MP gauge won’t go below 14”

[RobertGary1]

Just started. Reducing throttle below 14” the gauge then jumps up to 20” although engine response seems normal. Intake leak? Not many places for these to leak. Maybe a guage issue?

[RobertGary1]

Egt spread is 40-50F which is what it’s been since I bought this engine from Lycoming. Just went for a flight snd it did not happen jn the air but did on the ground consistently. Maybe it’s inconsistent though. 

[David Lloyd]

Check for loose fittings or a hole worn in the MP line.

[carusoam]

Expect that the calibrated hole in the MP line has opened up...

They suffer from crack propagation...it’s like a 1/8” aluminum line...

If you grab it while flying, it may complete the propagation and the MP will read atmospheric pressure... 29.9 or so...

This happened with my M20C...

Drilling a new calibrated hole isn’t too difficult...

It was the topic of my second phone call with Mr. Bill Wheat... 

Find the hole, take a pic... if it is broken open... there is excess line available or easy to replace... plan on buying a tiny drill...

PP thoughts only, not a mechanic...

Best regards,

-a-

[Browncbr1]

Check the flares on the MP line fittings for cracks or breaks..  

[Browncbr1]

9 hours ago, carusoam said:

Expect that the calibrated hole in the MP line has opened up...

They suffer from crack propagation...it’s like a 1/8” aluminum line...

If you grab it while flying, it may complete the propagation and the MP will read atmospheric pressure... 29.9 or so...

This happened with my M20C...

Drilling a new calibrated hole isn’t too difficult...

It was the topic of my second phone call with Mr. Bill Wheat... 

Find the hole, take a pic... if it is broken open... there is excess line available or easy to replace... plan on buying a tiny drill...

PP thoughts only, not a mechanic...

Best regards,

-a-

Could you please share the spec for the calibrated hole size?   Drill bit size?

[Ibra]

Does it show 29" when engine is OFF and about 12'' on idle run?
Does it not go bellow 14" no matter what you did? 
Do you get an RPM raise on idle shutdown?
What does MP show on idle shutwon? 

I would still check for air induction loss (highly unlikely) either from lose clamps on air intake hoses or gasket bolting to the cylinders (open engine cowling and turn the prop up/down) 

Otherwise it's just an MP gauge reading or lines issue 

[carusoam]

12 hours ago, Browncbr1 said:

Could you please share the spec for the calibrated hole size?   Drill bit size?

Hand written in a note in an old paper file...

But, better than that... I know where to look it up....   

It can be found in the parts catalog... in the instrument section... where each of the models has a different panel page for all the different lay-outs...

Only one of the models has the proper note to accompany it...

I think it is on the M20F instrument panel page...

Let me know if you are serious about finding the answer... more than happy to look for it...

I also have the drill sitting in my drill index...  

Best regards,

-a-

[RobertGary1]

Well this looks like it won't be that easy. Inspected the line and no extra holes anywhere. The hole its supposed to have looks great. I wonder if there is a way I could make an adaptor that would hook to my hand pump that I could draw a suction on it and see if it holds.

Also odd that it didn't happen in flight. Unless the leak is somewhere with low pressure in flight.

-Robert

Edited January 15, 2021 by RobertGary1

[PT20J]

My IA has a box full of old instruments. I generally borrow one from him when I want to test something. I recently borrowed an airspeed indicator to chase down a pitot leak. Maybe your mechanic has a MP gauge you could borrow. Or you could just get a vacuum gauge from an auto parts store.

Skip

[RobertGary1]

51 minutes ago, PT20J said:

My IA has a box full of old instruments. I generally borrow one from him when I want to test something. I recently borrowed an airspeed indicator to chase down a pitot leak. Maybe your mechanic has a MP gauge you could borrow. Or you could just get a vacuum gauge from an auto parts store.

Skip

I’ve got the gauge and the hand pump but lack the connector 

[RobertGary1]

Think I'll just disconnect both ends, cover up the pin hole and blow. That should tell me if it leaks or not. Worst case I have a professional mechanic's smoke machine but that's overkill.

