Help Diagnose Engine Problem - M20K TSIO-360-MB

[Z W]

Update to this.  New fuel pump was properly set up.  Plane now makes the proper MP.  However, the RPM problem kept getting worse, and it would only make 2400 RPM.  Some of you spotted this earlier which is impressive.  They sent the prop governor off for inspection and it was returned as in-spec.  It was overhauled not too long ago.  They then found a broken prop control cable (photo below).  After the cable was replaced, they reported RPM would only reach 2512 RPM on a static run, and requested that I come test fly it.  I went out today and saw about 2520 RPM on a full-power static run up.  I did two high-speed taxis down the runway up to 60 knots.  The RPM reached a maximum of 2600 RPM on the first run, and 2580 on the second.  2700 RPM is max and takeoff power.

This is not normal behavior for the plane.  Verifying 2700 RPM at the start of the takeoff roll is standard procedure and I've never seen the engine do this.

I did not take off.  The new shop manager is insisting the plane "needs to be flown" and that the RPM may not come up until it's in the pattern.  Supposedly they have talked to various other shops and mechanics familiar with Mooneys and they agree.

Questions:

1. What would cause low RPM other than a failing prop governor that could or should be checked?

2. Would you fly the plane in this condition, or allow someone else to fly it?  If I fly it, it will be to the nearest airport with another shop on the field, about a 15 minute flight away.

Any thoughts or suggestions would be appreciated.  Thank you.

 

[N201MKTurbo]

So, is the arm hitting the stop screw? Can you adjust the stop screw further out? Is there something keeping the arm from getting to the stop screw?

What MP does the governor hit max RPM? As you increase MP the RPM will increase until the governor starts controlling the RPM What is that MP?

[Z W]

I don't know about the stop screw.  This seems to be the page from the service manual, unfortunately without pictures:

[Guest]

53 minutes ago, Z W said:

Update to this.  New fuel pump was properly set up.  Plane now makes the proper MP.  However, the RPM problem kept getting worse, and it would only make 2400 RPM.  Some of you spotted this earlier which is impressive.  They sent the prop governor off for inspection and it was returned as in-spec.  It was overhauled not too long ago.  They then found a broken prop control cable (photo below).  After the cable was replaced, they reported RPM would only reach 2512 RPM on a static run, and requested that I come test fly it.  I went out today and saw about 2520 RPM on a full-power static run up.  I did two high-speed taxis down the runway up to 60 knots.  The RPM reached a maximum of 2600 RPM on the first run, and 2580 on the second.  2700 RPM is max and takeoff power.

This is not normal behavior for the plane.  Verifying 2700 RPM at the start of the takeoff roll is standard procedure and I've never seen the engine do this.

I did not take off.  The new shop manager is insisting the plane "needs to be flown" and that the RPM may not come up until it's in the pattern.  Supposedly they have talked to various other shops and mechanics familiar with Mooneys and they agree.

Questions:

1. What would cause low RPM other than a failing prop governor that could or should be checked?

2. Would you fly the plane in this condition, or allow someone else to fly it?  If I fly it, it will be to the nearest airport with another shop on the field, about a 15 minute flight away.

Any thoughts or suggestions would be appreciated.  Thank you.

 

The shop manager is full of beans, if it’s not making very very close to rated RPM stop and figure out why.  The engine should be able to reach full RPM in a few hundred feet of takeoff run, certainly well before lift off.

How is RPM being calculated/ displayed?  An electric tachometer, an engine monitor or an old mechanical tachometer?  Accuracy of the data first.

[EricJ]

Generally the three things that will affect max static rpm are the propeller fine pitch stops, the governor high rpm stop, and the engine's capability to produce power.    It is unlikely that the propeller pitch stops need adjustment if it was okay before.   If the governor was removed and replaced, it *should* be okay but may need adjustment on the high rpm stop.   The main other variable is whether the engine is making full power or not.

As mentioned above, noting MP and fuel flow during the next static or high-speed taxi runs may be useful, especially if you have pre-trouble data to compare to.

If the engine is fine, the next obvious thing may be the high-rpm stop on the governor.

If all of that doesn't point to something, then there's something unusual going on, imho.

[Z W]

1 hour ago, M20Doc said:

How is RPM being calculated/ displayed?  An electric tachometer, an engine monitor or an old mechanical tachometer?  Accuracy of the data first.

Good question.  It's displayed on a G500TXi installed in 2020 that has been trouble-free.  The old factory analog gauge is still installed on the far right side of the panel, but I did not think to observe it doing the high-speed taxi.  I will do that next time.  Thank you.

[Guest]

2 minutes ago, Z W said:

Good question.  It's displayed on a G500TXi installed in 2020 that has been trouble-free.  The old factory analog gauge is still installed on the far right side of the panel, but I did not think to observe it doing the high-speed taxi.  I will do that next time.  Thank you.

