Manifold pressure

My manifold pressure is unstable (ever since flight 1). I have G3X with the standard transducer supplied with that engine package. Unlike the Vans construction manual, I tapped the MP off #6 cylinder (closer to the connection point on the firewall). Does anyone think this could be a bad transducer, or just a bad choice of where to tap off the MP? Readings during flight bounce up and down +/- .5 in.

bob888 said:

My manifold pressure is unstable (ever since flight 1). I have G3X with the standard transducer supplied with that engine package. Unlike the Vans construction manual, I tapped the MP off #6 cylinder (closer to the connection point on the firewall). Does anyone think this could be a bad transducer, or just a bad choice of where to tap off the MP? Readings during flight bounce up and down +/- .5 in.

Click to expand...

Do you have any sort of a snubber/restrictor in the line? That might reduce or eliminate the fluctuation.

Most install a restrictor like one of these (or similar) to calm the readings down:

https://www.mcmaster.com/3820K26
https://www.mcmaster.com/3820K36

Thanks guys. Sounds like a plan.

Photos are gone, but here is what I did for the issue

http://www.vansairforce.com/community/showthread.php?t=96164&highlight=manifold+pressure

Those are pipe threads so make sure you use a thread sealant (high temp) on those threads.

Manifold Pressure

On several RV?s I have found some oil in the fitting or line right at the cylinder head which was causing erratic manifold pressure readings. In each case cleaning it out usually cured the problem. If the line runs downhill to the sender I turn the fitting and re plumb so it goes uphill first. Some production planes including my Pacer have a tiny vent hole on the bottom of the tube near the cylinder head. When the engine is running it?s always sucking in a little air which would tend to keep the line and fitting clear.

Don Broussard
RV9 Rebuild in Progress
57 Pacer

I also had a manifold pressure problem after about 100+ hours. I had installed the gage snubber as suggested by others here and by Garmin. I had installed it in the cylinder head port because I thought it gave more space to connect the angle fitting on the pressure line. The transducer is installed behind the firewall as recommended and there is a dip in the line to minimize the possibility of oil migrating to the transducer. Eventually, that snubber at the cylinder head almost completely plugged resulting in crazy manifold pressure readings. I replaced and moved the snubber to be at the end of the line just before the transducer. Be sure to use a gage snubber recommended for air, not liquid. As I learned, it is especially important to put it at the transducer, NOT in the cylinder head port.

bob888 said:

My manifold pressure is unstable (ever since flight 1). I have G3X with the standard transducer supplied with that engine package. Unlike the Vans construction manual, I tapped the MP off #6 cylinder (closer to the connection point on the firewall). Does anyone think this could be a bad transducer, or just a bad choice of where to tap off the MP? Readings during flight bounce up and down +/- .5 in.

Click to expand...

I have this same issue, due to this exact same setup.

Thanks for posting, I'll follow the fixes in this thread.