Strange Mixture Issue

[Socalmooneypilot]

Hi there, I’m a long time lurker on the forum hoping to tap the collective knowledge here regarding a strange issue I️ saw yesterday. 

Airplane is a ‘68 M20G with the carbureted O-360 engine  

Following a missed approach, I️ went to full throttle with mixture full rich. After a few seconds at full power, the engine began to vibrate and I️ noticed the EGT needle (factory single-probe) spike very high, past my normal cruise setpoint. 

I️ messed with the mixture knob for a second, and the vibration ceased and the EGT fell back down to normal. I️ can’t tell if messing with the knob actually fixed the problem, or if it was coincidence.

Everything I️ experienced seemed consistent with suddenly having the mixture leaned passed peak, but I️ didn’t touch the mixture knob before the problem occurred. 

Any thoughts as to what on earth happened?

Engine has about 9 years and 400 hours since major overhaul.  It has flown over 100 hours/year for the last three years without missing a stroke. 

Thank you all!

[Yetti]

where were you flying, how cold was it, what was the density altitude at the time?

[Socalmooneypilot]

where were you flying, how cold was it, what was the density altitude at the time?

Flight was at Bakersfield, BFL, and this was on climb out at a near standard day so density altitude was around 1,000’.


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[Yetti]

Boost pump on or off?  Done a leak down test on the mechanical fuel pump?

[Socalmooneypilot]

Boost pump on or off?  Done a leak down test on the mechanical fuel pump?

Fuel pump was off, something I️ forgot on my checklist, bad on me. I️ did check the fuel pressure gauge when the issue happened, and it was indicating normally (about 4”).

I️ have done a fuel system leak test by pressurizing the system with the electric pump, but not sure how to do it with the mechanical pump alone. Do you think this is a pump issue?


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[Yetti]

If you were lean, then it would point to a fuel delivery issue.   Not sure how the mixture works in the carb, but someone someone will be along soon who knows about carbs.  Would probably be checking linkages of the cables.

[Hank]

6 hours ago, Socalmooneypilot said:

Hi there, I’m a long time lurker on the forum hoping to tap the collective knowledge here regarding a strange issue I️ saw yesterday. 

Airplane is a ‘68 M20G with the carbureted O-360 engine  

Following a missed approach, I️ went to full throttle with mixture full rich. After a few seconds at full power, the engine began to vibrate and I️ noticed the EGT needle (factory single-probe) spike very high, past my normal cruise setpoint. 

I️ messed with the mixture knob for a second, and the vibration ceased and the EGT fell back down to normal. I️ can’t tell if messing with the knob actually fixed the problem, or if it was coincidence.

Everything I️ experienced seemed consistent with suddenly having the mixture leaned passed peak, but I️ didn’t touch the mixture knob before the problem occurred. 

Any thoughts as to what on earth happened?

Engine has about 9 years and 400 hours since major overhaul.  It has flown over 100 hours/year for the last three years without missing a stroke. 

Thank you all!

Where do you normally peak? What was your cruise EGT? What did it spike to?

My C generally reaches peak ~1500-1525°F, and I cruise 1450°-1475F. One flight it went nearly to 1600°, so I richened it up, and a little later it was actually above 1600°. I finished the flight on jjst the Right magneto. The dead left magneto was not burning all of the fuel (or not burning it as quickly), resulting in hotter gases and / or flame going into the exhaust.

So the more details you can provide, the better our troubleshooting can be. When were your magnetos last inspected? My failed ine had been rebuilt by Kelly about 50 hours before . . .

[goterminal120]

did you have carb heat on. for some reason the 0-360 in a Mooney runs realy rich with the carb heat on.

[Socalmooneypilot]

Definitely didn’t leave the carb heat on. That’s really interesting about the magneto failure causing those indications. I’m going to do an in-flight mag check to see if I️ can reproduce the symptoms when I️ get back in town this weekend.

Unfortunately I only have the factory single-probe EGT without the temp markings, so I️ don’t know what the actual temps are. I️ just know it was high relative to my normal cruise point on the gauge.

Frustrating to diagnose these things without a 4-Cylinder engine monitor. I’ve been trying to save that money for ADS-B, but maybe it’s time.


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[kortopates]

53 minutes ago, Socalmooneypilot said:

Frustrating to diagnose these things without a 4-Cylinder engine monitor. I’ve been trying to save that money for ADS-B, but maybe it’s time.

Engine monitors pay for themselves over time, assuming we learn how to use it. And most often an engine monitor will alert us to a problem before the fan stops and very often giving us enough warning to save the engine. That's pretty priceless in my book.

[Andy95W]

This sure sounds like an over-lean mixture to me.