If it’s on your Garmin, I’d believe that it’s accurate.

[cbarry]

I’m not an A&P and I realize you’ve posted the new fuel pump was set correctly, but are you absolutely certain the mixture control lever on the fuel pump travels freely from stop to stop?  This lever is typically reinstalled when then fuel pump is placed back on the engine and could be off a spline tooth or so causing the travel to be limited. 
 

You know your aircraft and when it’s right you’ll know.  When it’s not right, you’ll know.  When it’s right, is when it would leave the ground—if it was mine.

[N231BN]

32 minutes ago, Z W said:

Good question.  It's displayed on a G500TXi installed in 2020 that has been trouble-free.  The old factory analog gauge is still installed on the far right side of the panel, but I did not think to observe it doing the high-speed taxi.  I will do that next time.  Thank you.

Can you post a graph from the last ground run? Fuel flow and fuel pressure?

[philiplane]

Never fly a plane in the hopes that it will get better in the air. If it's not right on the ground, it's not right. Don't be afraid to tell your mechanic that lots of crashes happen in these situations.  

[Z W]

46 minutes ago, cbarry said:

I’m not an A&P and I realize you’ve posted the new fuel pump was set correctly, but are you absolutely certain the mixture control lever on the fuel pump travels freely from stop to stop?  This lever is typically reinstalled when then fuel pump is placed back on the engine and could be off a spline tooth or so causing the travel to be limited. 

No, I am not absolutely certain.  I located the engine set-up procedures in the factory service manual, highlighted the fuel pump calibration instructions, and provided them to the shop, with the help of Mooneyspace.  A little while later, we got the plane back making full MP, but not full RPM.  So I guess I'm assuming it was set up correctly.  Without an A&P license, and with no other shop on the field, I'm working with what I have.  Which makes all of your input that much more valuable.  Thank you.

[Guest]

It’s not in the manual, but here’s what your governor should look like.  The stop screw and stop pin high lighted.

[Will.iam]

What is your max ff at full throttle? Even though 24.7 is better than the low side of say 22 if you go much more than 24.7 you could be adding too much fuel and the engine would not make rated power. When i had to have my prop governor OH due to an AD they said they did not mess with the stops but upon reinstallation we did have to adjust the stops to get to 2700. If everything else checks out turn the stop screw one turn and see if the rpm’s change if they don’t you can put the stop back to the original setting and rule out the prop governor. 

[Z W]

I'll grab the engine log SD card out of the plane hopefully today and post some logs of fuel flow, fuel PSI, MP, and RPM.  I was expecting to leave the plane in the shop for them to continue troubleshooting, so I did not remove the log card.  Instead they basically threw their hands up and said they can't continue to diagnose until someone flies the plane to confirm there's a problem, since they believe it is fixed and will make 2700 RPM in the air.

Last night I sent the factory service manual instructions for adjusting the RPM to the shop manager and asked for confirmation that process was followed.  Got this in response:

"I can confirm that we followed the service manual as listed in your previous email.  The prop governor was bench tested by Aircraft Accessories of Oklahoma.  Their technician said it was running at 2800 rpm which we had adjusted it too before removing it.  When I contacted them about the bench test, they requested we leave the adjustments where we have them for the bench test. Once they perform the test they would let us know the findings.  At that point they contacted me with information that it was adjusted to 2800 rpm.  They stated it was operating properly.  They adjusted it down to the 2700 rpm where it was supposed to be.  The adjustment screw was safetied when we received it.  No adjustments from the 2700 rpm were made."

To me, leaving the governor stop screw set and safety wired at a pre-determined setting sounds inconsistent with the service manual instructions for prop governor rigging, which says the screw should be adjusted until the plane makes the proper RPM.  Am I misunderstanding that?

[N201MKTurbo]

2 hours ago, Z W said:

I'll grab the engine log SD card out of the plane hopefully today and post some logs of fuel flow, fuel PSI, MP, and RPM.  I was expecting to leave the plane in the shop for them to continue troubleshooting, so I did not remove the log card.  Instead they basically threw their hands up and said they can't continue to diagnose until someone flies the plane to confirm there's a problem, since they believe it is fixed and will make 2700 RPM in the air.

Last night I sent the factory service manual instructions for adjusting the RPM to the shop manager and asked for confirmation that process was followed.  Got this in response:

"I can confirm that we followed the service manual as listed in your previous email.  The prop governor was bench tested by Aircraft Accessories of Oklahoma.  Their technician said it was running at 2800 rpm which we had adjusted it too before removing it.  When I contacted them about the bench test, they requested we leave the adjustments where we have them for the bench test. Once they perform the test they would let us know the findings.  At that point they contacted me with information that it was adjusted to 2800 rpm.  They stated it was operating properly.  They adjusted it down to the 2700 rpm where it was supposed to be.  The adjustment screw was safetied when we received it.  No adjustments from the 2700 rpm were made."

