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Fuel System OverPressure 912IS

  • Paul Depperschmidt
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2 months 1 week ago #17445 by Paul Depperschmidt
Paul Depperschmidt created the topic: Fuel System OverPressure 912IS
First significant post after purchasing an S7 w 912iS (N52LR) a few months ago. I have been putting some good flight time in and getting comfortable with the plane. Love it so far. I have been flying since 79 with time in many planes, but primarily an SR22 I sold a few months ago. I really like this kind of flying.

A couple of weeks ago shortly after takeoff I got a fuel high pressure warning. At the time I had the main pump running and full power. I throttled back and it bumped up to the red and the Lane A light came on solid. Shortly after the Dynon suggested to land the plane. I was not far from the runway and landed without incident.

I was able to communicate with the builder and review the fuel system. The filter system is relatively new based on the recommendation from Kitfox/Rotax. It was installed in March of this year and has somewhere near 30 hrs on it.

I found what looked like an oil drip coming from the fuel pressure sensor. I was able to pull both fuel filters. It was kind of a mess, could not figure out how to drain the system to keep everything from leaking when disconnected. I let the large post pump filter drain as best I could and captured the fuel. There was some very fine particles suspended in the fuel. I lightly blew in the end and it held pressure for a couple of seconds then gave way. It seemed clear after that. I could not see a way to open the filter. When pulling the pre-pump filter it looked pretty clear.

I put everything back together and checked the pumps. Everything seemed to be fine. The pressure holds around 44-45. I did a full power start / roll out and pulled the cowl. No leaks. Then took it up in the pattern for 20 minutes. All good.

Last Saturday I changed the oil, filter and the fuel filter. I had purchased 3 replacement fuel filters. The original filter seemed to be working OK, but it had a problem prior, so just to be safe (maintenance induced problem coming) I replaced it. The pressure on pump 1 was in the 45 range. Seemed a little high, but in the green. The 2nd pump would put the system in yellow. I flew the plane for a while, running on the single pump one. It ran fine.

Last week I took my wife to Fredricksburg for lunch. All was well on the outbound trip. But 10 minutes in to the return the fuel pressure was in and out of yellow. Then it went red and Lane A light and the "land the plane" warning came on. I had to continually back off of the throttle to keep the RPM's down. I was close enough to my home airport to press on while keeping safety airports in range. I was able to land without a problem and on the ground turned the engine off and back on. The fault had cleared and the fuel pressure was back down to 44.5 or so. I noticed that using the pump 2 rather than 1 brought the pressure down to the middle range. Both pumps on would hit the yellow.

So... I am thinking the filter I had originally been running in the plane may have failed allowing the pressure to go down. The new one seems to just be too fine to allow proper flow. Yes, I could have accumulated trash in that short of time (1hr). But considering I effectively flushed the system multiple times by taking it apart, does not seem likely.

I could just run on pump 2 at the lower pressure and likely be OK. But this just does not seem to be working right. As a safety measure the fuel filter seems to be causing in-flight problems more than solving them. Any one else seeing anything like this? Suggestions?

Thanks in advance for any help and support.

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2 months 1 week ago #17447 by Bill Hertzel
Bill Hertzel replied the topic: Fuel System OverPressure 912IS
How are you measuring the fuel pressure?
Do you have a Differential sensor or is it calculated by the EMS from the MAP pressure?
What pressure do you see before engine start?
What pressure do you see at Idle on the ground?
What pressure is YELLOW?
What Pressure is RED?

I know! A lot of questions.
But without the actual numbers, we can only guess what you are looking at.

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2 months 1 week ago #17448 by Paul Depperschmidt
Paul Depperschmidt replied the topic: Fuel System OverPressure 912IS

Bill Hertzel wrote: How are you measuring the fuel pressure?
Do you have a Differential sensor or is it calculated by the EMS from the MAP pressure?
What pressure do you see before engine start?
What pressure do you see at Idle on the ground?
What pressure is YELLOW?
What Pressure is RED?

