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Engine misfire help...

Michael Burbidge

Well Known Member
I just finished my third condition inspection. Before the inspection my engine was running great. On the first flight after the inspection I was about 15 minutes into the flight, at full power, climbing through about 4000 ft. when I got what I can only describe as a misfire. I continued to climb to 7000 ft. and got 3 more misfires, on the way up. I then leveled off, pulled back the power and headed back to the airport. I did not get another misfire.

In the first 15 minutes of the flight I was cruising at about 2500 ft. and probably 50% power. I did not experience a misfire during that portion of the flight nor on takeoff.

I have basic Van's instrumentation, so I do not have recorded flight or engine data. I also did not have the presence of mind to do any diagnostics. I was focused on gaining some altitude and getting back to the airport.

Mag checks on the ground were fine.
Startup and idle were normal.

Here's the basics of my engine:

1 P-Mag, 1 slick mag
O320-D3G
TSMOH @1150 hours

Here's what I did during the condition inspection that could potentially be related:

Ignition related
Replaced auto plugs on the p-mag, gapped
Cleaned aviation plugs on the slick mag, gapped
Removed and checked p-mag, retimed.
Retimed slick mag, it was about 1 1/2 degree off (it was overhauled last condition inspection. I think some drift is normal.)
Slightly re-routed plug wires
Ohm check on auto plug wires

Induction related
Replaced intake pipe gaskets. (two were leaking)
Replaced intake pipe hose and hose clamps.
Cleaned air filter
Cleaned gascolator (no debri found)

Oil related
Removed oil cooler, repaired baffle cracks, replaced oil filter
Rerouted bottom oil cooler line to avoid chaffing
Replaced, rerouted breather tube to avoid new oil cooler line placement

I'm not sure where to start looking for the problem. So far I have rechecked p-mag and mag timing. Replaced the new auto plugs with another set. Looked for signs of intake leaks (blue stains, I'm not sure how long it would take so show up) Rerouted plug wires so that the aviation wires do not cross with the auto wires. (they were touching in one spot) My auto plug wires are separated as described in the installation instructions.

Having done this, my inclination is to go for another flight, staying close to the airport and if the problem persists, I will do a mag check at altitude, turn on the boost pump at altitude to see if the problem goes away.

I would appreciate any other diagnostics suggestions to help me find the problem.

Thank you,
Michael-
 
Could very well be some little pieces of carbon from cleaning the plugs that came loose and lodged to temporarily foul the plug or two. Have seen it happen and have had it happen. Something to consider?
 
Cleaned plugs again

I removed the 4 aviation plugs today and two of them had what looked like metallic debri from cleaning in the bottom. (I had the plugs cleaned and tested by an aviation plug cleaning service) I dislodged and blew out the debri. Perhaps that was the problem. We'll see.

Still looking for other suggestions on diagnosis.

Thanks,
Michael-
 
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Altitude related always makes me suspect the plug leads or plugs. Do you per chance have the auto plugs with screw on terminals?
If so swap them out for the solid top variety. Also make sure all the wires are seated properly.
Without an engine monitor it?s a lot harder to diagnose.
Tim Andres
 
Reading the list of things you did - rerouting the plug wires jumps out at me. I would look for chafing, looseness, possible induction issue.
I would also re-time both mags - for example, are you sure you timed the Pmag to TDC, and not 25 before or TDC with the Slick mag. I have heard of people making that mistake.
Curious why you would send your plugs out for cleaning? What can a service do that you can't, and does the guy working there take as much care as you? (Obviously not if you found debris in it!) If he drops your plug is he going to eat the cost of it, or hope nothing happened? If I drop a plug I throw it away and kick myself. A dropped aviation plug could lead to your symptoms.
 
can you do a mag check to see if you can isolate the problem to one mag or the other?
IMG_1635.jpg
 
I did have the screw type terminals. The solid terminals are hard to find. I have now found some and the second new set I put in are solid terminals.

I verified timing on p-mag at TDC and on mag 25 degrees.

Reseated all plug connectors.

I send my plugs out because I don't have a plug tester. They are pricey, and this place has always done a good job. But I hear your point and will reconsider in the future.

I'm planning on flying today to see if I've fixed the problem.

Thanks for the help,
Michael-
 
Good ideas in all the responses above. With respect to induction leak misfiring, I think it would be more likely to surface at lower power settings and at idle rather than full power. Like during taxi in from a flight. That's because with the engine at idle or low power the mixture will be extremely lean and out of whack. At full power when the carburetor butterfly is fully open the mixture wouldn't be as overly lean due to the increased fuel ratio in the mixture. A symptom of an induction leak would be no rise in RPM where there was one before as you pull the mixture to idle cut-off. Also, since you corrected some intake leaks during the condition inspection you should go through the process of re-adjusting your idle air mixture screw per the carburetor manual. I would consider this adjustment mandatory due to the work performed.

