300+ rpm mag drop, then engine quit

[matthew_william]

1966 Mooney M20E 200hp

I was with a CFI, doing a runup just prior to takeoff to shoot some practice instrument approaches when we noticed the engine running rough.

Both mags dropped 300 or so rpm several times in a row. Mixture was leaned out during prior taxiing. Then, after trying to run the engine lean and burn off any excess lead, we throttled back to 1,000 rpm, and the engine quit. We called ground control, started up, and went back to the hangar. Obviously not worth the risk.

We've had fouling issues in the past, and we lean quite aggressively during taxi to minimize this, and we also put an additive in our fuel to help reduce fouling.

Four days prior we flew, and did 4 ILS approaches totaling 1.5 hours. There was no leaning during this time.

I'm a <200 hour pilot, and as you can imagine, it kind of spooked me a bit and I'm trying to better understand why the engine ran so rough and quit during runup. I'd like to avoid an off-field landing! Any ideas?

[aaronk25]

What kind of condition are your spark plugs in? How want hours in them and when were they last cleaned?

When was last mag inspection?

[jetdriven]

Was the mag drop accompanied by rough running, or was it smooth?

[triple8s]

I was a Cessna pilot before the Mooney and one of the things I had to learn was the leaning required to keep the plugs from fouling while on the ground. Not saying you werent leaning properly but I know I wasnt in the begining, also, it is even more crucial in hot weather. SOP for me now? Lean till stumble at Idle and then richen slightly. Forget to lean and it will show up at the run up. I began to have starting problems and at run up last year, turned out the mags were getting weak so I had them OH last september during annual. You may have mag problems if you are already leaning after start up. Usually if you have any mag trouble it will show up during start up or run up. I flew a 172 with an O-360 lyc and leaning for taxi wasnt an issue in that plane even though it had the same engine, but in "my" Mooney you will lean for taxi.

[FloridaMan]

When the coil went out in my mag, it would idle, but the engine quit at run-up RPM when we cycled to that mag. Different, I know, but just thought I'd mention it. Did you notice anything weird with your EGTs?

 

Also, one thing that helped before I put in new plugs with the run-up was to lean at run-up RPM until the EGTs increased by 75 degrees before doing the mag check. 

[jfdez]

This sounds like it's time for new Mags.

[N601RX]

Take all the plugs out and look at them, especially the bottom. If they get bad enough leaning will not clear them. Also check the resistance of them. I've had a Champion plug go bad with less than 30 hrs on it. My IA has a cleaner/tester that will test fire them under pressure. The bad one I had would fire fine at ambient pressure, but as the pressure was increased it would stop firing.

If you want to get rid of your plug fouling issures buy a set of Tempest fine wire plugs. Not cheap, but worth it IMO.

[Riq]

If you want to get rid of your plug fouling issures buy a set of Tempest fine wire plugs. Not cheap, but worth it IMO.

+1

I lean aggressively during ground ops in my cherokee, and it still needs a plug cleaning/change every now and then, but the fine wires in my mooney have seemingly lasted forever and require no maintenance. This year the cherokee gets motor work and new fine wire plugs. 

[bumper]

"If you want to get rid of your plug fouling issures buy a set of Tempest fine wire plugs. Not cheap, but worth it IMO."

 

+2

 

And don't get the Champion fine wires. While Champ's non-sealed resistors may hold up long enough to see a set of massive plugs through their expected lifetime, the same cannot be said their fine wires. The Champion resistor design will fail the fine wire plugs long before the electrodes are worn. Just replaced an otherwise good set of Champion fine wires due to failed resistors - - and they arced and corroded the contacts inside the plug too, so stealing good resistors from a set of Champ massives wouldn't work either (I tried).

 

Tempest fine wires are the better deal. Less expensive too.

 

bumper

[FloridaMan]

I'm still not sure if this is helpful at all or if your issue has anything at all to do with spark plugs, but while people are on the topic of plugs, I had a rough running mag and a set of REM37BY took care of the issue for far less money than fine wires. 

[bumper]

Near term, I agree with Antares . . . and for my spare parts kit there's a couple of new massive. But for long term, I think fine wires more than pay for themselves in longer life, less maintenance (including far fewer lead balls to pick out), and maybe a tiny bit of improved engine effeciency.

[matthew_william]

Thank you all for the help!
 
I think we have found the issue.  The mechanic removed the plugs, and said they were some of the worst-fouled plugs he has ever seen, which is surprising since it had its annual from the same place late October.  We did have fine wire plugs put in at that time, and have been putting Alcor TCP fuel treatment in the fuel ever since (because of previous fouling issues).  There was also a bolt missing from the exhaust stack that was replaced; they indicated that this could have caused more air to be sucked in and contributed to the problem.  The plugs were cleaned, the missing bolt replaced, and the engine is running great.  Mag drops on both right and left are dropping 80 rpm (what it was before).
 
The problem came on suddenly after I had been doing practice ILS approaches with my instructor.  Very little time in between approaches to do any leaning, and the fuel pump was on quite a bit.  This surely could have contributed to the problem, but the main problem seems to be too much fuel on a cold start.  I was following the standard checklist procedure: fuel pump on, throttle one turn, mixture rich 3 seconds, mixture lean, fuel pump off, and crank.  I was told that this engine is easily flooded on cold starts, and that I should use the following procedure for a cold start: fuel pump off, mixture full rich, one turn on the throttle, and crank.  If it does not start, turn on fuel pump for 1-2 seconds, and crank again.
 
I will update again after a few more hours of flying time.  See attached photo.  This came out of the spark plugs (both top and bottom).  Don't worry, the pocket knife didn't come out of the spark plugs (just for size reference)!

[butchgilbert]

Take all the plugs out and look at them, especially the bottom. If they get bad enough leaning will not clear them. Also check the resistance of them. I've had a Champion plug go bad with less than 30 hrs on it. My IA has a cleaner/tester that will test fire them under pressure. The bad one I had would fire fine at ambient pressure, but as the pressure was increased it would stop firing.

If you want to get rid of your plug fouling issures buy a set of Tempest fine wire plugs. Not cheap, but worth it IMO.

 

 

 

Strongly recommend Tempest spark plugs.  Also the mechanic should check your idle fuel mixture setting, it really makes a difference.  Your engine should not foul the plugs that bad after 4 ILS approaches.  After 13 years of ownership I finally got mine set correctly, it makes a world of difference. 

 

IHMO.

 

Butch