Erratic MP readings - GRT

[Tankerpilot75]

Recently I had to replace my MP sensor. I’m now starting to experience erratic MP readings, particularly when I change power, but also at a steady power setting. When this happened several years ago it was determined that a loose hose connection was the likely culprit. I trimmed a few inches off the MP hose and reconnected it to the sensor and this significantly improved my MP readings.

It’s now erratic again and I plan on doing the same thing and trim off a few inches of the MP hose just prior to the sensor and reconnect it to the sensor. This time I’m also going to secure the hose on the sensor with a small spring hose clamp to hopefully prevent future leaks. I will need to be careful and not add any stress to the connection because the sensor connection is plastic and I don’t want to break it.

Here’s my question: a friend did a little research and discovered an old Dynon website posting that suggested installing a miniature oriface restrictor to settle down erratic MP readings. Has anyone done this and was it helpful?

 

[Ron B.]

Yes and yes. I put .040” orifice in mine.

 

[texdog]

Manifold

Yes it works with a restriction. I used a rivet with a .40 hole drilled in it. It has worked for 11 years in a 0-360.

 

[Bavafa]

As others have already said, you need one of these to dampen the flow. You can make one easy based on the above instructions.

https://www.aircraftspruce.com/catalog/eppages/oilfuelflow05-11908.php

 

[abuura]

Happened several years ago. The hose off the engine goes to a firewall fitting, then in the avionics area to a tee, which connects to the GRT MP sensor and to the LSE ignition box. On very close inspection I found that there was a crack in the tee. Replaced it, problem went away.

 

[petersb]

Yes it works with a restriction. I used a rivet with a .40 hole drilled in it. It has worked for 11 years in a 0-360.

Have 360 with GRT HXr, early model.

Did this, fitting top of cylinder #3 with very tiny hole in rivet, but still have fluctuations.

Also have fluctuations with my fuel flow , red cube between mechanical pump and carb, installed level on fire wall with in and out fittings at 45 degrees.

My friend has similar setup with 320 and Dynon Classic , fuel flow and manifold pressures displays steady.

Was wondering if dynon buffers the inputs and averages readings to make them display steady.

 

[lr172]

did this change when you installed the new sensor or some time after that. I have snubbers on both of my planes to tame the readings. However, I would still investigate for a leak to be sure you don't have one.

 

[Tankerpilot75]

did this change when you installed the new sensor or some time after that. I have snubbers on both of my planes to tame the readings. However, I would still investigate for a leak to be sure you don't have one.

It’s always bounced around a little but after replacing the MP sensor the erratic behavior has increased. As I said above I do think it’s probably leaking a little at the hose connection to the sensor and plan to tighten that up by trimming the hose back an inch or two to provide a tighter seal and installing a spring hose clamp on the connection.

But I suspect (based on prior experience) the readings will still be a little erratic and want to settle things down more if possible. I just reread the GRT MP sensor installation guide and it does suggest installation of a restrictor in the MP vacuum line near the engine to eliminate MP fluctuations.

 

[lr172]

It’s always bounced around a little but after replacing the MP sensor the erratic behavior has increased. As I said above I do think it’s probably leaking a little at the hose connection to the sensor and plan to tighten that up by trimming the hose back an inch or two to provide a tighter seal and installing a spring hose clamp on the connection.

But I suspect (based on prior experience) the readings will still be a little erratic and want to settle things down more if possible. I just reread the GRT MP sensor installation guide and it does suggest installation of a restrictor in the MP vacuum line near the engine to eliminate MP fluctuations.

Yes, it is likely the old sensor was a bit flawed and didn't respond as quickly as the new one did and why you noticed the change upon replacement. What you see now is normal and a restrictor will tame it.

 

[Finley Atherton]

I had fluctuating MP on my MGL Xtreme. Restrictor did not help. The Omega PS-8G snubber stopped the fluctuations.
Fin.
9A

 

[Richard Connell]

I had fluctuating MP on my MGL Xtreme. Restrictor did not help. The only thing that stopped the fluctuations was the Omega PS-8G pressure snubber.
Fin.
9A

I have a PS-8G snubber on my MAP transducer as well.
Unfortunately it appears to act as a partial one way valve as well and therefore overreads MAP significantly at cruise power settings.
Without it, it’s too erratic. So I was thinking of removing it and replacing it with an orifice type restrictor.
Seems I’m out of luck no matter what I try!

