Unusual - I think - engine trouble

[MooneyNoob]

Hi folks,

 

A couple of days ago, I flew a short 45-minute practice cross-country with my CFI in a 1979 Mooney m20k 231 (with a Turbo Plus intercooler and Merlin waste gate).   We flew at 8,500 and during the flight felt a couple of light kicks or sputters from the engine.   The other odd thing was that a VERY small movement of the throttle moved MP from 34 to 25 or so.   There was no easy way to keep move the power smoothly through that range.   And on final, when I wanted to pull the engine to idle, the MP would not fall below about 19.  At the destination airport, the line guy who welcomed us said that the plane sounded weird on approach.  When we were ready to head back to our home airport, we pre-flighted the plane and did a run-up VERY carefully, watching everything.  Once again, we could not get the MP on the indicator to go below about 18 or 19, but the engine sounded normal / idle with the throttle pulled back.   Note that I have two indicators: a JPI engine monitor and a steam gauge, both telling me the same MP numbers.  The other notable item was that oil pressure was a bit low (top of the yellow) below 1200 and the temp was close to the top of the green.      We decided to take off.   On climbout, at 105 MPH (~90 kt) and 34" MP, I normally get ~1,000 ft/minute climb.  I was getting between 200 and 300.    Note: the airport is just a few feet above sea level, and the day was warm but not hot, so D.A. was not an issue.   We made the call to return to the field immediately and landed without incident.     Oil was at 6.5 quarts on pre-flight inspection and no oil leaks were visible on post-flight inspection.

We left the plane at the FBO and will have the local A&P look at it this week but meanwhile, I wanted to post this here because it seems like an intriguing mystery and I wonder if we'll solve it here before the A&P does.

Any ideas welcome!

 

 

Edited April 18, 2022 by MooneyNoob

[carusoam]

Yikes!

How uncomfortable was that?

That is not normal…

When the throttle doesn’t work like expected…. Or the engine doesn’t supply power like expected… or the turbo stops working…

You know something is wrong because some guy on the ground said so…

you are describing something not right in great detail… 300fpm when expecting 1kfpm…

what if it was -100fpm?

 

it’s better to be on the ground to find out what stopped working properly…

Stuff doesn’t fix itself…

When it comes to TC’d engines… things get more complicated…

Deciding to take off with an unknown engine challenge belongs in another thread…

you have two challenges…

1) a mechanical one…

2) an aviation decision making one…

 

Start with the known mechanical challenges…

To solve things here without data and pics is a pretty tough expectation…

MP all over the place… you have a Merlin MP controller…. What is Mr. Merlin saying, anything?

Do you have any mechanical experience to draw from?

Turbos, oil pumps, V-clamps, MP controllers? (Automotive experience at all?)

Start with the JPI data and graphs…

1) Download them…

2) send them to Savvy…

3) click the share button…

4) post link here…

5) What JPI model do you have? (Is it full of data covering everything?) 

 

I make friends so slowly… I can’t afford to lose any…   
My newbie experience wasn’t handled any better…

i didn’t have an engine monitor…

let’s not fly that thing until the problem gets identified…

 

PP thoughts only, not a mechanic…

 

Best regards,

-a-

[aviatoreb]

Good call to return to the field.

Glad you are ok.

[chrisk]

Good call to return to the field.  If you have an engine monitor, download it and view the egt and cht.  It may give a clue.   Fuel flow observations would be good to know.

As for the line guys observation.  I would discount it some.   --Yes, a 231 does not sound like a Cessna when it idles.  I've overheard some folks commenting on a 231 saying it sounded horrible.  There is a reason. The 231 only has a turbo for a muffler.......   It sounds weird on the ground.

What would I do?  After changing my shorts, I would start with a full power static ground run.  I'd note fuel flow, RPM, and MP.     Then probably move on to pulling the plugs for an examination, doing a compression check, and a visual on the turbo compressor side.  (Mostly because all of that is easy).   If I didn't find anything, a functional check of the Merlin would come next.   Then a fuel flow check.   And when is the last time those mags were serviced?

 

 

 

 

[StevenL757]

@MooneyNoob, Absolutely a wise move to return and have things checked out.  I hope all turns out ok, and inexpensively for you.

[A64Pilot]

My swag is the boost controller is acting up, or the fuel servo. Be interesting to hear what it is.

Was the high manifold pressure at idle also with high or normal idle RPM?

Two things are off, fuel servo shouldn’t be able to change idle MP, and if you have boost then you should have power unless fuel flow is off.