It is possible for the mixture control to bind in the sheath that it runs through.  It looks like a tightly coiled spring that can collect dirt over the years and "grab" the wire that actuates the mixture arm on the carburetor.  The wire and sheath can then flex so the knob continues forward but the wire is still stuck.  So, even though you pushed the mixture knob full in, the arm on the carb wasn't. 

On the '68 M20G you should be able to easily remove the right side (copilot) cowling cheek panel, which will give good access to the carburetor.  The mixture control is at the top of the carburetor.  Have someone inside run the mixture control full out then full in, you need to be looking to make sure the mixture arm moves fully forward until it meets the stop.  While you're in there, spray some lubricant on the wire and sheath while your helper works the control.  Tri-Flow or LPS-2 is recommended.

Lastly, the mixture control itself, like all the engine controls, should not contact the instrument panel, but should stop about 1/16-1/8" short.  This ensures the control is actually hitting the stop and not stopping at the panel.  You'll need your A&P to make this adjustment, and while he's there he should disconnect the wire and ensure the mixture arm moves smoothly forward and aft.  He should also spray lubrication on the spring sheath inside the cabin under the panel and work it in there.

Your mixture control might have been intermittently binding for a while, but you never had a chance to notice before doing the go-around.  Under normal circumstances, your next move would be idle cut-off when you got back to your hangar.

Good luck, let us know haw you make out.

[Socalmooneypilot]

Went for a test flight today, now I'm more confused.

Takeoff was normal- nothing unusual about the EGT or engine vibration levels.

I did a mag check in my climb configuration- WOT, 2600 rpm, 100 mias, full rich mixture.  When I turn off one of the mags I get engine vibration and an EGT spike, identical to what I saw before.  Behavior is the same on either mag.  Going back to both mags makes things go back to nominal.  Awesome, found the problem.  It's been 8 years/400 hours since the last mag inspection, probably time for a mag overhaul.

Then, back in the pattern, things get weird again.  I'm at 17" MP and 2300 RPM, full rich mixture, and the EGT suddenly climbs again.  This time it climbs right to my normal cruise temp, which I estimate is somewhere around 1450 deg.  But here's the thing- I do another mag check, and I still get a drop on either mag.  So both mags are functioning even when the issue is occurring, as far as I can tell.  Additionally, I make the EGT go up and down with the mixture, but it still returns to the too-high temperature when I go back to full rich.  I was able to make the EGT return to normal by throttling to idle, then back to 17".

So in summary-

• Issue is intermittent, seems to come and go without warning, and happens at multiple power settings
• Mags and mixture control seem to be functioning normally, even when the issue is happening

I wonder if this is a fuel distribution issue, or something going on with the cylinder that has the single EGT probe?  I suppose the next step is to borescope the cylinders and look at the plugs?

Other thoughts are appreciated!

[carusoam]

Oddly...

Sounds like....

• three plugs working properly on one mag.
• three plugs working properly on the other mag.
• runs moderately well on both mags.
• two bad plugs are not in the same cylinder.

The odd EGT rise will indicate which cylinder has an issue under which mag is operating...

No engine monitor to diagnose the problem...

Do you have Champion spark plugs?

Do you run full rich on the ground?  

Have you removed the lower plugs to see what is in them?  Wet with oil or large lead beads?

pullng plugs is easy and low cost. Take pictures of what you find. Keep good track of what cylinder they come from...

Without data, you kinda get stuck in the swapping out things that may not be bad...  been there before...

Best regards!

-a-

 

[Socalmooneypilot]

Thanks for the quick reply Carusoam.

I'm using Champion plugs, standard type (not the fine wire ones).

I lean religiously on the ground, and in flight at low power settings.  I've never had any fouling issues, and my A&P always says the plugs look good at the annual.  Last clean/gap was three months ago.  Plugs have about 300 hours on them, all changed at the same time.

I would love this to be a plug fouling issue, simple and cheap to fix.  We did notice an ignition wire chaffing on the doghouse at last annual and wrapped it with electrical tape.

I'm going to call the A&P and see if he has some time to check out the plugs and wiring next week, and get his thoughts on troubleshooting.

Really wishing I had an engine monitor to help with this.

[carusoam]

Champion plugs have been known to go bad...

Their internal resistance increases over time.  Easy to remove and measure...

If your plug wires are original, they are really easy to swap out, and not too expensive.  The kit comes with parts all the way back to the mag...

The ignition wires are armored beneath the outer rubber or braided/fiber (original) layer...

I think Aircraft Spruce May still have a drawing of how the ignition wires are constructed.  Their biggest challenge is making a good connection at the plug.  They get removed and reconnected so many times in their life.

How many hours on the carb?  Any history of OH?

Ever Check the induction tubes to see what is growing inside there?