I know! A lot of questions.
But without the actual numbers, we can only guess what you are looking at.


Hi Bill,

Thanks for the questions! Let's see how far I get.
There is a pressure sensor set up post pump and prior to the fine filter. The filter is the Bosch - F5021 I got from Kitfox
Will have to check on before engine start. Never actually seen it as the Dynon is still booting up at start usually. Might change that from now on.
AT idle it would be in the 44 psi range except when I changed the filter where it was 45+
Will need to check exactly for yellow level - I think it is 46
And will need to check specific on red level -

Just considered that the alarms on the Dynon can be set. What are the expected levels post pump and pre-filter? Maybe it is just set too low?

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2 months 1 week ago #17450 by Bill Hertzel
Bill Hertzel replied the topic: Fuel System OverPressure 912IS
The Manual calls for 3.0 +- 0.2 Bar on the Fuel Rail.
That is ~40-47 psi nominal. Your engine's reference point could be anywhere within that range.

I think you are asking too much of the regulator.
What the pressure is, is not as significant as that it holds relatively steady.

A 1-2 psi variation between pumps is insignificant.
The pressure sensor is placed between the filters as a diagnostic device.
If the pressure goes UP significantly, it indicates that the second filter is clogging.
If the Pressure goes Down significantly, the first filter or the pumps need a look.

The pumps have a lot of excess capacity until you get into real trouble.
If the second filter were extremely clogged, the pumps have the capacity to push 4 times the needed fuel at >90 psi and the fuel rail would still be at 3 bar.
The filter could present a 50 psi restriction and the engine would still not miss a beat.
The Pressure gauge might indicate 100+psi, but the regulator would still be happy at 45.
It would be time to land without delay but making the airport would not be a problem.

If you typically see a 1-2psi restriction across the Second filter,
48psi would be a reasonable YELLOW alarm point with 50psi as a RED.
46psi is just not giving you any wiggle room.

Get used to what you see on a regular basis and then don't worry about it until something changes significantly.
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2 months 1 week ago #17459 by Paul Depperschmidt
Paul Depperschmidt replied the topic: Fuel System OverPressure 912IS
Thanks for your help with this Bill,

I thought I had the answer yesterday by adjusting the limits in the Dynon yesterday to your suggestions. But unfortunately it looks like I have something else going on.

I took the plane up to check everything and it was all fine for about 20 minutes of pattern work and so I started out to explore locally. With both pumps on the pressure was in the 45 range. A couple minutes after leveling in to cruise with an RPM of 5300 and fuel pressure of 45 I felt a surge in the engine RPM. I looked down and the RPM had yellow lined and the fuel pressure went to 47. The Lane A light came on and the Dynon warning to land came on. I did not see the pressure hit 50, but it might have for a second. I had to continually pull the throttle back to keep from RPM running up. The pressure backed down to 45 with pump 2 on only. This was exactly the same as before I adjusted the settings the previous week.

This new information had me pull the cowl and take a look at everything. I specifically looked at the throttle linkage. Everything looked OK. As I worked the throttle the action was not smooth at first. The wire seemed to bunch up a bit and then snap to full open. I worked that a few times and did not see it again.

I am hoping this is a simple, mechanical thing. Of the big 3 / fuel / air / spark, it still seems to be the fuel. But I am not sure what to investigate at this point.

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2 months 1 week ago #17461 by Bill Hertzel
Bill Hertzel replied the topic: Fuel System OverPressure 912IS
You never did answer the, "How is the Pressure Measured" Question.

If you have a "Gauge" type sender instead of a "Differential" Sender, When the Manifold Pressure suddenly changed as the throttle cable jumped, you could get a situation where the Dynon read the Fuel Pressure before the Throttle change and the Manifold pressure after the change.
It would then calculate a bogus number and sound the alarm.
What was the reason associated with the Dynon "Land Now" recommendation?
"Land Now, Because....??? "

Have you downloaded the Rotax EMS log to see why the Lane "A" light came ON?