I haven't heard of a professional spark plug cleaning service before. There must be as many piston airplanes as there are boats in the Puget Sound area to support that business model. I have no doubt the owner of that business is 100% trustworthy. But maybe he had an intern working for him who isn't as cognizant about the reprocussions of a dropped plug.
Be aware that if a dropped spark plug is re-used the cracked porcelain can lead to catastrophic engine failure due to pre-ignition. Not detonation (although that can exacerbate the problem) but pre-ignition. Pre-ignition is the worst of the two. The porcelain insulator inside the plug shields the cylinder chamber from the hot parts of the spark plug so if this porcelain is cracked and compromised the fuel-air mixture can be ignited by the hot plug prior to the magneto-controlled spark. The same pre-ignition trouble could have been caused by the debris you found in two of the plugs.

Also, if your 4 aviation spark plugs are Champion name brand and are over approximately 4 years old then change them. Champion plugs had an issue causing pre-ignition and the company re-designed them and re-vamped their production line since then. Same part numbers. Doesn't apply to the Tempest or other brand(s).

Jim
 
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Yeah, I often have order the correct plug, but I?ve fixed the same issue you are having by using the solid top. You can crimp the threads a bit to provide a ?self locking? feature, but I?d suggest you just order up a dozen NGK 3961 plugs from amazon or Napa and have them on the shelf for later.
The 3961 is the p/n for ngk solid top BR8ES plug many of us run.
I have also had to replace a Pmag harness that tested fine as to resistance, but keep giving me a little high altitude misfire.
BTW, you may find the silicon dielectric grease in the plug boot blackened if the plug screw top or connector was loose.
Tim Andres
 
I went flying today and did not get any misfires. So one of the several things that I did has fixed the problem. It would be nice to know which one, but that would have taken awhile.

As a recap here's what I did:

? Replaced the auto spark plugs with the screw terminal top with solid top.
? Cleaned out metallic remnants from the aviation plugs.
? Rerouted the mag plugs wires so they do not cross with the auto plug wires.
? Reseated all the auto plug wire terminal connections.
? Rechecked mag and e-mag timing, but did not change.


Thanks for the help!
Michael-
 
My vote is #3. My understanding is that Pmags run so well that you can't really tell when the aviation plugs are not firing. Try a mag check at altitude to see if it replicates your issue under the same condition.
Glad it seems to be fixed!
 
Not knowing your wx conditions, I may be way off base.
Low power cruise followed by full power.
With that said, your description sounds like you swallowed a small piece of ICE.

I also wonder if you were cruising at a very lean and low power setting, and then went full power, and climbed. This can cause an unnerving crackle.

You can do this during run-up too. Lean to LOP at 1700/1800, then forget to enrich the mixture and go to full power for take off.

Snap Crackle Pop.

Good luck

Joel
 
Your mixture gets quite a bit richer as you climb but you can't see it with steam gauges. I have a Dynon EMS and keep leaning to the same egt as on takeoff.

I was amazed at how much you can lean. Your plugs may be fouling from an over-rich condition.

On my T-6, if I didn't lean, by 3,000' it would start misfiring like crazy until and old T-6 instructor pointed that out to me.

Best,
 
Dumb question, are you starting on?

1. P-mag
2. Slick Mag
3. Both

You may have answered this, but I have missed it.
 
Joel, I was cruising a full rich, about 2500 feet, low-medium power setting. It was a clear day, about 32 degrees at 2500.

Bill, Start on p-mag only.

Michael-
 
Plugs and or wires

I had this happen before. Everything normal but flying along in cruise just felt a slight stumble of the engine. Passenger couldn't feel it but I know my plane. Since I was headed to a remote place in Western Alaska I decided to turn around and have mechanic look at it...took a lot of looking but found an intermittent fault on one plug wire. Replaced them all and never encountered issue again. It's usually the simple things...
 
Joel, I was cruising a full rich, about 2500 feet, low-medium power setting. It was a clear day, about 32 degrees at 2500.

Bill, Start on p-mag only.

Michael-

It is good that you are starting on the P-mag.

Burke has some good advice.

One of our first EIC customer called to tell us he had an intermittent stumble and we weren’t sure if it was the EIC or his ignition.

That customer was great and went through a bunch of tests for us.

Then one day, he felt it stumble and noticed that one of the coil pack condition bar graphs was dipping during the stumble and coming back up.

It turns out he had recently replaced his prop governor and didn’t secure the plug wires. When he pulled the cowl, he found one if his plug wires touching the engine mount and he could see a small mark where the spark energy was shorting through the wire to the engine mount. He moved his wire away from the mount and the problem went away.

Good luck solving this one. Feel free to give me a call, if you want to talk it over.
 
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