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Manifold Pressure Issue

R

rdg

Member
I have been experiencing an issue with sensing manifold pressure. RV-8A with IO-360-M1B Lycoming, constant speed prop, Garmin G3X display configured by SteinAir with the appropriate Garmin engine display feed. Airplane has 6.4 hours in flight with about 13 total hours on the engine. Shortly after takeoff and during climb out my manifold pressure stopped indicating. At the time I was trying to set climb throttle/prop setting. I started to return to the field. During return, the manifold pressure indication returned, but stopped a short time later. As you can tell I was able to return safely.
When I removed the cowling, to check on the pressure line from the engine to the sensor and the wiring, all checked out. I turned on the G3X and I had pressure indication. I started the engine and ran through power changes and the indication worked as expected. I replaced the cowling and started the engine to do a taxi test. The indication quit mid taxi and I returned to my hanger.
My next thought was to replace the sensor which I did. I continued to have the same issue; pressure indications with the cowling off and no indications with the cowling on. I am at the end of my thoughts and possible fixes to this problem. I am reaching out to see if anybody has any suggestions as to what to check next. Any suggestions will be greatly appreciated. Would it be worth a call to Lycoming?
 
rdg said:
I have been experiencing an issue with sensing manifold pressure. RV-8A with IO-360-M1B Lycoming, constant speed prop, Garmin G3X display configured by SteinAir with the appropriate Garmin engine display feed. Airplane has 6.4 hours in flight with about 13 total hours on the engine. Shortly after takeoff and during climb out my manifold pressure stopped indicating. At the time I was trying to set climb throttle/prop setting. I started to return to the field. During return, the manifold pressure indication returned, but stopped a short time later. As you can tell I was able to return safely.
When I removed the cowling, to check on the pressure line from the engine to the sensor and the wiring, all checked out. I turned on the G3X and I had pressure indication. I started the engine and ran through power changes and the indication worked as expected. I replaced the cowling and started the engine to do a taxi test. The indication quit mid taxi and I returned to my hanger.
My next thought was to replace the sensor which I did. I continued to have the same issue; pressure indications with the cowling off and no indications with the cowling on. I am at the end of my thoughts and possible fixes to this problem. I am reaching out to see if anybody has any suggestions as to what to check next. Any suggestions will be greatly appreciated. Would it be worth a call to Lycoming?
Click to expand...

Check for a bad wiring connection or loose ground.

Larry
 
Out of curiosity, where are you picking up your MP? I'm picking mine up from the primer port on #4 through a restrictor to the sensor manifold on the firewall.

I agree with the other posters, look for a bad electrical connection as the first culprit. Did the MP peg at some value (perhaps to do a clog in line) or did it show no indication (with a big red X on the G3x)?
 
Mine did same. The wiring from the MP sensor is pretty big maybe 16ga and the wiring in the shielded cable connected to it is 22ga in my setup. When I did a solder splice it didn?t properly set on one connection. It was loose. Fixed it 40 hours ago no problems now.
 
I had a similar problem. Turned out to be a longitudinal crack in the hose from the intake port area to the transmitter. During initial start up, all was normal, but eventually the side wall of the hose would collapse into the interior of the hose causing a leak.
 
Hose?

I had a cracked hose also. I would sometimes get a reading and then it would quit. I replaced the gauge, sender and finally the hose.
 
What's the difference between in the hangar and in-flight?

Have you considered that when you apply power the front of the engine moves slightly? You need to do what's called the Wiggle test. Have someone sit in the cockpit while you wiggle the harness starting from the front, working your way back to see if the display cuts out.
 
I do not have a restrictor in the MP line. I am taking the MP as provided in the firewall forward instructions from the #3 cylinder. I will check out the hose. I have looked at the wiring to see if there was a loose or bad connection.
 
Last edited:
bkervaski said:
My MP was all over the place before I installed a restrictor. Cheap and easy to rule out as an issue.
Click to expand...

Sure, a restrictor will eliminate the oscillations, which are typically only a couple of inches. The OP is getting no reading, not oscillations and therefore the fix for him is not a restrictor. a restictor is not required to get an MP reading that is reasonably accurate.

Larry
 
Problem Solved

Thanks for all that posted. It was the manifold pressure line that was the culprit. Dead give away was when I disconnected the line and pushed air through it, it went through fine in one direction but backed up in the other direction. Removing the ends afforded me the opportunity to see the split line. Ordered a replacement and should be back together this weekend.
 
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