Mooney 231 - reduced manifold pressure

[M20Kid]

I have a strange issue that I wanted to bring to the group.  I have a 231 that I regularly take to the flight levels.  It's got a stock TSIO-360LB engine and a Merlyn wastegate STC; no intercooler or other mods on the system.  In the past I've been able to climb to FL210 (the highest I've taken it) and still been able to run 30 inches of manifold pressure running lean of peak with a fair amount of throttle still available.  The other day I was at FL200 and could only get 27 inches with wide open throttle - something has obviously changed.  My mechanic was able to pressurize the manifold and discover a small air leak, which he sealed.  I sent pictures of the turbo charger to Main Turbo in California and they didn't immediately see anything amiss.

It seems to me that this must be either a turbo not putting out full pressure or a leak in the manifold after the turbo.  Could be a manifold pressure gauge issue, but then the CHT, EGT, and TIT would all be out of whack and they're not.

Has anyone experienced this same type of issue?  I'm at a loss for even what type of mechanic to call - engine shop? General Maintenance? Turbo overhauler?

[N201MKTurbo]

I don’t have a 231, but that happened to mine once. It was the pop off valve. 

[M20Kid]

I thought about that valve.  The mechanic pressurized the system to 40 inches and the valve didn't leak, but I realized with 40 inches on the ground the pressure differential is essentially 10 inches.  That same pressure at altitude is over 20 inches of differential.  Perhaps it does open in flight but not on the ground.

How did you test the pop off?  I understand it takes a specialty shop.

[jlunseth]

Also check the wastegate. One difference between the Merlyn and the factory fixed wastegate is that the Merlyn can fully close. Perhaps it is not fully closing and you are wasting exhaust overboard. This would probably not be seen on the ground because there is no need for the wastegate to close.

[N231BN]

The "pop-off" valve pressure is absolute, it wouldn't be very useful if it wasn't.

In addition to the other things you mentioned check the alternate air valve and of course check for exhaust leaks.

[tmo]

Probably a stupid q, but the DA at FL200 wasn't some obscene value that would put you that far above critical altitude? It is awfully hot this summer.

[N231BN]

The density altitude today at FL210 was 23,760.

I think Merlyn advertises critical altitude of 19k but seems like most get 20-21.

I haven't had my 231 that high but I'm surprised you could get 30" while LOP. I'm usually full throttle at 15k while cruising LOP. What RPM were you running?

[Warren]

I have the same operating experience.  I can usually get it to run LOP at 210 even on a warm day with about 30" MP ( I have the Merlyn wastegate controller and the intercooler).  This is the limit for me to run LOP.  By 220 I need to be ROP and burn a lot more fuel - the winds have to be really good to be worth it or blue sky on top.

Not sure what your issues are but I agree that the MP is low.  I think it is likely to the be the wastegate or the turbo efficiency.  It would take a pretty big leak to lose 3" of manifold pressure.

Take a look at the waste gate.  I think Merlyn will inspect and refurbish for a pretty reasonable price if needed.

Good luck

[carusoam]

Hmmmm….

Lots of good input here…

Something has changed, including the weather…

Waste gate control could be affecting the turbo rpms…

There is a lot written about using ‘mouse milk’ to lubricate the really hot parts of the MP controller…

It sounds like the gate isn’t closing the way it used to… when the parts gets hot.

 

See if @M20Doc is passing overhead…. (Turbo output lower than expected)

 

If it is only the warmer weather…   We have the intercooler guy around here as well….

Best regards,

-a-

[mooneyflyfast]

On my Seneca 11 (same engines without Merlin wastgates) when I had an engine not making critical altitude it was usually an exhaust leak.  Hook up the blow side of your shop vac to the exhaust and do a soap bubble test on it.

[carusoam]

Hot side exhaust leaks happen….

And are very dangerous….  Blow torch under the cowl dangerous!

Be on the look-out for exhaust leaks, between the engine and turbo…

PP thoughts only, not a mechanic…

Best regards,

-a-

[jlunseth]

On 8/17/2021 at 6:50 PM, N231BN said:

I haven't had my 231 that high but I'm surprised you could get 30" while LOP. I'm usually full throttle at 15k while cruising LOP. What RPM were you running?

Something is not right then. I can get full throttle into the 20s depending on the day. I am generally at 34” and 11.1 GPH LOP at 16k, if the engine temps will stay reasonable. That is about my altitude limit with that mixture though. But not the throttle limit, the throttle in my plane has several thousand more feet before it maxes out. 15k is more like the non-Merlyn, fixed waste gate critical altitude.

