MP holds low in climb

[Ragsf15e]

Hello,

 I have a 252 (encore conversion) with an SB engine that should hold 39” in climb.  Taking off Ive been seeing 38.5” and figure that was fine.  Climbed to 16,500 today and noticed a slow decline down to about 36.5” through 10,000’ but it held that up to 16,500.  I will download engine data and look at past flights, but is there an adjustment to the wastegate controller?  The engine and turbo are only ~200 hours since oh…

[Falcon Man]

Great question. This AVWeb link might help answer your question: https://www.avweb.com/ownership/troubleshooting-the-turbo-system/.

[Jim F]

1 hour ago, Ragsf15e said:

Hello,

 I have a 252 (encore conversion) with an SB engine that should hold 39” in climb.  Taking off Ive been seeing 38.5” and figure that was fine.  Climbed to 16,500 today and noticed a slow decline down to about 36.5” through 10,000’ but it held that up to 16,500.  I will download engine data and look at past flights, but is there an adjustment to the wastegate controller?  The engine and turbo are only ~200 hours since oh…

I would check for an intake or exhaust leak.

[Ragsf15e]

4 hours ago, Falcon Man said:

Great question. This AVWeb link might help answer your question: https://www.avweb.com/ownership/troubleshooting-the-turbo-system/.

Thanks that was a good read.  

[Z W]

First thing I would do is apply Mouse Milk to the wastegate linkages. A sticking wastegate can cause lots of issues. I Mouse Milk mine every 25 hours at oil change time.

While the lower cowl is off to do that, maybe also remove the air filter, reach in, and spin the turbo by hand, checking for abnormal axial and radial play and making sure it's not contacting the housing. Wouldn't expect that at 200 hours since overhaul, but it's a quick check.

Looking for intake and exhaust leaks is a good idea. I'll also add - make sure your alternate air is not coming on, as it can cause this problem. Check for intake obstructions, which can cause the alternate air to come on. Make sure your intake plenum boot is fully drawn up onto the cowl flange and not bunched up and partially obstructing your intake. This is an important step every time the lower cowl goes back on that is sometimes missed, and as far as I know, unique to the 252 engines. You can quickly check that by removing the access door on the copilot side of the lower cowling.

After that, if you're still having the problem, I would have the fuel system setup checked in accordance with SID97-3G (attached), now a part of Continental's M0 maintenance manual, by a Mooney-specific shop. I've had multiple non-Mooney shops fail to get this right. Maxwell is really good at it. What were your fuel pressure and fuel flow showing while you observed the decrease in MP? The fuel pump has an altitude compensating function that has to be set up correctly.

I think of it this way - to make maximum power, your engine needs sufficient air, fuel, and spark. Taking spark out of the equation, you're missing either some fuel or some air.

SID97-3G - Continuous Flow Fuel Injection Systems Adjustment Specs and Instructions.pdf

[Pinecone]

Answer to your question, yes, there is an adjustment.  

But do check for leaks also.

[Ragsf15e]

11 hours ago, Pinecone said:

Answer to your question, yes, there is an adjustment.  

But do check for leaks also.

I have gone through both the mooney and the TCM troubleshooting guide for low MP.  They agree with what you guys said about checking the intake for obstructions and checking the intake manifold for leaks.  Those are the first steps on the TCM troubleshooting guide for MP issues.  I'm only seeing a very slight reduction, but I've looked back through my engine data and it's definitely there.  I get 38.5 on takeoff and then ~36.5 at ~12,000' climbing with all three full forward.  I should see 39 throughout.  One thing I noticed, at idle, it looks like I get ~14.5" mp.  In my old IO-360 it was more like 11" at idle.  Do you guys see any less MP than that in idle?  If my MP is higher at idle, that might show an intake leak.  Saavy has a published flight profile to check for induction leaks but it doesn't seem to be a part of their normal test profile anymore.  I'm thinking it's difficult to assess them in the air.  Probably have to pressurize it on the ground and use soapy water.

[Z W]

9 hours ago, Ragsf15e said:

Probably have to pressurize it on the ground and use soapy water.

I believe this is the standard practice, using a shop vac to pressurize.

At one point chasing a low MP issue we did this and the A&P said he found several leaks around the intake. We replaced all the intake connecting rubber hoses at a reasonable cost. It made no difference. All of yours should have been replaced 200 hours ago at overhaul though.

[Ragsf15e]

3 hours ago, Z W said:

I believe this is the standard practice, using a shop vac to pressurize.

At one point chasing a low MP issue we did this and the A&P said he found several leaks around the intake. We replaced all the intake connecting rubber hoses at a reasonable cost. It made no difference. All of yours should have been replaced 200 hours ago at overhaul though.

What eventually solved your issue?  Was that the one i read about where the intake was constricted and closing down incoming air near the filter?

[exM20K]

15 hours ago, Ragsf15e said:

I'm only seeing a very slight reduction, but I've looked back through my engine data and it's definitely there.

So if this is not a new problem, have a look at how the MAP drop correlates with oil temperature. It is not unheard of to see warmer, less viscous oil exert less force on the wastegate controller.  Considering it holds MAP above 10,000, then maybe not an induction leak after all.

2” is a lot to lose, though.

-dan

[Ragsf15e]

15 minutes ago, exM20K said:

So if this is not a new problem, have a look at how the MAP drop correlates with oil temperature. It is not unheard of to see warmer, less viscous oil exert less force on the wastegate controller.  Considering it holds MAP above 10,000, then maybe not an induction leak after all.