To me, leaving the governor stop screw set and safety wired at a pre-determined setting sounds inconsistent with the service manual instructions for prop governor rigging, which says the screw should be adjusted until the plane makes the proper RPM.  Am I misunderstanding that?

No, you have it right. The governor is a proportional pressure controller. Different propellers take different pressures to move the blades. With a proportional controller, there is always an offset between the set point and the process variable. To get the proper process variable the set point must be changed to account for different process gains.

After all that jibber jabber, it means it has to be adjusted for each individual instillation.

[Will.iam]

2 hours ago, Z W said:

I'll grab the engine log SD card out of the plane hopefully today and post some logs of fuel flow, fuel PSI, MP, and RPM.  I was expecting to leave the plane in the shop for them to continue troubleshooting, so I did not remove the log card.  Instead they basically threw their hands up and said they can't continue to diagnose until someone flies the plane to confirm there's a problem, since they believe it is fixed and will make 2700 RPM in the air.

Last night I sent the factory service manual instructions for adjusting the RPM to the shop manager and asked for confirmation that process was followed.  Got this in response:

"I can confirm that we followed the service manual as listed in your previous email.  The prop governor was bench tested by Aircraft Accessories of Oklahoma.  Their technician said it was running at 2800 rpm which we had adjusted it too before removing it.  When I contacted them about the bench test, they requested we leave the adjustments where we have them for the bench test. Once they perform the test they would let us know the findings.  At that point they contacted me with information that it was adjusted to 2800 rpm.  They stated it was operating properly.  They adjusted it down to the 2700 rpm where it was supposed to be.  The adjustment screw was safetied when we received it.  No adjustments from the 2700 rpm were made."

To me, leaving the governor stop screw set and safety wired at a pre-determined setting sounds inconsistent with the service manual instructions for prop governor rigging, which says the screw should be adjusted until the plane makes the proper RPM.  Am I misunderstanding that?

Well realistically an encore SB engine is the MB engine with turned up MP and more fuel to get 220HP at 2600 RPM. So it’s not going to over stress your engine if it’s at 36” and 2600rpm just less power. With just you and a light fuel load you should have enough power for a test flight. I would use the same rule as i do for all takeoffs if I’m not up to 70% of my rotation speed by 50% of the field abort. To be extra cautious make it rotation speed at 50% for extra margin. Then when it doesn’t go to 2700 at flying speed you can get that issue out of the way so they will stop using it as an excuse. Since you know in the past this is not how your engine performed. 

[Pinecone]

Hmm, M20 was designed for 150 HP O-320.  That is 75% power out of a 200 HP engine.

When I was doing a lot of formation work in my friend's T-34 (IO-550), I would lead 4 ship takeoffs at 23 inches, 2300 so the wingmen had plenty of power. Airplane flew fine.

 

Would not do that off a real short runway though.

 

[Z W]

I would have no problem doing a regular takeoff limited to 2600 RPM, solo and light and with plenty of runway.  The plane was flying just fine at 60 KIAS doing my high speed taxis and was up to that speed before using 1/3 of the 5000 foot runway.  There's plenty of power.  My concern comes from not knowing why it's suddenly only making 2600 RPM.  

A replacement cable and lack of adjustment of the governor screw stop is starting to sound likely.  If that's it, I would have no concerns flying the plane, but would rather just get the screw set properly.

[EricJ]

50 minutes ago, Z W said:

A replacement cable and lack of adjustment of the governor screw stop is starting to sound likely.  If that's it, I would have no concerns flying the plane, but would rather just get the screw set properly.

It won't be making full power at less than rated RPM, so it's definitely worth getting it sorted.   I agree that the likely culprit is the governor high-rpm stop adjustment, assuming the cable is rigged properly to get full travel and there's nothing preventing the engine from making full power. 

[Z W]

Spent all day working on this today.  Finally solved the original low manifold pressure problem.  During last annual, they repaired/rebuilt the air filter intake housing.  New/replacement not available.  Someone repaired the cloth boot by adding about 3" of rubber extension:

When the lower cowl was installed, this extended boot bunched and shoved down into the air filter housing, completely blocking off the intake, causing the alt air to come on.  Trimming this rubber boot down to remove the excess material fixed the intake problem.  After that, the engine was making full MAP (38", then settling to 35.5" which is perfect) and running great.  Turns out we probably did not need to spend months replacing the fuel pump and getting it calibrated.  But at least now we have a new fuel pump.

