Manifold Pressure Mystery

[dlloyd3]

If it is a snubber or restrictor, it should flow air when you blow in it. Maybe it is plugged up. With the directional arrow, it seems more like a check valve. Again, when you blow in one end it should flow. Maybe it is plugged up.

[az_gila]

Originally Posted by Tango Mike

Thanks, Gil, I have to disagree.

Unlike the other three fittings, which you can blow through and see light from one end to the other, the fitting labeled with EA, FLOW, and the arrow has what looks like a solid plug inside. No light passes from one opening to the other, and if I blow on either end, I can't get any air to move through the fitting.

It's a different fitting altogether. I think it might function as a "snubber" that prevents full, unregulated manifold pressure from the engine entering the MPT on the cockpit side of the firewall. It allows pressure variations to pass through the "plug", but slowly so as not to damage the MPT.

The tubing isn't exactly all the same size, but my understanding is that pressure variations don't care. Pressure and vacuum alike fill all the space they can reach.

Simple test, remove it, replace it with an Ace Hardware coupler and test again, nothing bad will happen.

If it's a restrictor it wouldn't normally have a flow direction, however you should be able to blow air through in all directions.

My comment on the tubing size wasn't to say all sizes should be the same, just that the upper leg nipple looks small, perhaps smaller than 1/8 inch...

[Pat Hatch]

Tosh,

I think you found your problem. If that's the source for both of your MAP senor vacuum pressures, it very well could be blocked. Consider ditching that whole contraption and just go with a simple "T" off your manifold pressure line. [Ed. looks like you need some kind of 3/way fitting since you also have to go to your VM-1000 indicator.] If you need a restrictor you can put it in the fitting on the cylinder. From the "T" to each of the MAP sensors with a good quality silicone hose and you should be in business.

[Tango Mike]

The component in question with the flow arrow is a snubber fitting, designed to be one-directional, with the pressure side pushing in the direction of the arrow on the fitting. This is very clearly stated in the product description.

According to VM-1000 expert Reggie, use of a snubber fitting can prevent manifold pressure readings from jumping around, especially during power changes. They dampen oscillations by making the gauge respond a little more slowly.

The original valve in this airplane was installed properly with the flow arrow pointing toward the engine. The reason is that in relation to atmospheric pressure, manifold pressure in a normally aspirated engine is a vacuum, so any air outside the engine is always at higher pressure than that inside.

To confirm that a plugged-up snubber fitting is the source of the problem, I've removed it and will start the engine tomorrow. If the manifold pressure reading on the VM-1000 is normal at idle, I'm going to do a runup and check the manifold advance. If it's operating normally, the problem is solved. If the manifold pressure readings jump around too much, I'll install a new snubber fitting.

Report to follow . . .

Tosh

[Tango Mike]

Manifold pressure on the VM-1000 and MAP advance were both normal at idle and run-up RPM.

I haven't yet checked manifold pressure readings inflight, but on the ground I didn't notice any "jumping around" during power changes without the snubber fitting installed.

Lessons learned:

1. Don't use a snubber unless your MP readings on the VM-1000 are too skittish and it's distracting.

2. If a snubber fitting is used, install it first in the sequence so it affects only the VM-1000's manifold pressure transmitter. The flow arrow on the fitting will point toward the engine.

3. One forum member commented that he cleans the snubber fitting in his airplane every so often. I tried cleaning this one during troubleshooting, but it made no difference, probably because it's been installed since the airplane was built in '97. The fact that I cleaned it a year or so ago when I followed Reggie's procedures for avoiding the manifold pressure transducer "blues" obviously didn't help avoid this problem

Thanks to all who joined in this thread. This a great resource, and I'd hate to fly without it.

Tosh

[Don]

Thanks for posting the solution! I can't tell you how many problem threads I read where the solution is never posted and I'm left guessing as to which of the possible suggestions was the right one - or if none of them were right.

This post hit close to home - I've started getting some peculiar readings on take-off and landing on my VM-1000C. While my situation appears different from yours, I have gotten a few ideas about where to start looking for gremlins.

[Tango Mike]

Don,

If you'll PM me with an email address, I'll send you the .doc file I built from Reggie's information about how to avoid the MPT blues.

Tosh