further for a better explanation...

Recently there is a thread around here... for IO360s that have been factory OH’d... their fuel pressure is too high, from the factory... causing the FP to go past redline... with no solution available (at the time)...

I believe it turned out the factory was supplying a good pump, but didn’t match the paperwork of the plane... a real paper snafu...

MS did a great job of sleuthing the problem through... to get to a satisfactory result...  proper paperwork or new parts? I don’t recall...

Sounds like a similar challenge, but on a different engine... O360 vs IO360... 6psi vs 35psi...

Compare that to the M20R... There is no fuel pressure gauge.... just FF... can’t get one without the other...

Fuzzy PP thoughts only, not a mechanic....

Best regards,

-a-

 

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There could be a difference between the M20B and our M20C fuel pressures simply because the B has a different fuel system design.

The fuel pumps in the M20B are parallel to each other, while in the later M20C they are in series.

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Guest Spike Kavalench
On 6/14/2018 at 6:42 AM, David Herman said:

Spike ... 

The fuel pressure (high) red line is 6.0 on our C.

Our fuel pump is pretty strong: we’d see up to 7ish with the boost pump and 5.5 - 6.5 ish with the engine driven pump. That’s on the ground at idle ...

With the original MP/FP Gauge even though the needle was way up there at times ... I never gave it much thought. It was always in the center of the green in cruise and I could see a slight rise when I turned to boost pump on to switch tanks ... etc.

After installing the EDM-930 ... that intermittently slightly high FP began giving us intermittent warning messages. Particularly at low power setting ... e.g. taxi and short final. That was bothersome ... so I asked our IA what could be done ... thinking there might be an adjustment?

The answer we got was : that the FP from the engine driven fuel pump is not adjustable ... you just get what you get ... and if we replace the pump ... we could get more of the same. Additionally, it’s a pain to replace due to the location. He reasoned ... it’s better to have a little stronger pump ... than a little weaker pump. Better to just live with it.

So we’ve just learned to live with the intermittent high FP warnings that come at low power settings. 

Having said that ...8.5 seems very high? Not giving any advice ... just trying to share the information - ymmv

I wonder if  a “crimp in the line” can cause a reading like that? Doesn’t a fuel line go into the (original) gauge in the panel to get that reading? 

Our shop actually put another (hand-held) gauge on the source of that line to verify the pressure reading was accurate and it wasn’t the gauge itself ... and that gauge read the same. 

 

Thanks David and everyone for the replies. I looked at the engineering drawings for the turbo installation and they refer to a redline of 8psi, and also provide a recommended pressure adjusted to 6psi at 2300 RPM, so maybe the turbo mod has an adjustable fuel pressure... in any case, thanks all for the input.  Here’s a pic. She cleaned up pretty nice!

714A7302-7C5A-4A96-B1CC-921BEB6EFB14.jpeg

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  • 3 weeks later...
Guest Spike Kavalench

So it turns out there is an adjustment you can make on the fuel regulator which is integral and unique to the turbo installation. Rayjay engineering drawings show how. In case anyone was wondering.

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