Still have a problem though.  RPM will not go above 2520 or so during static runup, or 2600 at 60 knots during high speed taxi:

Fuel flows are 24-26 GPH which is good, maybe a little high even.  MAP is 36" or even a little more, which is great.  RPM just won't go any higher.  Prop governor was previously sent out for IRAN, came back from the repair shop as in-spec.  Today the governor limit screw was adjusted all the way out, and the brand new prop cable was properly adjusted and reset for maximum travel.  Resulted in no change to engine performance.  Photo proof:

Still no more than 2500ish static RPM, and 2600 down the runway with takeoff power.  This is displayed on the G500Txi that is 2 years old.  The analog factory RPM gauge shows about 100 RPM lower at any given time, e.g. it was reading 2400 during runups and 2500 down the runway.  That's normal for it to read lower than the Garmin.

The engine runs, sounds, and feels great, as normal, now that the air intake problem is fixed.  Plane seems to have the usual amount of power.  Everything seems fine except for the RPM readout.

Any suggestions?  Running out of ideas. The service manual's "Propeller Governor Trouble Shooting" section's remaining suggestions are "Low engine power - Consult engine manual" and "Sticky pilot valve - Remove head and clean pilot valve..." which would appear to be a part inside the prop governor.

Another data point - discovered the shop changed the oil with 100W, instead of the 15w50 we've been running for years.  Asked them to change it which they say they did.  No idea if that would explain some of the trouble.  We plan to change it again ourselves here shortly.  Filters arrived this afternoon.

If anyone wants to review the full Savvy logs for more info, they are here:

https://apps.savvyaviation.com/public_account/da323253-5fc0-4292-8b9c-75ae9ddd8305

Had a great day working on the plane and learned a lot about it.  Thanks in advance for any suggestions anyone can give that we can either do or provide to a shop.  

 

[EricJ]

100W and 15w50 are the same viscosity (50 weight) at operating temp, so that shouldn't have mattered.    The "100W" isn't the spec "weight" for viscosity, it's a marketing number.

Whatever happened seems to have happened between 7/29 and 10/08, which doesn't narrow it down much.

FWIW, I was recently going over the engine data for my airplane and noticed a gradual loss of takeoff and climbout rpm starting when I did some intake maintenance a few weeks ago and also which included removing and reconnecting the prop governor control cable.    I was worried I'd affected the rigging or affected engine power or something, so on my most recent flight I just manually pushed my prop control forward again during climbout and my missing rpms came back.   I have a throttle quadrant and maybe the friction lock just got loosened up or something or there's something new interfering with the control that pushes it back a little bit that I didn't notice, but it might be worth trying to just make sure it isn't getting pushed back somehow.

Your latest engine data seems to show a runup before the last static pull, but the results look odd when looking at the EGT as well.    I couldn't tell what was a mag check and what was a prop pull, but it looks like maybe the prop pull was done on only one mag?   It's hard to tell looking at the data.

[Guest]

Has anyone checked/ measured propeller blade angles?  On a Hartzell propeller it can be adjusted in the field, on a McCauley sadly it can’t.

[Will.iam]

I change 100w in summer and multi weight in winter and no effect on prop governor. Are you sure the prop governor control arm is set to hit the stop in the govenor before reaching the end of the arc motion of the arm? My A&P made sure of that and that it reached that limit with another 1/8 inch of travel left in the control knob in the cockpit to make sure i could get the control arm full travel to the stop. If you adjust the rpm screw and get no changes then your control arm is limiting or something is not setup internally correct. 

[Z W]

4 hours ago, M20Doc said:

Has anyone checked/ measured propeller blade angles?  On a Hartzell propeller it can be adjusted in the field, on a McCauley sadly it can’t.

Not to my knowledge.  How would those get changed?  The prop has not be reinstalled.

[Z W]

11 hours ago, EricJ said:

Your latest engine data seems to show a runup before the last static pull, but the results look odd when looking at the EGT as well.    I couldn't tell what was a mag check and what was a prop pull, but it looks like maybe the prop pull was done on only one mag?   It's hard to tell looking at the data.

What seems odd about the EGTs?

Some of those runups look strange I believe because when they were done with the cowl off, the air intake boot collapses, blocking off the air intake completely and the engine starts running off alt air.  It won't idle correctly like that, by the way, and will only make 33-34" of MAP.  It was doing the same thing with the cowl on, intermittently, depending on how good of a job you did cramming the extra intake boot material up onto the inlet on the lower cowl during install.  I was the first person to realize this yesterday after several months of attempted troubleshooting when I noticed the alt air light was on during runup.  Verified it by having someone watch from outside the plane. The engine will absolutely suck that boot flat against the air filter and choke itself onto alt air.

Also in the latest data we were cycling the prop a lot, both at low and high power settings, trying to see if flushing some oil through the prop / governor would make any difference.  It did not.  Lots of high and low power prop pulls.  May make for odd engine data graphs.