 

[Tankerpilot75]

I have a PS-8G snubber on my MAP transducer as well.
Unfortunately it appears to act as a partial one way valve as well and therefore overreads MAP significantly at cruise power settings.
Without it, it’s too erratic. So I was thinking of removing it and replacing it with an orifice type restrictor.
Seems I’m out of luck no matter what I try!

I found this restrictor on the internet and placed an order yesterday. I plan on installing one next time I go to the airport but, unfortunately, that will likely be a couple weeks out due to weather and my own availability.

BTW, shipping cost three times the price of the small part!

https://www.universalpowerconversion.com/miniature-inline-orifice-restrictor-p-7419.html

I did reread the GRT MP Installation instructions and in the notes it suggested putting an orifice (restrictor) on the engine side near where the hose connects to the vacuum source.

 

[Sl2trike]

Fuel filter

I have an MGL panel and installing a small fuel filter inline to the RDAC eliminated the pulsing and erratic readings. My MAP is now rock solid and replaced at every CI. The best $5 investment for my 6.

 

[petersb]

I have an MGL panel and installing a small fuel filter inline to the RDAC eliminated the pulsing and erratic readings. My MAP is now rock solid and replaced at every CI. The best $5 investment for my 6.

I like that...I do have a .040 restriction but it did not help. Im assuming the filter material would buffer out the pulses

 

[Finley Atherton]

I like that...I do have a .040 restriction but it did not help. Im assuming the filter material would buffer out the pulses

It is likely that the air volume of the filter also helps damp the pulses. Before using the Omega snubber I temporarily added about 10 ft of 1/4” ID tube to the MP line which greatly reduced the fluctuations.
Fin
9A

 

[petersb]

It is likely that the air volume of the filter also helps damp the pulses. Before using the Omega snubber I temporarily added about 10 ft of 1/4” ID tube to the MP line which greatly reduced the fluctuations.
Fin
9A

Thanks, will install it tomorrow

 

[ljpappas]

Fluctuating MAP radings

This happened to me back in 2006 when I installed my first Dynon system.

I called Dynon and was told by tech support, (believe it or not), to break a filter off of a cigarette, remove the brown paper wrapper, and insert the white filter material in the tube near the MAP sensor. Whatever! I am not a smoker so I had to source a single cigarette from someone that did.

Well, I did as told and it immediately resolved the problem. I put about 300 hours on that plane, and as far as I know, the filter is still in that tube. Go figure!

I am guessing a more "aviation" solution exists by now.

 

[Tankerpilot75]

I installed a .04 restrictor in line before the sensor. I also put some tension clamps on the hoses on top of the barbs where the hose connects to the sensor and the newly installed restrictor to ensure tight connections. NO CHANGE! The readings still jump around. I’m at a loss.

Also it seems my MP readings are high with this new sensor. I did update my EIS settings for the new sensor.

Today’s takeoff MP indicated 30.8” (Runway alt 1,450; Altimeter setting 30.27; temperature 55*; IO360, 2670 RPM C/S prop). This seems high MP to me. What MP readings are you seeing on your TO run?

Any new suggestions on the erratic MP readings?

 

[lr172]

I installed a .04 restrictor in line before the sensor. I also put some tension clamps on the hoses on top of the barbs where the hose connects to the sensor and the newly installed restrictor to ensure tight connections. NO CHANGE! The readings still jump around. I’m at a loss.

Also it seems my MP readings are high with this new sensor. I did update my EIS settings for the new sensor.

Today’s takeoff MP indicated 30.8” (Runway alt 1,450; Altimeter setting 30.27; temperature 55*; IO360, 2670 RPM C/S prop). This seems high MP to me. What MP readings are you seeing on your TO run?

Any new suggestions on the erratic MP readings?

an .04 is not enough. On my 6, I tapped the base of the servo for MP source and is rock solid. On the 10, I think that I used an .008 restrictor. This does produce a small lag in reading changes, but not significant. Can get them from mcmaster carr. When I added a custom EFI to the old porsche, I made a small plenum in the line out of PVC pipe. This also worked. I am not a fan of the cigarette filter approach.