fuel pressure adjustment advice

[jamesm]

Hi,

I have a 1967 M20C with a Lycoming 0-360-A1D carbureted engine.

A few years back I have installed a  Electronics International CGR30C  and then about another year later installed CGR30P. These Engine monitors come with Red & Yellow Annunicator lights. When I look at the CSV files from the engine monitor  it shows max fuel press  ~6.9 psi is pretty much the peak pressure.  So I went to Lycoming Engine Operator Manual and the TCDS (Type Certificate Data Sheet) E286 both show 8.0 PSI and the max however the  Mooney in the Owner's Handbook shows 6 psi as the maximum. So the Red annunicator light comes on  since I am over 6 PSI max limit  set by Mooney.  And of course the Red annunicator goes out if I turn off the the electric boost pump.

I saw the tempest video doesn't sound like there is much to adjust. I am looking at spare a Duke Electric fuel boost pump  It has a label stating " pressure relief preset DO NOT ALTER".  Since Mooney Owner Manual  states 6.0 PSI as the maximum and E.I. is obligated to comply since Air frame manufacture supersedes the Engine operating manual which states 8 PSI as the  Maximum PSI. 

What is the standard practice of adjusting the fuel pressure ? if any? 

Is it wise to be adjusting the fuel pressure so that  an annoying red annunicator  light to go off?

Please advise,

James '67C

 

 

[N201MKTurbo]

The fuel pressure is set by the spring in your fuel pump. There is no easy way to change it.

If you chip the epoxy from around the seal disk on the electric pump, there is a disk with a screw slot that allows you to set its pressure anywhere you want. No authorization to do that. Not sure what regulation would stop you....

[PT20J]

If it were mine, I'd leave well enough alone. You only get the high indication with the boost pump on and it's only slightly high. Fuel flow, not pressure is what keeps the engine running. The flow is fine if the engine runs well. The only reason these things have fuel pressure gauges is that it was the cheapest thing to measure back in the day to give you an indication of a pump failure or a leak. Of course, the usual caveat applies: whenever you get an erroneous indication from a cockpit gauge, the first thing to do check the accuracy of the gauge.

Skip

 

[jamesm]

Thanks,

I would agree leave well enough alone. I wanted make sure I wasn't  overlooking something.

It is  kind annoying when Engine TCDS says 8 PSI but the air frame is 6 psi and I am stuck with the lower.

James

 

[PT20J]

13 minutes ago, jamesm said:

Thanks,

I would agree leave well enough alone. I wanted make sure I wasn't  overlooking something.

It is  kind annoying when Engine TCDS says 8 PSI but the air frame is 6 psi and I am stuck with the lower.

James

 

I know this is frustrating. The manufacturers of replacement primary instruments are bound by what the airframe manufacturers specified as limitations. Everyone that was at Mooney when they set 6 psi max is dead or gone: who knows why they specified that. Maybe the original boost pump put out a bit less. With old analog gauges, we didn't pay so much attention to such things, but now red lights flash and make us nervous. I know; I'm not immune. It's a mismatch between equipment designed 60 years ago when close was good enough and modern digital technology. 

[N201MKTurbo]

I don't think it would be difficult to get a field approval or one time STC to have your instruments re-configured. You have plenty of data. After you have the approval, there should be no problem getting the instruments reconfigured for new ranges. Call the FSDO and ask, It doesn't cost anything.

[jamesm]

I hadn't thought of that maybe I will give it a try and get a STC.

 

Thanks,

[N201MKTurbo]

32 minutes ago, jamesm said:

I hadn't thought of that maybe I will give it a try and get a STC.

 

Thanks,

Before you call, do your research, find the appropriate ACs and regs. They will be much more willing to help you if you have your ducks in a row before you call.  

[carusoam]

Don’t feel alone...

There are a few things Where redline is the operating point...

Mechanical things operate with oscillations...

Approaching the redline quickly, guarantees exceeding it...

 

Also don’t feel alone because it isn’t just an EI challenge...

JPI suffers the same pain...

 

During the T/O run MP and RPM are being pressed to the limit, oil P, and fuel P are increasing...

Things light up like X-mas trees during times of change...

 

But...

This is an artifact of adding simple digital technology on top of mechanical systems...

A more complex digital technology could average things over time... to help... or recognize the overshoot and ignore it temporarily...

See what Cies does to make mechanical devices digitally svelte... lots of data handling, averaging over time...

 

The last thing you would want to do... is change the mechanical device to make the gauge appear better... 

 

One of the cool things about MS.... we have an EI guy we can ask..!

Paging @oregon87 (question about alarms during high power T/O and other times of minor exceeds...)

PP thoughts only not an instrument technician...