There are a couple of air leak possibilities and seals that can be replaced for a few dollars... four seals and a few small bolts and a can of carb cleaner... check with your mechanic to see if he will look over your shoulder for this... the bolts may be a challenge to remove because the environment they are in.

Can you tell, I had the lowest cost M20C, with very little instrumentation to work with, and NO MS to help...?

PP thoughts only...

Best regards,

-a- 

[Browncbr1]

If you’re doing a lot of approach practice at low power settings, you’ll be lead fouling plugs if not leaned.   Full rich when not full power will eventually lead to lead balls in your plugs.  

Mid you’re planning on doing low power much, I recommend running a little TCP.  I find that it helps things during training.   

[RobertGary1]

Before spending real money I’d pull the plugs and inspect them.  This could just be lead fouling and you need to lean more aggressively when not at wot. 

Could also be oil fouling  

-Robert

[kortopates]

Sounds like ignition system is working - but truth is with only one EGT sensor you can't tell if you had a plug missing on both mags giving similar like drop so I would pull and check plugs to eliminate the simple stuff. But your symptoms make me suspicious of your carb with a possibly leaking seal or gasket. 

[Socalmooneypilot]

I love MooneySpace.  In the end we found two smoking guns, and fixing both together solved the problem.  Long writeup below for future searches.

My A&P and I pulled off the cowling last weekend and found major problem #1.  There was a fairly major leak at the intake flange for the right front cylinder.  Blue 100LL dye was all over the intake, cylinder, and had even sprayed onto the inside of the cowling.  Closer inspection found that one of the bolts securing the intake flange had backed out, and contributing to the problem was a cracked gasket.  My A&P says a leak like this will cause the cylinder in question to run excessively lean, which likely explains the majority of the engine roughness.  @kortopates gets credit for that one.

What was interesting is that this was not the cylinder with the single EGT probe, and a subsequent test flight showed a smoother engine but still high temperatures.

So we pulled the cowling again and focused on the cylinder with the probe, the left rear.  As many of you suggested, we found significant lead deposits on the bottom plug.  We elected to pull all eight plugs for a very thorough cleaning and gapping.  The following test flight showed the temperatures were back to normal and the engine felt better than before.

A few lessons learned-

1. Next time I'll at least pull the cowling off before spooling up the experts at MooneySpace.  It didn't take a rocket scientist to find the intake leak.
2. My A&P suggests I clean/gap the plugs at each oil change, rather than at each annual.
3. I'm in talks with my local avionics shop to install a used EDM-700.  The quote I got for install was more reasonable than I expected and it is worth the piece of mind.  It would have really helped in this situation.

[carusoam]

Nice follow-up, SoCal!

The awesome power of MS at work!

Best regards,

-a-

[Bob_Belville]

SoCal, I'm glad your problem is identified. A couple of suggestions:

• It's a good idea to remove the top cowl with some frequency and to look around for "stuff". You don't have to be a mechanic to see things amiss. At oil changes valve cover screws need to be snugged and any leaks should be identified.
• If you've gotten 300 hours out of Champion massives you've done better than average. You should do the resistance test suggested above. ttps://blog.aopa.org/aopa/2015/03/19/champion-from-denial-to-acceptance/
• An A&P can install an EDM. A&Ps have a lower shop rate and are more convenient, probably the mechanic that does your maintenance on your home field. 

[Socalmooneypilot]

Thanks Bob for the link to Mike’s article. I️ did not know the story behind champion. I will do a resistance check at the next oil change, about 20 hours to go, unless something starts acting up sooner. 

Have most people switched to Tempest plugs at this point? I️ don’t mind replacing all 8 if there’s a compelling quality story to it. 

[carusoam]

Tempest fine wires...  seem to be the way to go...

Not perfect, but not long term terrible like the other manufacturer...

Fine wires for better handling lead build up and oil that is often present in the lower plugs...

If you only change four, do the lower ones first...

 

Other plug problems have been found with screw off connectors at the top of the plug.  They get loose and unscrew over time...

can't remember the details on this one...

Best regards,

-a-

[Hank]

16 hours ago, Socalmooneypilot said:

Thanks Bob for the link to Mike’s article. I️ did not know the story behind champion. I will do a resistance check at the next oil change, about 20 hours to go, unless something starts acting up sooner. 

Have most people switched to Tempest plugs at this point? I️ don’t mind replacing all 8 if there’s a compelling quality story to it. 

The Tempest fine wire plugs are very popular with our fuel-injected brethren. As part of solving the magneto issue I had last spring / summer, I purchased 8 new Champion massive plugs and a new wiring harness with 90º elbows premade instead of having to fab something or add connectors. My old harness  cable to the left lower plugs was pinched between the battery box and the doghouse, but I couldn't tell until I took it off. Through the rubber and the woven shielding . . .