Erratic CGR-30P Manifold Pressure Readings

[Leo Langston]

My MP sensor is behaving erratically again and providing artificially low or zero reading even though the engine is running fine. My plane is a ‘49 Navion with the E-225 engine. This is the 3rd MP sensor that has been installed in my plane since the CHR was installed in April 2022. The first sensor became erratic and was replaced under warranty, the avionics shop replaced it and the replacement failed during the first test flight and was also replaced with the sensor currently installed. This sensor has worked fine until this past week when after takeoff and reducing power to stay in the pattern the MP reading dropped to zero. Thinking the MP line was blocked we blew it out with air. On my subsequent flight the MP readings were still erratic. I tried power cycling the instrument to no avail. Finally I did some WOT run ups by my hangar and was able to get the MP reading up to 26” at WOT and around 12-14 at idle. From the attached graphs you can see the MP reading slowly coming back up to nearly normal readings before I shut the engine down.
I have seen posts on other forums about erratic EI MP sensors. Is there a generic problem with these sensors? Some say it’s due to fuel getting into the sensor but how does that happen other than fuel vapor condensing in the line? The engine when running should suck out anything liquid in that MP line. My old analog gauge never had a problem.
Given this problem tends to occur in flight and not before takeoff what are the best ways to continue flight without a reliable MP reading. If staying in the pattern I can just leave the prop forward and fly like a fixed pitch prop but if I need to fly to my avionics shop for a replacement how would you judge power? By fuel flow?

thanks
Leo

 

[NordicDave]

Is there oil fouling back to the sensor? Some installations have a pinhole in the MAP pressure line to suck back fluids while not large enough to change the pressure value. Refer to EI's installation manual.

Where is the MAP sensor located? Hopefully behind the firewall on a stable mount?

Investigate the entire MP line for leakage using standard techniques. Hopefully a new AN hose from intake manifold to firewall and firewall to sensor was part of the install. Also the shop should have a mechanical MP gauge to test if it's the engine/hoses or to suspect the sensor.

Comparing MP to RPM is more helpful than EGT.

Power and fuel flow can be estimated via the POH flight performance tables using RPM.

 

[Leo Langston]

Not sure how oil would migrate back to the sensor. That little box is mounted just under the instrument panel. It’s just zip tied to other electric cables and the clear tube that come out of the sensor box is connected to the hard line running up to the firewall and then there is hose in the engine bay up to the intake manifold. When the first sensor failed they did find some sort of liquid in the clear line (red circle). I checked after this last incident and saw no liquid. I also replotted the graphs with MP against RPM for the last 2 flights. Everything on the first flight looks normal through the mag check and prop cycle and take off. The MP goes weird when I reduce power after I reach pattern alt on the downwind leg.

 

[jbrrapa]

Speak to Dave (as opposed to David) at EI. He is well aware of the problem.

 

[Leo Langston]

Speak to Dave (as opposed to David) at EI. He is well aware of the problem.

Any insight into exactly what they (EI) knows? Is this a common problem? I have Navion friends with the MVP-50 that have never had an issue with their MP. Is it a carb versus Fuel injection issue? I have sent EI a email describing the issue so I will see what their response is.

 

[Dan Thomas]

Is the sensor mounted directly to the engine or is it on the firewall and fed by a hose from the manifold? Vibration (and heat) can wreck sensitive stuff like this.

 

[Salty]

@Dave Arata

 

[Leo Langston]

My MP sensor is mounted inside the cabin up under the instrument panel. The original MP line from the engine to the old analog gauge was left in place and the shop just coupled the EI sensor’s clear tube to that old line. So any oil or fuel has several feet to cross to get up to the sensor.

 

[jbrrapa]

Any insight into exactly what they (EI) knows?

I have a twin and am now on my fifth and sixth MP sensors in the last year and a half. I made two trips from the Bahamas to Florida with no MP readings on either engine. The four failures occurred after the plane was parked in the Bahamas with very humid and wet conditions. EI was able to duplicate the problem by exposing new sensors to very high humidity. Dave told me the problem was in the "coating" and the last wo sensors he sent me have the appropriate revision. I don't know if the problem has been solved because the plane has not been parked outside since the last two sensors were installed.

I suggest you call Dave rather than wait for an email response. He is very knowledgeable and customer friendly.

 

[Leo Langston]

So just call EI and ask for Dave? Is there a last name?

 

[Salty]

@Dave Arata

 

[Dave Arata]

@jbrrapa I remember your situation well and am looking forward to a positive report upon your return from your next trip to the Bahamas.

@Leo Langston I received your e-mail and will be sending a reply shortly.

Regarding the question about how fluid can infiltrate the transducer, fuel can work its way though the tubing by way of capillary action. This is why many manufacturers include a small weep hold in the plumbing, using that small pressure to keep the fuel at bay. Alternatively, a service loop in the polyurethane tubing can have the same affect as the capillary action can struggle against gravity.