Bad Fuel Pressure Sensor? Or bad main fuel pump?

[Saville]

Originally Posted by BillL

Ya can even use a small portable tank.

That's what I meant a couple of posts above when I said:

"Go from the NPT plug on the top of the manifold block to a gauge..."

[lr172]

I don't see how you can have a sensor issue, as it continues to deliver clear and consistent results in your tests. how can a sensor only give bad readings when you turn the boost pump off and provide correct readings when it is on?

Your pump is failing and it cannot deliver the volume it should or once did. Remember, a pump will only produce pressure when it's supply has exceeded demand. This is why you sometimes see lower pressure at higher fuel flows. Your pump is flowing enough to satisfy demand at full power, but not producing enough excess flow to produce a positive pressure (a bad situation, as your margin is gone). At lower power settings, supply is exceeding demand and you see pressure. In my opinion, your tests very clearly show a pump that is not producing enough flow.

Larry

[Saville]

Originally Posted by lr172

I don't see how you can have a sensor issue, as it continues to deliver clear and consistent results in your tests. how can a sensor only give bad readings when you turn the boost pump off and provide correct readings when it is on?



Larry

Perhaps because the lower pressure moves the sensor off the worn part as others have suggested.

Also, your analysis forgets that after a while the reading came on and stayed on at all power settings. Now it's true that could mean the pump is intermittently bad. But it can also mean other things are intermittently bad.

Also remember that the readout is behaving as if there is no signal... NOT zero. It's not displaying zero pressure. It's displaying no reading.


I'm not saying it isn't the fuel pump. But a simple gauge test will say whether the problem is upstream from the sensor (hose or pump) or downstream ( sensor, wiring EFIS).

[lr172]

Originally Posted by Saville

Perhaps because the lower pressure moves the sensor off the worn part as others have suggested.

Also, your analysis forgets that after a while the reading came on and stayed on at all power settings. Now it's true that could mean the pump is intermittently bad. But it can also mean other things are intermittently bad.

Also remember that the readout is behaving as if there is no signal... NOT zero. It's not displaying zero pressure. It's displaying no reading.


I'm not saying it isn't the fuel pump. But a simple gauge test will say whether the problem is upstream from the sensor (hose or pump) or downstream ( sensor, wiring EFIS).

Sorry, must have mis-read your initial post. Have you ever seen your pressure reading display 0? Are you sure that --- is not synonymous with 0?

[Saville]

Originally Posted by lr172

Sorry, must have mis-read your initial post. Have you ever seen your pressure reading display 0? Are you sure that --- is not synonymous with 0?

Firstly, remember that at startup, I have a reading - then sometime during the taxi to the run-up area, the reading goes to "---". Taxi MP is very very low. So if your theory was correct, I would expect the display to NOT be "---". But it is.

I have never seen it display 0.0...or 0.1 or 0.3. This is a very subtle but important distinction. I interpret this to mean either the input signal is grounded, or - possibly - it's open.

Here's why:

My EFIS GP FM is a TruTrak which is a couple of generations old. IT has a module called the Engine Data Module (EDM) which allows you to connect various signal lines to the EFIS display - such things as CHT's, EGT, Fuel Pressure, and RPM.

Important fact: There is ONLY ONE input to the EDM for RPM.

Important fact: The P-lead on the Left Magneto is directly connected to this input. Both P-leads go to the ignition key.

During run-up, when I check the left mag, I see 1800 on the display, as expected.

But when I check the right mag, the display stays at 1800 for about a half a second and then rapidly counts down to .....

you guessed it: "---"

As we know, when testing the right mag, the left P-lead is grounded.

Exact same behavior with the fuel pressure indicator...aux pump on and I get a reading; aux pump off and it hangs for a second then quickly ramps down.


Watch this video and you will see precisely what it does. Let me know if you cannot view the video.


The yellow band of the fuel pressure indicator is set from 0.0 psi to 1.0 psi.
The video begins with the aux pump on, then I turn it off - watch the needle and the numeric value. The needle disappears, and the numeric vale goes to "---".

Then I turn the aux pump back on - you see the needle re-appear and move up to 3.8 again.

I would HOPE that if ground means 0.0psi on the sensor that the EFIS would display 0.0.

[lr172]

I watched your video and also saw the fuel flow drop with the pressure. As it returns, the fuel flow goes well above normal, presumably because the carb bowl got low as flow was previously decreased below the engines requirements. This still points to an intermittently failing fuel pump. I suppose you could have some wide scale problem with your elms, affecting multiple sensors, but the odds don't support it.


I don't want to seem that I am arguing with, so will not post further.

Larry

[Saville]

Originally Posted by lr172

I don't want to seem that I am arguing with, so will not post further.

Larry

I don't see your posts as being argumentative. I appreciate all insights because I learn a lot that way.

[jdeas]

1. I bet the EFIS shows --- when the signal gets outside the expected range (such as zero)
2. A second instrument (FF) backs up the pressure sensor anomaly.

Real enough for me to buy a fuel pump before next flight....

[lr172]

Originally Posted by jdeas

2. A second instrument (FF) backs up the pressure sensor anomaly.

In my opinion, the behavior of the fuel pressure readings are not anomalous. They are relatively classic for a failing fuel pump. Yes, some intermittancy thrown in, but pretty classic. The fuel flow behavior fully backups up the diagnosis. I also believe that "---" means 0 or out of range.

Larry

[Saville]

With some very interesting results.

Quick reminder:

Last time out I was getting a fuel pressure reading at startup but by the time I got to the run up area it was "---"

So today I started up and taxied a very short distance to a place where I could run up.

At startup, I got an FP reading of 4.8psi.

Taxied over to the area - 4.8psi

After a little warmup I went to 1800rpm as if I was doing a runup

4.8psi.

So I watched and waited. As things warmed up I could detect a slight wavering of the FP. it would bobble between 4.7 and 4.8......

Things got warmer and the FP started to ever so slowly drop.

The warmer the engine got, the faster the FP went down. At around 300 CHT you could see it winding down though not as fast as the movie up above in an earlier post.

The warmer the engine got...the faster the FP dropped. It was approx around 328 where the FP got to 1.9......

then 1.8.............

then "---" in a matter of 2 seconds. And it stayed that way.

Pulled the power back to idle which was about 9" Hg:

stayed at "---"

MP didn't matter - heat did.

Ok so I taxied back to the hangar and shut down. A few moments after the engine shut down I flipped the EFIS back on and saw this:



Well whaddya know.......Note the oil pressure dial on the lower left - 0psi (as we would expect). The fuel pressure? "---"

The EFIS is perfectly capable of displaying 0 or 0.2 or 1.1. Therefore "---" DOES NOT mean zero. Also recall that in all experiments, the EFIS never displayed a FP of less than 1.8....it would drop to 1.8 or 1.9 and then "---"

So this test has shown 2 things:

1) Whatever the problem is - it's heat related.

Twice, now, I noted an FP reading at startup but not after the engine had time to warm up.

I now think the reason the FP display was giving me a reading as I was letting down, on my last flight, was because the engine was cooler than cruise.

I wish I had the presence of mind to flip onthe EFIS before I started to see of the FP read zero. I will do that at the next test.

2) "---" does not mean zero. And the fact that the FP display will never show less than 1.8 but always goes to "---" means something.