Manifold Pressure line.....weep hole

[steve@innoviator.com]

Does anyone have an explanation for a MAP line weep hole? Typically weep holes are near the bottom of lines, not the top.

[MB65E]

My understanding is that it's a hole for excess moisture, oil, residual fuel to escape. I've had an install on a MVP-50 that didn't have the weep hole... Constant Manafold pressure issues! I'm sure there is more technical reason.

-Matt

[cliffy]

OK I guess I need an education, MAP line weep hole? 

[MB65E]

Usually with in about 6in on the #3 or 5 cylinder manifold pressure line. In the 1/4 alu hard line, Bottom side, sometimes right next to a clamp.

Maybe too it's a faster way for the guage to read Ambiant pressure?

-Matt

[carusoam]

On my C, the small 1/8" aluminum line from one cylinder's intake ran directly through the firewall to the back of the instrument...

The intake is full of dark blue gooey deposits coating everything...

The tiny hole (vent) is drilled in the 1/8" line. It is a calibrated hole dimension. Too big the gage will read low, too small the gage reads high.

My experience comes from the vibrations cracked the soft aluminum. While searching for the problem, I broke the line open (in flight).

This is also the moment that I lost faith in instruments that bring hot or flammable liquids into the cockpit. OK for high priced planes back in the 60's. Not so good for a family flying on a budget...

I had heard of the weep hole, but could not find any documented evidence. Mine was broken in half and really hard to measure.

Bill Wheat was probably my best resource for this... The hole is documented in one of the IP drawings for the older Mooneys, but not in any of the C drawings...

Bought drill bit, had mechanic take care the details...requires cutting the tube, flanging the end and reinstalling...

The instrument is under vacuum until the engine shuts down. Without the vent hole, fuel/air mixture would be drawn towards the instrument...

Overall, with the proper details, it's a fifteen minute job....?

Best regards,

-a-

From my hand written notes.... 0.023" drill. Keep in mind 1/32" is .032". This is tiny....!

Also keep in mind, I am not a mechanic....

[Guest]

The weep hole allows fuel, oil etc to drain out of the map line on shut down. It is usually at the bottom of the rigid MAP line as it exist the intake port fitting.

Clarence

[steve@innoviator.com]

Thank You Gentlemen,

Very good explanations..

steve

[cliffy]

OK learn something new every day

Now my question is- If it only shows on earlier than C model drawings why would we assume it would be used (required) on Cs and later? 

[carusoam]

Cliffy,

I was less than clear...

C,D,E, and F were produced concurrently. All had the same weep hole.

I believe the F drawings have the detail captured on the drawing. I was looking as an owner of a C. The 1/8" flexible aluminum line is barely documented, never mind the weep hole.

I tried to buy the line from my MSC, they sent me one for a J? instead... 1/4" rigid tube?

I believe it may have the same weep hole. My memory wasn't that good then either...

Best regards,

-a-

[cliffy]

This is interesting as I've never heard of it before. At first glance I would think the hole would affect the MP reading at lower MP setting. I would think it would act like any other induction leak but would have a direct effect on the MP gauge reading. I'm going to ask a real old timer A&P friend of mine if he ever heard of such a thing. Interesting, very interesting. 

[carusoam]

Cliffy,

Now you're being funny...

It does act like any other induction leak allowing excess air into that particular cylinder.

Unlike any other induction leak, it is directly affecting the instrument with the improper air flow.

Without the hole the MP indicates off the instrument...

With the hole cracked open slightly, the MP is off a couple inches...

With the hole broken open, it reads atmospheric pressure even while idling...

Hence the word calibrated being used. The instrument is sensitive to the small hole.

Knowing MP and RPM is key to proper power management.

My C lost the MP this way.

It lost The rpm gauge via a worn drive cable, it wobbled until the needle fell off...

Great trivia for the non digital instruments guys...

You'll need to find that drawing for proof.

My digital instruments still have analog needles...

Best regards,

-a-

[Andy95W]

Cliffy / Carusoam - thanks for posting. Nice to learn new things.

Does anyone have a copy of the engineering drawing that has this weep hole?

Cliffy, please let us know what your "real old timer A&P friend" has to say.

[cliffy]

Waiting to hear back from the "old timer"  but this is something I want to learn about. 

Lost my MP once due to broken 1/8" line  Makes no difference to as I'm full throttle or pulling back for landing :-)

I may also have a contact for the drawing on the line but it might take a while to get it. 

[Piloto]

The orifice on the manifold pressure line is for  dampening the pulsating vacuum caused by the intake stroke cycles. Without the orifice the MP reading would be fluctuating about 1"rapidly, specially below 1,000 RPM.

 

 Check: www.grtavionics.com/MANPRES.pdf par. 4

 

José

[Guest]

Jose,

Two different animals. A restrictor orffice is within a fitting in the MAP line, the weep hole is drilled into the side of the line.

Clarence

[carusoam]

Additional info...

From the parts Manuel...

Figure 17 Instrument Panel Assembly. Page 74

This drawing depicts all the wires and hoses going to the backs of all the instruments.

Figure and index nos...

17-41 FP and MP gauge

17-42 hose, snubber, and hardware list...

17-43 MP line and hardware list... Gives two part numbers for the lines....

660061-3 for C,D,E

660121-3 for F

I think the part numbers may give a hint to a drawing number with the reference.

Now, how does one get an engineering drawing for the individual part?

As José points out above the instrument manufacturer would be the one with the knowledge of the hole dimension.

Note that the FP indicates a need for a snubber, the MP, nary a hint!

Why is the F different than the others?

Hope that helps,

-a-

[Piloto]

Clarence has a point, but the hole is too small to drain any oil plus it will need positive pressure to get it into the line. On my engine the hole is located on the top side of the line. It looks to me more like the hole is for vacuum bleed off.

 

José

[Andy95W]

Carusoam said his hand written note said .023 drill; Poloto's PDF noted a #70 drill bit, which is .028. Think we are talking about the same thing here.

Still curious to see what Cliffy's old time A&P buddy says.

[MARZ]

Timely. My large tube broke in half at the adel clamp. I didn't see any weep hole on inspection have to take a closer look

Sent from my iPhone using Tapatalk

[carusoam]

Mike, what year is your F?

At some point in history the F evolved away from C and E...

From dusty memory...

The C's weep hole is about 2" from the instrument.

-a-

[MARZ]

67. Guess I'm slow. I've got a larger tube off the engine ends just back of the right rear cylinder. Then a much thinner tube that connects and goes to the gauge. Where is the hole?

Sent from my iPhone using Tapatalk

[carusoam]

From dusty memory...

The C's weep hole is about 2" from the instrument.

-a-

[triple8s]

The hole may be obscured by a clamp. The C I owned had a tiny hole that was very close to an adel that helped support the line as it made its way back towards the firewall.

[Yetti]

Per chance does anyone know anything about snubbers say on an F model

[MB65E]

They leak, they don't seal. They are supposed to drain excess fuel in the intake sump. 

Don't over prime and they are not needed. 

The ball type ones are better than the Piper diaphragm type. 

-Matt