Best regards,

-a-

[oregon87]

James-

As mentioned above and per the FAA, primary functions displayed on the CGR must be programmed with the POH limits as they supersede the engine manufacturer's manual.  We absolutely understand your frustration and will try to do everything within our ability to help.  Do you use the boost pump for start, takeoff and landing only?  Basically, is the boost pump only turned on conditionally?  I guess another way to ask; is the fuel pressure normal operating range under 6.0psi in cruise?  The CGR includes a delay feature for the external warning lights.  We could try to set a small delay and see if that helps your problem.  Worth noting, the delay is not conditional.  As such, if fuel pressure were to exceed the maximum allowable in flight, you would not get the external warning until after the delay period expired.  Alternatively, we could turn off the external annunciator entirely.  The fuel pressure indication would still change color if a limit was violated, but it would not trigger the annunciator lights.  

[carusoam]

While awaiting James’ return...

 

For some Mooneys, the electric fuel pump is used procedurally... and as needed...

1) Building pressure in the fuel system for engine start...

2) Sometimes used after a cold starts for a minute or two to allow the engine to warm enough to evaporate enough fuel...

3) Used for T/O and departure, usually off by 1k’ AGL...

4) Some Mooney Electric fuel pumps are automated, and FP isn’t always measured... (FF is used as a substitute)

5) During the landing procedure... in preparation for the Go Around...

 

PP thoughts only, not a mechanic, generalized Mooney ops only, not intended as an answer for James....

Best regards,

-a-

[jamesm]

I understand E.I. is obligated to comply with Air frame manufacture  fuel pressure specifications.

I would like  to make clear that I am NOT trying to bash on EI or JPI or  any other manufacture Engine monitor or any other  engine component manufacture product .

if I construed my post to create such  a reaction. I am sorry I hope there is no hard feelings. 

I don't want things to get out hand here. My intentions of my post was to see if any others M20 B,C,G i.e. Lycoming 0-360-A1D with  carbureted engine owners  experience same  or similar  fuel pressure readings when the electric fuel boost pump is on.  I was curious to see if other engine monitor  manufactures read same or similar values.

The fuel pressure indication is fine when the  electric fuel boost is off. I  turn on  the electric fuel boost pump on during starting, take offs , landings, and switching  fuel tanks.

I was pretty sure  before posting that  there wasn't any real way to adjust the fuel  pressure that didn't involve  breaking seals and other lock out devices. So I posted to be sure and that I wasn't over looking anything. Beside I would be extremely reluctant to change a fuel pressure setting  that has always worked.

My other point is that if I have to  explain to the  FAA check airman or CFI , CFII or equivalent or passenger   as why the red light is coming on. It is possible that could make or break my next rating or could be one contributing  factor  of failing  a check ride or their confidence in my airman ability. So If I have go into some long diatribe conversation as to why to  the Airframe says 6 psi  this and Engine manufacture says 8 psi.  I rather alleviate the  conversation all together by not having it. As N201MKTurbo suggested applying for a STC could  be a solution.

 

 

James

 

 

[carusoam]

All great detail, for good reasons, James!

 

You have a tough question to ask... you handled it perfectly...

We have the opportunity to ask the tough questions directly to the manufacturer some times... this often nets the best answers...

Where we have seen some strange fuel pressure behavior was with IO360s where some new pumps were going past redline... and not matching the Aircraft manufacturer’s requirement...

There are a few threads for that challenge and the solution was in there as well...

So if you have a new fuel pump that is causing this challenge... we can find those threads....

Best regards,

-a-

[cliffy]

OK folks Back to School time-

Mooneys have 2 types of fuel delivery to the engines. The 180 HP carbureted  engine and the fuel injected engines, They have different fuel pressure requirements. We have here the carb'd version-

With a carb'b engine the only function of either the engine driven fuel pump or the electric driven fuel pump is to supply the carb float bowl with a constant amount of fuel (under a relatively low pressure) so that as fuel is used out of the float bowl going to the engine it can be replaced as the float in the bowl drops lower and pulls open the needle and seat to allow the new fuel in to keep the fuel in the bowl at a constant height. Once that height is achieved the float moves up and the needle closes off the entry jet to stop more fuel from coming in and flooding the carb. This goes on constantly while running. In reality the needle sits a little off the seat all the time while running.  It doesn't take much pressure to do this as the engine uses fuel from the bowl and sucks it up into the throat of the carb and then into the engine. The pump pressure to the carb does NOT feed the engine directly as it does in a fuel injected engine. 

In a fuel injected engine high pressure fuel (@30 psi) is forced into a small orifice so as to spray into the cylinder right near the intake valve. The pressure varies  (controlled by the fuel servo) to provide the amount of fuel the engine needs. 

With the carb engine the actual pressure does not really have much affect on the running of the engine. 2 psi difference  really makes no difference to the engine or carb. You will find that as you go higher in altitude with the electric pump off your fuel pressure will drop (especially on a hot day)  and can drop down to 1 or 2 psi where upon you may need to run your electric pump in order to deliver enough fuel to the float bowl fast enough to keep the engine running smoothly. 

The question about how the pressure is perceived by others is something else. This is not an uncommon occurrence with carb engines.   

[yvesg]

There are so many threads about this issue... here are some:

 

I guess this might be generalized across the C fleet.... lets see, do we have any M20C that do not have this behavior? Please state your airframe year if you respond.

Yves