2” is a lot to lose, though.

-dan

Thanks, that’s an interesting observation because my oil temp has been pretty low this winter.  Im going to block off part of my oil cooler this week and see if that makes a difference, so I’ll keep an eye on the mp as well.

[Z W]

2 hours ago, Ragsf15e said:

What eventually solved your issue?  Was that the one i read about where the intake was constricted and closing down incoming air near the filter?

Yes, related to a poor repair of the air filter housing, which had cracked.

[Z W]

16 hours ago, Ragsf15e said:

One thing I noticed, at idle, it looks like I get ~14.5" mp.  In my old IO-360 it was more like 11" at idle.  Do you guys see any less MP than that in idle? 

I happened to be flying today and checked. I was showing 16" MP at 1000 RPM idle. FYI.

[Ragsf15e]

Just to show what I'm looking at with my "low MP"... it's one of those things you might not notice without digital engine monitors.  It's definitely there, but is it significant?  I went back through the engine data through Nov 2023 (which is farthest back I had).  It looks like it's always been ~38.5" on takeoff.  It's impossible to tell what happened on climb before I owned it because the previous owner climbed at reduced power.  However, initial takeoff MP is still the same as the oldest data I have. 

This is a full power climb I did a couple weeks ago and you can see it slowly degrades through climb.  It started close to 39" on this one and I have all 3 forward during climb to 12,500'.  It ends around 37.5".  

[Pinecone]

You can tweak the full power MAP number.   In some weather conditions, my hits 40 on take off.  The POH says up to 41 inches for max of 2 minutes.

But mine holds the MAP to at least 10,000 (only flight data I have handy).  

[wombat]

My engine (TSIO520-NB) has the opposite problem.  As I climb, the MP increases slightly.  Maybe 1" per 5,000' of altitude gained.

[Bolter]

On 2/28/2025 at 6:26 PM, Ragsf15e said:

I have gone through both the mooney and the TCM troubleshooting guide for low MP.  They agree with what you guys said about checking the intake for obstructions and checking the intake manifold for leaks.  Those are the first steps on the TCM troubleshooting guide for MP issues.  I'm only seeing a very slight reduction, but I've looked back through my engine data and it's definitely there.  I get 38.5 on takeoff and then ~36.5 at ~12,000' climbing with all three full forward.  I should see 39 throughout.  One thing I noticed, at idle, it looks like I get ~14.5" mp.  In my old IO-360 it was more like 11" at idle.  Do you guys see any less MP than that in idle?  If my MP is higher at idle, that might show an intake leak.  Saavy has a published flight profile to check for induction leaks but it doesn't seem to be a part of their normal test profile anymore.  I'm thinking it's difficult to assess them in the air.  Probably have to pressurize it on the ground and use soapy water.

I listened the Ask the A&P's podcast the other day, and they talked about the induction leak in-flight test.  It would target if a single cylinder had an induction leak, not an overall leak.  Essemtially (from memory), operate at a steady condition at high alitutude.  When satsified with the numbers, make a large power reduction with throttle.  I think he said 8".  Watch for a cylinder that does not track like the rest, and that one has an induction leak and is keeping up power by sneaking in its own air.  

As I finished writing, I found that this link, end of the article, has the test: https://www.savvyaviation.com/wp-content/uploads/articles_eaa/EAA_2012-01_flight-test-profiles.pdf

I liked that article on Turbo Troublshooting someone shared.  I have a normally aspirated engine, so it was just academic to read it.  If I understood the wastegate function, it is spring loaded to maximum bypass when there is no oil pressure.  For as long as you have sufficient oil pressure, the wastegate will drive to the setpoint, regardless of the oil viscosity.  In other words, it will always try to achieve its goal.  If you don't get full MP because of the wastegate, then I think you have to have one of these conditions:

1. pressurizing oil is leaking past the piston applies pressure to, assuming you have oil pressure in the green - look for oil leaks?
2. you have reached min bypass already point already, and the turbo is already giving all she got. - check settings on wastegate since you are too low to reach min bypass?
3. wastegate sensing pressure point is higher than MP gauge measurement point, perhaps due to temperature profile under the cowling - put a temp measurement gauge at the wastegate sensing point?

Just a brainstorming exercise, certainly no expert on this.

[exM20K]

It could also be a single cylinder or one bank of cylinders and not the wastegate or controller.

The simple version of an induction leak test in a turbocharged engine is to climb to an altitude of, say, 10,000.

Run it hard.  Record data, maybe a gami lean sweep.

Then run it at ambient MAP, using a 1" / 1000' of altitude lapse rate as "close enough." Eg: if you're at SL and 30.00" altimeter, run 20" MAP and record data, maybe a GAMI lean sweep.

If a single-cylinder or even/odd cylinder anomaly disappears in the low-power test, you have a single-cylinder or single-bank  issue.  NB: the issue could also be the upper deck pressure line(s) to the injector(s).  Or the upper deck MAP line to the fuel pump, which isn't supposed to leak fuel back via a faulty aneroid.   Ask me how I know...

-dan

 

[Fix]

On my 252/Encore SB engine  my intake plenum boot is a little wrinkled, more than I think it should be.
But what I can see I have full power 39" to 8000ft without problem.

Will try to fix the attachment on the annual, since I think someone did some cowling work.
Would be more easy when propeller is sent for overhaul.