Another Item for your CI Checklist

[bjdecker]

For those of you who have an Omega PS-8G snubber installed in your MAP line, don't forget to clean it out every so often (e.g. at each oil change, Condition Inspection/Annual Inspection).

I encountered "odd" behavior of the MAP reading in my Garmin G3X Touch over the past few flights and while on the ground with the engine off. Readings were physically impossible unless I was in the eye of a hurricane or in a 1000ft deep hole in Death Valley.

Upon removing and inspecting the MAP transducer, I noticed that the MAP port in Van's manifold (VA-168) was loaded with engine oil, and the snubber was also clogged with engine oil. I removed the hose from the #3 cylinder primer port and flushed it with air. Cleaned out the manifold with brake cleaner, and replaced the snubber with a .008" orifice fitting instead of the Omega snubber.

Since I am not in the mood to move the pressure tap to the intake plenum/sump, I will add inspection/cleaning of the MAP line/snubber to my CI checklist.

Cheers,

Brian

 

[Freemasm]

For those of you who have an Omega PS-8G snubber installed in your MAP line, don't forget to clean it out every so often (e.g. at each oil change, Condition Inspection/Annual Inspection).

I encountered "odd" behavior of the MAP reading in my Garmin G3X Touch over the past few flights and while on the ground with the engine off. Readings were physically impossible unless I was in the eye of a hurricane or in a 1000ft deep hole in Death Valley.

Upon removing and inspecting the MAP transducer, I noticed that the MAP port in Van's manifold (VA-168) was loaded with engine oil, and the snubber was also clogged with engine oil. I removed the hose from the #3 cylinder primer port and flushed it with air. Cleaned out the manifold with brake cleaner, and replaced the snubber with a .008" orifice fitting instead of the Omega snubber.

Since I am not in the mood to move the pressure tap to the intake plenum/sump, I will add inspection/cleaning of the MAP line/snubber to my CI checklist.

Cheers,

Brian

Interesting. If the sensor is mounted high, I'm wondering how the oil got that far. Capillary action has it's limits. Wondering if a small leak at the sensor was a contributor. it's been removed so, will be interesting if you see it again.

 

[cajunwings]

MAP fitting clogging

My 1957 Piper Pacer has a tiny vent hole in the MAP line right at the low point where it comes off the cylinder. It is a very small air leak that allows any oil to get sucked into the cylinder. How much it affects the MAP reading I can’t say, but it’s never gotten clogged with oil.

Don Broussard
RV9 Rebuild in Progress
57 Pacer

 

[vic syracuse]

Some Pipers even have a push-to-drain valve on the MP gauge which you are supposed to activate when the engine is running. It sucks the oil back into the cylinder.

I recommend checking and cleaning these lines at every CI.

Vic

 

[bjdecker]

...But wait...

..Theres more...

I should have asked this question in the OP -- WHY is there oil/fuel residue in my manifold pressure line/snubber/sensor?

The line is connected to the primer port on the #3 jug -- this port sits right at the intake valve opening so it sees "schmutz" (oil, fuel) coming off the intake valve stem and intake manifold charge air.

Under all operating conditions (i.e. when the pistons are moving), there is a vacuum present at this port -- from a very little (full throttle, < 1inHg less than ambient) to a whole bunch (idle, ~11inHg < ambient).

So how does fluid work its way uphill for over 2 feet away and more than a foot higher in elevation? Is this from the pressure pulses from the intake valve setting up some kind of standing wave in the hose?

...mysterious happenings inside the intake manifold...

 

[bjdecker]

Interesting. If the sensor is mounted high, I'm wondering how the oil got that far. Capillary action has it's limits. Wondering if a small leak at the sensor was a contributor. it's been removed so, will be interesting if you see it again.

...A leak, especially farther downstream (or upstream) would cause the contents of the hose to be sucked back into the cylinder...

No?

 

[Stevea]

..Theres more...

I should have asked this question in the OP -- WHY is there oil/fuel residue in my manifold pressure line/snubber/sensor?

The line is connected to the primer port on the #3 jug -- this port sits right at the intake valve opening so it sees "schmutz" (oil, fuel) coming off the intake valve stem and intake manifold charge air.

Under all operating conditions (i.e. when the pistons are moving), there is a vacuum present at this port -- from a very little (full throttle, < 1inHg less than ambient) to a whole bunch (idle, ~11inHg < ambient).

So how does fluid work its way uphill for over 2 feet away and more than a foot higher in elevation? Is this from the pressure pulses from the intake valve setting up some kind of standing wave in the hose?

...mysterious happenings inside the intake manifold...

(not so) wild guess here.....fuel and oil are vaporizing in the intake manifold system, and re-condensing (after engine shut down/cool down) in the line near the manifold pressure line/snubber/sensor (and BTW everywhere else in the pressure sense line). If things are in vapor form, the vapor is going to float around anywhere it feels like in there.

Also, yes there is a vacuum in the line....but the entire sense line is at the same vacuum (except for the brief pressure transient that occurs when changing throttle position). So, there's very little (essentially zero) pressure differential induced flow in the line.

 

[Freemasm]

...A leak, especially farther downstream (or upstream) would cause the contents of the hose to be sucked back into the cylinder...

No?

Yeah. Trying to think of a transient situation. Was a reach and a swing/miss. Just couldn’t think of a scenario that would cause your condition. The line should have no flow per se’ so it the sensor is high and away, I’ve got nothing.

Edit = It would be interesting to see if this repeats itself in your next inspection after verifying the sense line connections are bubble tight.

 

[Mikeyb]

I don’t think this failure is the type of thing that warrants being added to a CI.

From a reliability centered maintenance point of view on a typical RV the risk assessment would show:
1) The probability of having a manifold pressure sensor blockage is low.
2) The risk of having a blocked manifold pressure sensor is low.

Removing and installing the line could do more harm than good.