Bad Fuel Pressure Sensor? Or bad main fuel pump?

[lr172]

Originally Posted by 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.

This test further supports a failed fuel pump. It's capacity is reduced with increased engine heat. Rubber becomes more flexible as temp increase and the slit/tear/hole allows more fuel through (i.e. less flow/pressure out of the pump) as it warms. I just can't see a likely scenario where a sensor consistently changes it's behavior based upon temp. The sensor isn't mounted on the engine and will heat much slower than the engine itself.

You really need to work with the probabilities here. Troubleshooting rarely produces 100% definitive actions. You have to follow the path that has the greatest probability and in your situation, I feel that path is clear. Either way, it's your engine. Let us know how you finally resolve this. Maybe we'll all learn something.

Larry

[Steve Iacoviello]

No reading means bad sensor. I had those same sensors in my RV and was advised by Dynon to change them as they do not last and are unreliable. My opinion, change the sensor not the fuel pump.

[Sam Buchanan]

I'm having a hard time figuring out all the dilly dallying around. Fuel delivery problems account for most engine outages. If this was my airplane (and it was a few years ago) I would (did) replace the fuel pump with no second thoughts (my plane also got a rebuilt carb). If the problem still existed, then the sender would be replaced. That would yield a 'new' fuel delivery system that should work fine for several more years without all the protracted hand wringing about how an EFIS works.

Fuel pumps wear out, consider it an item that needs periodic replacement. Replace it and the sender before they put you in a farmer's field somewhere in the middle of nowhere with a bent airplane.

[Lenny Iszak]

It may come in handy to know how the Efis works for that exciting moment when the oil pressure sender fails and there's no backup to just switch it on, like with the fuel pump, and you're not exactly sure if you have 5 more seconds before a piston comes through the cowling or it's just a joke courtesy of VDO.

Btw, also consider that the Efis doesn't show instant pressure values but averages over a few seconds, that's why it comes down gradually.

Now i'm too really curious what this ends up being. So please do let us know.

Either way, no need to be annoyed by differing opinions. We're all learning.

Lenny

[Saville]

Originally Posted by Sam Buchanan

Fuel pumps wear out, consider it an item that needs periodic replacement.

After 152 hours???

[Steve Iacoviello]

Originally Posted by Sam Buchanan

I'm having a hard time figuring out all the dilly dallying around. Fuel delivery problems account for most engine outages. If this was my airplane (and it was a few years ago) I would (did) replace the fuel pump with no second thoughts (my plane also got a rebuilt carb). If the problem still existed, then the sender would be replaced. That would yield a 'new' fuel delivery system that should work fine for several more years without all the protracted hand wringing about how an EFIS works.

Fuel pumps wear out, consider it an item that needs periodic replacement. Replace it and the sender before they put you in a farmer's field somewhere in the middle of nowhere with a bent airplane.

Sam, the sensor is bad as evidenced by the "blank" reading. With a good sensor installed one can determine if the fuel pump needs changing.

[lr172]

Originally Posted by Steve Iacoviello

Sam, the sensor is bad as evidenced by the "blank" reading. With a good sensor installed one can determine if the fuel pump needs changing.

Where did you come up with that? The OP has posted numerous tests with readings other than blank. You're dealing with intermittent pressure readings here that, in theory, could point to either the sensor or the pump.

Regardless of who you believe here, I think most will tell you that flying this plane without replacing the pump, just isn't smart given the risks.

Larry

[lr172]

Originally Posted by Saville

After 152 hours???

It is a sandwich of rubber slices. Someone posted a teardown of a pump. I encourage you to look at his pictures. LOTS of small rubber parts. failure can happen at any time. It is not just a wear issue. I just had a pump fail with only 400 hours on it.

Larry

[Steve Iacoviello]

I came up with that by looking at my own RV with a new Lycoming. Now at 450hrs. The fuel pressure moves around with temperature, and rpm but always well above the minimum of 0.5 psi required by the engine. What I do know is that sensor was junk. Much more stable readings when I accepted Dynons advise and replaced it with a Klavico.

[Saville]

Originally Posted by lr172

Where did you come up with that? The OP has posted numerous tests with readings other than blank. You're dealing with intermittent pressure readings here that, in theory, could point to either the sensor or the pump.

We now know there's a problem with the sensor/EFIS because that a reading is flat out wrong. It should display zero, not "---". It should continue to count down to zero below 1.8..I should see 1.7, 1.5 1.2 0.9 etc. and then zero. I do not see that.

That is not right.

It doesn't mean there's not a problem with the fuel pump. But I think today's tests show there's a sensor or wiring problem.

We also know that the readings come and go with heat. So the "intermittency" has to do with heat. What the heat is affecting has yet to be determined. I know what you THINK. But I'm going to determine that.



Regardless of who you believe here, I think most will tell you that flying this plane without replacing the pump, just isn't smart given the risks.

Who said anything about flying the plane before finding the exact problem and fixing it? I knew I could cause the problem to occur on the ground, as I showed this morning, mainly because of the failure during the taxi to the run up area.

The only difference between my position and yours (and others) is that I'd like to actually determine where the problem is instead of just starting to replace expensive parts and keep replacing parts until the problem goes away.

Perhaps you are thinking that my resistance to diving in right now and replacing the fuel pump is that I don't believe the problem is with the pump.

That's not it.

My way is to figure out what the problem *IS* and then fix it. And I never said it couldn't possibly be the pump.
.

Larry

Ten more characters so that I can make the post