[N231BN]

Something is not right then. I can get full throttle into the 20s depending on the day. I am generally at 34” and 11.1 GPH LOP at 16k, if the engine temps will stay reasonable. That is about my altitude limit with that mixture though. But not the throttle limit, the throttle in my plane has several thousand more feet before it maxes out. 15k is more like the non-Merlyn, fixed waste gate critical altitude.

What RPM are you running?

[jlunseth]

2450

That mixture is 71% HP and produces really good cruise speeds. 

I will tweak the fuel flow back a little to keep the TIT at or under 1600, if TIT goes over 1600 I do something about it, and if I can’t control TIT, which happens in the high teens with that mixture, depending on day, then I will just go ROP. Out in ND, a lot of the year it is cool enough that I can get quite high though. 

There are also circumstances where LOP does not work in the winter because temps are just too cold. -54 dF at 21k for example. You need a warmer mixture, but I probably don‘t need to tell you that.

[N231BN]

5 minutes ago, jlunseth said:

2450

That mixture is 71% HP and produces really good cruise speeds. 

I will tweak the fuel flow back a little to keep the TIT at or under 1600, if TIT goes over 1600 I do something about it, and if I can’t control TIT, which happens in the high teens with that mixture, depending on day, then I will just go ROP. Out in ND, a lot of the year it is cool enough that I can get quite high though. 

There are also circumstances where LOP does not work in the winter because temps are just too cold. -54 dF at 21k for example. You need a warmer mixture, but I probably don‘t need to tell you that.

Now that I think about it, I was looking for a power setting that was WOT so I had the RPM down around 2250. Pure experimentation, trying to see if the fuel balance changed while WOT. I also don't think I have tried higher altitudes(15k+) since getting my Merlyn overhauled.

[M20Kid]

I've been away from this thread for some time because I thought I found the problem and was waiting to report facts.  My mechanic found the turbine vanes had shifted in the housing and were rubbing the top of the case, obviously setting up a situation where the turbo could not spin as fast.  I sent the unit to Main Turbo in California and he did overhaul the unit.  However, he said the rubbing was not significant enough that he could say for certain that it was a smoking gun.

My plane is currently down for annual.  My mechanic is double checking the exhaust manifold and intake manifold looking for leaks, as well as checking the Merlyn for losses.  The only part he can't readily check is the pop-off valve.  We are going to try to pump up the manifold pressure using shop air and see if we can get it high enough to verify leaks - hopefully none found.

Anyway, I'll report back when I know if the turbo overhaul fixed the issue.  Fingers crossed.

[carusoam]

Kid,

Did you get any feed back on what was left of the turbo’s vanes?  A pic would be interesting…

 

aside from the unusual alignment issue…

When the vanes get extra hot…. They have a tendency to grow in length, enough to contact the case…. When this happens they get shortened, and stop working as well as they need to in normal flight conditions…. One of the reasons people keep extra tight control of their TIT…

So… contact and friction with the case will definitely keep the turbo from spooling up properly…  Shortened vanes can allow exhaust gasses to get by without driving the vanes….

PP thoughts only, not a mechanic…. Good luck with the next steps.

Best regards,

-a-

[PeteMc]

Have you reached out to Merlyn?

Maybe sending your wastegate in to them as part of your Annual might me a consideration.  I was actually in Spokane years ago and actually had them take a look at mine while I was there.

[M20Kid]

I spoke with Merlyn about this issue and they said while it's a possible failure mode, it's not a common failure mode.  They sent instructions on how to check the unit locally without sending it off to them.  A simple procedure that is being done during the annual.  I'll report back whatever I find.

[M20Kid]

Here's a picture of the exhaust side of the turbocharger.  Rubbing at the top is obvious and, not so obvious from the picture, the bottom gap is greater than usual.  The sides appear normal.  It appears the turbine wheel was pulled up or case was pulled down slightly causing minor contact.  Again, Main Turbo said they were unsure if this was enough to slow it down and register the manifold pressure loss.  We'll see in a few days when it comes out of annual.

[M20Kid]

I got the turbo back and was so excited to fly that I forgot all about posting here.

The results are in and Gary Main at Main Turbo was spot on; the amount of contact between the compressor blades and the case was only responsible for half the pressure loss.  When I took the plane back to 21k with the fresh turbo I only got half the lost pressure back.  Still unsure where the leak is occurring but the pressure difference is small enough now that I don't want to throw large bundles of cash around to chase it.  My mechanic thinks it may be lower compression as the engine wears - I have over 1500 hours on the engine and about 1000 hours on the cylinders so this could just be normal aging.  Hmm ...

At least the plane is flying again!

[N231BN]

Good news about your turbo. I really doubt that low static compressions would be causing your issue.

Did you check out the alternate air system to make sure it doesn't have any leaks?