‘Probably find out it isn’t either, it will be mag timing or who knows

[Browncbr1]

Could be your MAP sensor line has broken a flange or come loose, thus allowing excess air into the relevant cylinder.   That would explain some popping and proofing on final and on the ground as well as the MAP that won’t go to normally low indications.  
 

however, I don’t think that could possibly cause such a degradation in climb performance as you described.  

[kortopates]

@MooneyNoob what your describing is the Merlin pneumatic wastegate controller sticking on you. Based on its inability to regulate manifold pressure it needs to be pulled  and sent to Merlin for overhaul/repair. 

Sounds like you're going through 231 transition training with a CFI?  

"The other notable item was that oil pressure was a bit low (top of the yellow) below 1200 and the temp was close to the top of the green." 

Your CFI should have been able to answer this very basic concern. The Yellow arc is normal for idle rpm, the only time it permissible to see the oil pressure drop into the yellow is at idle, Its not permissible to operate the engine in cruise though below the green. If ever drops below the green in flight into the yellow, its time to pull back the power and make a precautionary landing before the engine or turbo seizes because it not getting sufficient oil pressure for more than idle power. Luckily, usually the turbo seizes before the engine does. So best to think of yellow as a redline for cruise - which is see. See Engine limitations section of your POH for more. 

[MooneyNoob]

22 hours ago, A64Pilot said:

My swag is the boost controller is acting up, or the fuel servo. Be interesting to hear what it is.

Was the high manifold pressure at idle also with high or normal idle RPM?

Two things are off, fuel servo shouldn’t be able to change idle MP, and if you have boost then you should have power unless fuel flow is off.

‘Probably find out it isn’t either, it will be mag timing or who knows

The high indicated MP was at idle RPMs (18"-19" MP at 1,000 RPM).

The mags looked alright ...each cylinder was firing as expected, temps were in line, etc.

[MooneyNoob]

11 hours ago, kortopates said:

@MooneyNoob what your describing is the Merlin pneumatic wastegate controller sticking on you. Based on its inability to regulate manifold pressure it needs to be pulled  and sent to Merlin for overhaul/repair. 

Sounds like you're going through 231 transition training with a CFI?  

"The other notable item was that oil pressure was a bit low (top of the yellow) below 1200 and the temp was close to the top of the green." 

Your CFI should have been able to answer this very basic concern. The Yellow arc is normal for idle rpm, the only time it permissible to see the oil pressure drop into the yellow is at idle, Its not permissible to operate the engine in cruise though below the green. If ever drops below the green in flight into the yellow, its time to pull back the power and make a precautionary landing before the engine or turbo seizes because it not getting sufficient oil pressure for more than idle power. Luckily, usually the turbo seizes before the engine does. So best to think of yellow as a redline for cruise - which is see. See Engine limitations section of your POH for more. 

Yep, BFR (completed after a 20-year flying hiatus), 231 transition, and instrument training (ongoing).

[kortopates]

4 hours ago, MooneyNoob said:

Yep, BFR (completed after a 20-year flying hiatus), 231 transition, and instrument training (ongoing).

Good luck!

One more thing I'll mention for the sake of thoroughness, is that the long air reference hose between the Merlin and the upper deck should be checked that its secure and in good shape and not leaking. But Merlin supplies such a strong hydraulic hose for this application that will last much longer than the engine, so its pretty unheard of to see that be the problem unless its chaffing against something. Barring an issue there, that leaves the wastegate unit.  

[carusoam]

Interesting explanation for why sharing data is important…

OP included this statement….

“we could not get the MP on the indicator to go below about 18 or 19”
 

1) If this were real…. You could taxi at near T/O speed…. If you didn’t hold the breaks….

2) If this was a faulty instrumentation reading… the plane would barely want to move…. As usual…

 

Being the helpful friend on the internet… I probably misjudged the statement, expecting that it was #1…. 

Pictures and graphs are worth 1k written posts…

PP thoughts only, not a mechanic…

Best regards,

-a-

[Austintatious]

I have a thought on what it might be...

You may have an induction leak.  So while you are seeing 34 inches of MP, some of it is leaking out instead of going into the engine and causing a loss of power.

When you are pulling the power back and can not get MP below 19 inches, air is rushing in through the leak.

This would also explain erratic MP movements with throttle movement.

[A64Pilot]

Usually if you have an induction leak MP stays low, you can’t get normal boost, because of course it’s leaking.

The high MP at idle confuses me, as a turbo at idle isn’t doing anything, many don’t even rotate slowly, much less fast enough to build boost, so if the turbo isn’t compressing air, how can the MP be high? Lends logic to the leak theory, vacuum will be low if there is one, so yes thinking it out there is credence to the leak theory.

Thankfully leaks are usually easy to diagnose