M20J MP Gauge doesn’t go below 19”

[Lax291]

I’ve got a 1990 M20J and my first complex plane. I’ve been noticing the MP gauge does not go below 19” with the throttle fully closed. I notice this when coming in to land as well as taxiing. Not being familiar with the Mooneys or planes with MP gauges, I’m thinking this should go lower and even my CFI during transition training mentioned he sets the MP to 15” when abeam the numbers configuring for landing. 

If it should be going lower, any idea what the issue might be?
 

 

[EricJ]

Something's not right.    My settings for instrument approaches run between 12" and 15", and it'll certainly run below those levels.

If the engine is running and idling okay and you can reduce rpm sufficiently in the pattern (which I'm assuming you can), then there may be a leak in the instrument or in the line to the insturment or some other issue with the instrument.    

[Ragsf15e]

3 hours ago, Lax291 said:

I’ve got a 1990 M20J and my first complex plane. I’ve been noticing the MP gauge does not go below 19” with the throttle fully closed. I notice this when coming in to land as well as taxiing. Not being familiar with the Mooneys or planes with MP gauges, I’m thinking this should go lower and even my CFI during transition training mentioned he sets the MP to 15” when abeam the numbers configuring for landing. 

If it should be going lower, any idea what the issue might be?
 

 

As @EricJ said, there’s a leak.  An intake leak will do that, but 19” seems very high for a simple intake leak.  I’m guessing there’s a leak along the line to the gage or in the gage itself.  You should be able to idle close to 10” or so.

If the engine really had 19” on taxi, you’d really be scooting!

Edited November 5, 2023 by Ragsf15e

[Lax291]

Thanks for the help. I looked at the braided stainless line and right where it connects to the firewall it can spin freely before the fitting. The other end of the line which is connected to the rigid tubing coming from the engine does not turn freely so wonder if the connection to the firewall is bad. Going in for an oil change and will have the mechanics look at it. 
 

[PT20J]

2 minutes ago, Lax291 said:

Thanks for the help. I looked at the braided stainless line and right where it connects to the firewall it can spin freely before the fitting. The other end of the line which is connected to the rigid tubing coming from the engine does not turn freely so wonder if the connection to the firewall is bad. Going in for an oil change and will have the mechanics look at it. 
 

Is the B-nut on the end of the hose at the firewall end tight?

[Lax291]

I did check to see it the connector was hand tight and it was. I couldn’t turn it either way with my hand, but if you think I should put a wrench to it let me know. 

[PT20J]

4 hours ago, Lax291 said:

I did check to see it the connector was hand tight and it was. I couldn’t turn it either way with my hand, but if you think I should put a wrench to it let me know. 

I would. It’s hard to tell from your description what’s  loose, but if the B-nut is snugged down, the hose should not be loose. 

[Lax291]

Alright, i removed the line and looks like the threads on the hose might be a little cross threaded and didn't/couldn't seat all the way. Going to see if I can clean it up a little and see if it will snug up more. 

[PT20J]

Threads are galled. I would replace the hose and firewall fitting. 

[N201MKTurbo]

That almost looks like a hard line you are holding. If it is, cut the flare off, get a new B nut and reflate it. You will lose about 1/4 inch in length. If you can handle that, it will cost $5 and take about 10 minutes.

[Lax291]

Thanks guys, it's a flex line unfortunately. 

[Fly Boomer]

48 minutes ago, PT20J said:

Threads are galled. I would replace the hose and firewall fitting. 

How could that have been prevented?

[N201MKTurbo]

I always put those fittings on with my fingers and then snug them with a wrench. If they won’t go on with fingers, there is something wrong.

[N201MKTurbo]

Take a picture of the label and send it to Aero Performance in Chandler Az.they will make you a new one.

Or go through Aircraft Spruce and it will be made by Aero Performance in Chandler AZ.

[Ragsf15e]

I agree with replacing the bulkhead through fitting too but it’s harder to get at.  You’ll need to get at in on the instrument panel side too…

[N201MKTurbo]

52 minutes ago, Ragsf15e said:

I agree with replacing the bulkhead through fitting too but it’s harder to get at.  You’ll need to get at in on the instrument panel side too…

With the F you just have to take off the access panel in front of the windshield and it is right there.

[Ragsf15e]

3 hours ago, N201MKTurbo said:

With the F you just have to take off the access panel in front of the windshield and it is right there.

I always forget about that.  My F has the 201 windshield mod which makes that 1000% more difficult.

[Lax291]

I took the other side (good side) and threaded it on the firewall fitting and it threaded on fine and snugged up, think I still need to replace it?

 

@N201MKTurbo thanks for the recommendation on Aero Performance!  I was going to stop at my local MX shop to see if they had one but sounds like this is custom made? (Apologies for the ignorance, green Mooney owner here). 

[Ragsf15e]

6 minutes ago, Lax291 said:

I took the other side (good side) and threaded it on the firewall fitting and it threaded on fine and snugged up, think I still need to replace it?

 

@N201MKTurbo thanks for the recommendation on Aero Performance!  I was going to stop at my local MX shop to see if they had one but sounds like this is custom made? (Apologies for the ignorance, green Mooney owner here). 

Fitting on the firewall is probably ok. Don’t be frustrated if the new hose doesn’t fix it.  I swear mine had a bigger leak and would still get down to 12-13”.

[N201MKTurbo]

5 minutes ago, Lax291 said:

I took the other side (good side) and threaded it on the firewall fitting and it threaded on fine and snugged up, think I still need to replace it?

 

@N201MKTurbo thanks for the recommendation on Aero Performance!  I was going to stop at my local MX shop to see if they had one but sounds like this is custom made? (Apologies for the ignorance, green Mooney owner here). 

All hoses are made to a spec. If someone has one laying around it would be pure luck if they had what you need. And it would probably be used.

Everything the hose shop needs to know to make that hose is on that tag.

[Lax291]

Well the new hose did it and finally able to see MP below 19" which is making landing a lot easier? Thanks for all the help on this, much appreciated. 

[PT20J]

Sometimes it's something simple 

[Ragsf15e]

That’s the best thing it could have been, cheap and easy!