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2 months 1 week ago #17481 by Paul Depperschmidt
Paul Depperschmidt replied the topic: Fuel System OverPressure 912IS
Apologies for the delay in reply. International work travel is slowing me down.

I did not build it, but I think the sender is a gauge type. It is positioned after the pumps and prior to the fine filter. It is in a position to allow the bypass valve to work in case of an over-pressure. The Dynon gauge is looking at Fuel Pressure only as far as I know. If it is using the MAP, I am not aware of it.

Unfortunately the Dynon provided no additional information other than "land the plane". I have not used the download engine function yet. I used to do it on the Cirrus all the time. Looks like I need to figure that out.

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2 months 1 week ago #17498 by Bill Hertzel
Bill Hertzel replied the topic: Fuel System OverPressure 912IS
If the sender has ONLY one connection to the Fuel line, it is a "Gauge" pressure Sender.
If the sender has one connection to the Fuel line, AND a smaller Vacuum Line going to the intake Manifold, it is a "Differential" pressure Sender.
If you have a Gauge Style Sender and it is Not being corrected for MAP pressure by the Dynon, you will be seeing pressures in the middle 30psi range at idle.

If the Dynon is calculating the Differential Pressure it may give intermittent erroneous readings during rapid throttle changes as it collects and recalculates the updated data from multiple sources with various update delays.
Change the throttle position and give it a second for the display to stabilize.

The fuel pressure system is extremely reliable. When it goes bad it is going to go terribly bad.
It can go Plus or Minus 10-15 psi before the engine starts losing power.

You may want to open the alarm window even wider to prevent you getting used to alarms that you will keep ignoring.
Determine a nominal number for your individual engine and give the alarm window 5-7psi of wiggle room.
You do not want to get used to it "Crying Wolf!"

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2 months 1 week ago #17509 by Paul Depperschmidt
Paul Depperschmidt replied the topic: Fuel System OverPressure 912IS
Thanks again Bill,

Is there an absolute not to exceed number for the fuel system, assuming it is a gauge type? It would seem possible to simply set the red level to that number? Or is it system specific?

I would post a picture, but can't figure it out in this system.

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2 months 6 days ago #17513 by RotaxOwner Admin
RotaxOwner Admin replied the topic: Fuel System OverPressure 912IS
Use the Action button just below a post, and select Reply. This opens up the full message editor that lets you add attachments and format text.

(The "Quick reply" box at the bottom is just to jot down some quick lines of text...)

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1 month 3 weeks ago #17587 by Paul Depperschmidt
Paul Depperschmidt replied the topic: Fuel System OverPressure 912IS
I thought I had things fixed. I took the fuel system apart and blew in the filter with minimal restriction. Put it all together and flew for around 3hrs. One pump w pressures a little over 44. With multiple stops/ starts. On my last short flight when leveling for cruise the rpms started running up on their own, the fuel pressure went up and the red “land the plane” warning came on. As usual when I land and restart the engine is fine.

The higher revs came up suddenly. It would seem that a fuel pressure increase due to blockage would not cause higher revs. If anything the revs would fall due to a lack of fuel flow. If something blocked an injector, would it cause this? No EGT issues seen, so not thinking it is likely. Maybe throttle linkage?

For curious minds have included images. And I cut the old filter to see what was i it. Pretty basic.

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1 month 3 weeks ago - 1 month 3 weeks ago #17596 by Bill Hertzel
Bill Hertzel replied the topic: Fuel System OverPressure 912IS
Are you getting a "Lane" warning light from the engine?
Download the logs from the Dynon and the engine ECM.
Look at the data closely. The cause of the problem is in there.

