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Low O-360 Manifold Pressure

BigTex

By BigTex
in Vintage Mooneys (pre-J models)

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BigTex Mentor

BigTex

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This week I had my mechanic looking into an occasional engine stumble when at idle or when pulling manifold pressure and he noticed that at idle my engine is showing a negative pressure. Other than this observation the engine seems to run fine (other than the occasional stumble). He seems to think that since the engine has around 1500 hours, it's likely the carburetor but he's talking to some rebuild houses to get their thoughts.

Does anyone have any thoughts on what's going on?

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BigTex Mentor

BigTex

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I did some additional trouble shooting over the weekend. With the engine off, the manifold pressure was around 29". At idle, the MP reads zero. At max power, the manifold pressure come to life and appears to read normally.

I'm thinking it might be the gauge. Thoughts?

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BigTex Mentor

BigTex

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Since this gauge is a dual MP/FP gauge, does anyone have any recommendations on a replacement? I see that Aerospace Logic has a combo unit (FM200K) that might be a good replacement.

Also, can it be serviced/IRAN'd and if so, any recommendations on service centers?

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carusoam Grand Master

carusoam

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There is a small calibrated hole in the tube that runs from MP gage to the nearest cylinder. The hole lets the line breath to keep the gage from sucking fuel from the intake upon shutdown. This is a tiny hole that is not well documented. In my C, the hole cracked open.

Emergency fix, had no hole. Similar operation as you describe above.

Getting the right size hole is the key to keeping the MP gage operating properly. Anyone know the size of the hole? I can look it up if needed. I found it by speaking to the factory. It is a note on a drawing from one of the old Mooneys, but not all of them, C, F or G...

Your description indicates that it is probable that your hole has been plugged or removed...

Let me know what you find. If you need more info about finding the hole dimension, I have the notes I took from the conversation...

Total cost for this fix.... A few bucks for the small drill bit. The original line was long enough to extend, re-drill and reconnect...

I may have copied it to the forums here once before.... Searching might find it.

Best regards,

-a-

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danb35 Mentor

danb35

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Air Parts of Lock Haven should be able to repair the gauge. I don't have any personal experience with them, but I've heard good things about them. If the gauge is reading zero at idle, and/or negative MP under any conditions, there's a problem with the gauge. There's no way the line could cause that--a leaky line would tend to make the MP read high under low-power conditions, and about normal at high power.

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carusoam Grand Master

carusoam

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Gary,

I have .023" diameter for the hole. Keep in mind 1/32” is .0312”, so it's pretty tiny.

I spoke with the following people while trying to determine what to buy...

Lone Star aero, Dennis Bernheart

MAC, Bill Wheat

Weber Aircraft, Les Jayne

That was 10 years ago...

The hole was approximately 1” behind the instrument. The tube itself is only like 1/8” diameter.

I got another hint at a reference from a piper instrument. Many mechanical MP gauges use a similar technique to protect the gauge itself.

You may be able to contact Stacey Ellis at Mooney.

My M20C manuals do not have the proper reference in them. I think it came from an M20F drawing...

Best regards,

-a-

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carusoam Grand Master

carusoam

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Dan,

Keep in mind that the hole is there for a reason and the gauge is calibrated to have it there.

Hole too large, instrument reads too high.

Hole too small or missing, instrument reads very low at low MP.

I get to share this archaic expertise about once every ten years! Thank God for MooneySpace!

-a-

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M204ever Contributor

M204ever

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Gary,

this hole is there to avoid MP oscillating of the indicator caused by the piston strokes, its kind a damping nozzle.

Neverthelesss, w/o a nozzle or hole restriction the instrument should read correctly.

Restrictor fittings may also be installed directly at the cylinder intake port or the instrument itself.

The MP line routing should be such to prevent fuel contamination.

(Cheap) restrictor fittings can be found directly from Lycoming, btw they are installed in the fuel pressure port of the carburettor as well.

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danb35 Mentor

danb35

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Hole too large, instrument reads too high.

Hole too small or missing, instrument reads very low at low MP.

Doesn't work that way. If there were significant flow throug the MP line, there would be a pressure drop across the restriction, and the size of the hole would make a big difference (because that, along with its shape and the rate of flow, would determine the pressure drop). However, in this case, there's no real flow through the line, and the pressure on each side of the restriction will quickly reach equilibrium. The reason it's in there (as M204ever posted) is that "quickly" isn't the same as "instantly"; there is a brief delay, which ends up smoothing the gauge response.

If the MP gauge responds slowly to throttle changes, the restriction might be plugged or too small. If the gauge is really twitchy, the restriction might be missing entirely or have too large a hole. However, if the gauge reads inappropriately in a steady state, that couldn't be caused by the restriction.

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carusoam Grand Master

carusoam

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Gary,

From my C's parts manual...Figure 17-41. P/n 660012-1. Gage, fuel and manf pressure... For C and D

No manufacturer name available....

Dan,

I did the best I could with what I had... When I got the hole replaced, the guage worked correctly. Too big, missing, or too small are not acceptable either way. Vibration of the IP made rpm and MP more of a suggestion on such old devices. Both of mine Independently broke around 5,000 TT. Shared experience, not deep, non-linear flow of compressible, Non-Newtonian fluids, in unusual geometries, pHD thesis. Ymmv.

Check with your mechanic for identifying and replacing the hole.

JPI930, or similar, would make a nice replacement....And eliminate the deep mathematical challenge...

Hope this helps...

Best regards,

-a-

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BigTex Mentor

BigTex

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I'd like to update the group on the resolution to my manifold pressure issue. As it turns out it was debris in the line blocking the vent hole. We blew the line out and now the gauge operates as it should. Thanks again for everyone's thoughts on this.

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