Lycoming O-360 Low Manifold Pressure

[Fawaz]

Hi Ya'll,

New member to the Maule community here. I just bought a '04 MXT-7-180 that has been sitting in a hangar for 3 years. first thing I did was replace the engine with a new rebuilt engine from Lycoming cause the previous one needed an overhaul (O-360-C1F).

New engine, new prop, new governor, new tires, replaced a few parts here n there, did the annual... All good. Except, when I did the engine run!

The problem:

While the engine isn't running the Manifold pressure gauge shows 26-27" while the airplane is situated 1800' ASL. After engine start the needle goes down to 14" and stays there. Upon throttle advancement up to 2000 RPM the needle doesn't move one bit. At full throttle it moves up a little bit. Kill the engine and the needle goes back up to 27".

I removed the copper tube between the gauge and the engine, cleaned it with compressed air and re-tightened everything. made sure all the bolts are tight around the induction system and that there are no leaks. Restarted and still same problem.

At this point I was beginning to think the gauge is faulty so I brought in another gauge from another Maule that has also been sitting around for 3 years (different type gauge) and fitted the copper tube to it. There is improvement, the needle does go up easily but is limited to 20" even at full throttle!

I am becoming a bit frustrated, am I really dealing with 2 broke gauges here? or do I have a problem in the engine?

Has anyone had this problem before? I could use all the help I can at the moment, and sorry for the long post!

Regards,
Fawaz

[montana maule]

I would bet you have a gauge problem. Both setting for years. I have had to send mine in for overhaul and calibration twice now do to needle sticking and faulty readings.

[crbnunit]

Agreed. If the engine runs well, suspect the gauge or the tubing.

[Ron Hallmark]

I'm hoping someone with the right knowledge will expound on this problem. It seems that the gauge is okay if it's reading 27" at 1800 ft above sea level before start up. That seems to be accurate. And if it goes back to 27" after shut down, that also seems to be the way a normal gauge would function. It's the stuff in between that's confusing. Dropping down to 14" and moving up only a"little bit" at full throttle, that doesn't seem right. My M-5-235C is having the engine rebuilt right now and Gann insisted that along with the NEW Lycoming cylinders, plugs, wiring harness, oil cooler and hoses, (oil pump too,but found that out when we opened up the engine) the CARBURETOR had to be overhauled along with the magnetos and fuel pump as a part of the rebuild. Now, I have to admit, I don't really know how anything works inside carburetors. Could it be that somehow the carburetor got put back on the '04 MXT-7-180 without an overhaul? Maybe the carburetor did not need an overhaul? Please someone 'Splane this thing!

[gbarrier]

There is a shunt (a small adjustable valve) inside the fitting on the gauge. With those symptoms I'm not so sure but should be checked. Remove the hose and look inside the fitting on the actual gauge. There should be a screwdriver slot there which will allow you to loosen or tighten the little valve. I would loosen it or even take it out for testing purposes. If that works, put it back in and adjust till needle movement is smooth. Might not be it at all but worth a try. And yes, sitting around that long and both gauges could have crudded up.

[Gary Raser]

I just had a problem with my manifold pressure gauge not reading properly. At first I thought it was a lose throttle cable, but the engine felt very responsive and fuel flow was also responding like it should. It is a M7-235 with the manifold pressure near the top of the dash. I now understand the post about the panel being one of least favorite places to work. The only thing that needed done to get it working better then it has in a long time was a little adjustment on the little sunbber screw. (the little adjusting screw inside the fitting after the copper line is removed)

[Fawaz]

Thanks guys! I will verify with a 3rd Gauge and let you know.

In case I need a new gauge, what is required to replace it with the one in the link below from Aerospace Logic? They have an STC for my plane, but I'm new to the aviation world does it automatically come with an STC and I just order it from spruce and replace it and fly the next day? Does anyone recommend another gauge?

https://www.aircraftspruce.com/pages/in ... fm200k.php

I live overseas and sending the old unit for repair will cost me a lot of ground time.

[Fawaz]

I'm hoping someone with the right knowledge will expound on this problem. It seems that the gauge is okay if it's reading 27" at 1800 ft above sea level before start up. That seems to be accurate. And if it goes back to 27" after shut down, that also seems to be the way a normal gauge would function. It's the stuff in between that's confusing. Dropping down to 14" and moving up only a"little bit" at full throttle, that doesn't seem right. My M-5-235C is having the engine rebuilt right now and Gann insisted that along with the NEW Lycoming cylinders, plugs, wiring harness, oil cooler and hoses, (oil pump too,but found that out when we opened up the engine) the CARBURETOR had to be overhauled along with the magnetos and fuel pump as a part of the rebuild. Now, I have to admit, I don't really know how anything works inside carburetors. Could it be that somehow the carburetor got put back on the '04 MXT-7-180 without an overhaul? Maybe the carburetor did not need an overhaul? Please someone 'Splane this thing!

Hi Ron, since mine is rebuilt from lycoming it comes with a new Carb.

[Fawaz]

There is a shunt (a small adjustable valve) inside the fitting on the gauge. With those symptoms I'm not so sure but should be checked. Remove the hose and look inside the fitting on the actual gauge. There should be a screwdriver slot there which will allow you to loosen or tighten the little valve. I would loosen it or even take it out for testing purposes. If that works, put it back in and adjust till needle movement is smooth. Might not be it at all but worth a try. And yes, sitting around that long and both gauges could have crudded up.

Thanks will give it a try.

[andy]

Since you already tried replacing the gauge with another and it still didn't work properly and you already cleaned out the tubing, my guess would be that the problem is in the connection of the tubing to the engine. It's probably fouled by something. It's possible that you encountered two bad gauges but I think it's unlikely.

[DeltaRomeo]

Is the copper line that feeds the manifold pressure gauge attached to the backside of the #3 cylinder head?

[Fawaz]

Is the copper line that feeds the manifold pressure gauge attached to the backside of the #3 cylinder head?

If by Cylinder 3 you mean the rear right from the pilot’s view then yes. It says so too on the Lycoming IPC.

[Fawaz]

Problem Solved!

It turns out that the original MP gauge was faulty, but the secondary was working properly. And the reason it did not exceed the 20" MP on full throttle was because the engine was too rich!

I took it to a good mechanic and he noticed that when I pull the mixture to full lean for shutdown the engine was reluctant to die and kept turning until i increase the RPM to 1200 that's when it shuts down.

And he advised me that he read in the manual that if this happens it means the mixture setting was too rich, so he re-set the mixture screw on the carb. from 6 turns to 3 turns (i think) and after that the engine MP worked perfectly!

On the test flight i noticed that the engine would start running rough after leaning the mixture approximately 1inch from full rich! is that normal? my EDM is sent out for repair so I don't have an EGT reading atm.

Would appreciate your inputs if any.

Thanks!

[andy]

It depends on the density altitude, your power setting and the mixture control cable installation but my mixture control usually is about 2" from the full forward setting when the engine starts to run rough.

[benflyn]

With a field elevation of 6700 ft and up in every direction, I never use full rich on a NA engine, I even lean at idle during warm up.

I guess what I am trying to say is, it not the position of the mixture that matters it is what the engine is trying to tell you is where the mixture needs to be.