 

-Robert

[Browncbr1]

16 hours ago, carusoam said:

Hand written in a note in an old paper file...

But, better than that... I know where to look it up....   

It can be found in the parts catalog... in the instrument section... where each of the models has a different panel page for all the different lay-outs...

Only one of the models has the proper note to accompany it...

I think it is on the M20F instrument panel page...

Let me know if you are serious about finding the answer... more than happy to look for it...

I also have the drill sitting in my drill index...  

Best regards,

-a-

I’ve looked all over the parts catalog and MM, but unable to find it.   I would really appreciate your help if you could dig up this spec.  Thanks craig

[PT20J]

The original question had to do with determining whether it's an indication issue or an engine issue. When a instrument reads abnormally, but everything is working properly, I first suspect the instrument (but I don't just assume that and keep flying until I verify it).

An internal combustion engine is designed to run at relatively high speed. At idle, piston speed is slower and there is not as much turbulence within the cylinder and that affects combustion. To get it to run requires a pretty rich mixture and that yields a slower flame propagation. Autos advance the timing at idle to compensate but our timing is fixed. This is one reason your car idles better than your airplane. What this means is that any significant induction leak should make the idle rougher. Depending on how big the leak is, and how many cylinders it affects, it might not show up at higher powers at all. I once flew a Beaver with a R-985 that flew fine but wouldn't idle worth a damn. Turned out to be a loose carburetor that wasn't properly torqued after replacement (mechanic still owes me a beer!)

In this case, I would first look everything in the engine compartment over with special emphasis on the intake tubes (and bolt torque) and the manifold pressure gauge plumbing. If that all looked OK I would replace the gauge with a known good one and see if the problem goes away. I wouldn't try to test the gauge itself because I would not have a calibrated way to do that (unless I went to a lot of trouble to build something), and even if it worked when I sucked on it, I would always wonder.

Skip

[carusoam]

10 hours ago, Browncbr1 said:

I’ve looked all over the parts catalog and MM, but unable to find it.   I would really appreciate your help if you could dig up this spec.  Thanks craig

You are in the right area....

I looked in my parts manual and didn’t see it either...  I know it is a small note in one of the drawings... possibly service manual... the parts manual only has one drawing to cover the four different birds...

So I dug through paper files instead....

It has the answer...

But, proper homework would require cross checking with Mooney to make sure it is the right answer...

I had tried to buy the part from Weber back in about ‘05... they sold me a part for an M20J... assuming it was the only part.  So all the older Mooneys must have used it as well...

the older Mooneys use a standard piece of tubing, and drill the hole...

 

If you look at the plethora of post it notes....   

1) There is a hole size 0.023” this is what we wanted...

2) The next hole size is 1/32” this is a common tiny drill... which is 0.0312”...  this will be close, if you can’t get the 0.023... (not recommended)

3) What you are drilling into is really soft aluminum, get the right drill size... 

We have the internet now... back then it wasn’t as easy...

PP thoughts only, I was working with a mechanic back in the day as well...

Best regards,

-a-

[EricJ]

On 1/14/2021 at 11:25 PM, RobertGary1 said:

Think I'll just disconnect both ends, cover up the pin hole and blow. That should tell me if it leaks or not. Worst case I have a professional mechanic's smoke machine but that's overkill.

 

-Robert

I wouldn't put a smoke machine in it since they can leave an oil film that might work its way into the instrument.   I don't know how sensitive it might be to that.

Also check the hard lines if you haven't already, as cracks there can make them wonky as well.    The hard line on mine broke apart right at the cylinder once with no warning.   It must have had a crack for a while before that.

[RobertGary1]

I was able to put a hose out on the line at the cylinder side snd suck it down to 14” and the gauge held indefinitely so I don’t think I have a bad line. I could try with my hand pump to get it lower but the leak can’t be there