When the issue occurs; have you tried cycling the throttle through its full range in flight?
Or do you just throttle back and land?
What happens to the fuel pressure when you throttle back?

Was the fuel filter replaced BEFORE last flight?
The fuel pressure sensor is measuring the pressure in the fuel line before the fine filter.
Its purpose is to give you an ear warning of a potential problem.
This does Not mean the fuel pressure in the fuel rail of the engine has changed.

Just asking the obvious...
In the Fuel filter picture; The fuel comes OUT the Port shown in the picture.
Fuel flows from the outside to the inside within the filter.
It is installed correctly.

You had a previous issue with the throttle cable binding.
It was never actually resolved, the problem just disappeared. It may be returning.
Throttle UP is produced by giving slack to the cable and having the throttle spring take out the slack by opening the throttle.
If there is a delay in the cable being re-tensioned, The throttle may open a little more at a later time.
Last Edit: 1 month 3 weeks ago by Bill Hertzel.

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1 month 3 weeks ago #17602 by Paul Depperschmidt
Paul Depperschmidt replied the topic: Fuel System OverPressure 912IS

Bill Hertzel wrote: Are you getting a "Lane" warning light from the engine?
Download the logs from the Dynon and the engine ECM.
Look at the data closely. The cause of the problem is in there.

Yes, I am getting Lane A warning light when it happens. I will figure out how to get that information over the holidays.

Bill Hertzel wrote: When the issue occurs; have you tried cycling the throttle through its full range in flight?
Or do you just throttle back and land?
What happens to the fuel pressure when you throttle back?

I did not cycle to idle other than to land. But I did cycle it. And it seems the rpm's keep creeping up and I have to back off the throttle. After the initial run up the fuel pressure settles down to normal. About 44psi.

Bill Hertzel wrote: Was the fuel filter replaced BEFORE last flight?
The fuel pressure sensor is measuring the pressure in the fuel line before the fine filter.
Its purpose is to give you an ear warning of a potential problem.
This does Not mean the fuel pressure in the fuel rail of the engine has changed.

Yes, the fuel filter had been changed. I pulled the system apart and checked it prior to the flight but did not change the filter. I did not think the fuel pressure prior to the fine filter could change the pressure at the rails (other than starving them, which is not the case here), so you confirmed that.

Bill Hertzel wrote: Just asking the obvious...
In the Fuel filter picture; The fuel comes OUT the Port shown in the picture.
Fuel flows from the outside to the inside within the filter.
It is installed correctly.

Fuel comes from the pump to the manifold holding the gauge. From there it normally goes to the fine filter and then to the rails unless the pressure forces the safety valve open. In which case it will bypass the fine filter.

Bill Hertzel wrote: You had a previous issue with the throttle cable binding.
It was never actually resolved, the problem just disappeared. It may be returning.
Throttle UP is produced by giving slack to the cable and having the throttle spring take out the slack by opening the throttle.
If there is a delay in the cable being re-tensioned, The throttle may open a little more at a later time.

What you described is what it feels like is happening. The RPM rev is sudden and after I have settled the throttle. The friction knob is snug enough to not allow movement there. And a couple of times when the event happened my hand has been on it and it did not move. It feels like the throttle cable is bunching up right after the sleeve and after a few minutes it snaps out. I am guessing that would potentially mess with the fuel pressure gauge and start the alarms. Then everything settles out. And when the engine is restarted all is well. It just does not seem like fuel filter blockage to me. I am leaning to the throttle linkage as the issue.

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1 month 2 weeks ago #17685 by Paul Depperschmidt
Paul Depperschmidt replied the topic: Fuel System OverPressure 912IS
I think we may have found it. In the Kitfox forums a builder was seeing the bunching up of the throttle cable. The short story seems to be an over torque on the nut securing the throttle cable. It has to move freely or it will bunch up. Based on the evidence I think this is the culprit on my Kitfox. I will check it this week. Some pictures will help explain.

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