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PMag Issue (final diagnosis - bad spark plug)

dweyant

Well Known Member
Got my plane back from paint on Friday at GLO. It looks great, I'm quite pleased with the job Mark did!

However, I noticed a bit of roughness in the engine flying home Friday. It was about an hour flight, and it wasn't that bad, but noticeable. I generally replace the spark plug about every 150 hours or at annual, and since my condition inspection is coming up in April I decided to go ahead and replace the plugs over the weekend.

I got a chance to fly her again last night, and now I'm getting about a 200 RPM drop and very rough running when I do the mag check (left mag).

I've got a call into Emag Air, but figured it wouldn't hurt to ask for suggestions on things to try while I'm waiting for them to get back with me. I did check the timing and both mags are still timed correctly, and I have 8 brand new spark plugs.

Might be unrelated, but going back and looking at #2 CHT's they usually run at about 340 in cruise, the last few flights they have been mid 320's.

And just to be clear, I don't think it is anything GLO did or didn't do with the plane, it is likely either something that has been slowly getting worse and I'm just now noticing, or since the plane hasn't flown in 6+ weeks that caused it (I almost never go more than a week without flying it).

Thanks for any suggestions.

-Dan
 
It is unlikely to be the P-mags. I only say that because you just replaced the plugs.

If they took the cowl off to paint it, there is a chance one of the plugs isn't seated properly.

What plugs are you running?

Also, we recommend replacing the plugs every 100 to 130 hours. The reason is that the ceramic insulation tends to breakdown over time.

If you have an EICommander installed, it will display the condition of the plugs, plug wires, and coil packs and over time you can see this deterioration. (It would also tell you if there is an issue with a specific plug.)

One other thing, what plugs are you running? Are you running the NGK BR8ES solid tips, part #3961? NGK makes two versions of this plug, one with removable tips and the solid tips, both are BR8ES. If you installed the removable tip plugs and one of them started unscrewing, that could account for your issue.

Good luck! (Check your PM's.)
 
When you are on the left mag and get the 200 RPM drop, what are the EGT's doing? Is there any one that looks to be misbehaving, or is it a average change.

Also, do you lean to peak RPM before doing the mag check?
 
I had a very similar issue last year on our -3...turned out to be some corrosion on the end of one of th plug wires. Emagair calls for resistance checks on the plug wires (I think at each annual), and when I did that, it was obvious which one was bad. New wire, and the engine was once again as smooth as glass.

Check the P-Mag manual for the test procedure.
 
An additional possibility is plug wire shorting. Make sure your plug wires are not touching each other or some other metal such as engine mount or engine case. I found that these automotive plug wires don't insulate as well as aviation plug wires.
 
Have had similar issues very recently. Brad Dement of EMagAir suggested I do an in-flight mag check while monitoring EGTs. Sure enough, when running on the suspect P-Mag my #1 and #2 EGTs would drop dramatically and the engine power output likewise would drop. The poor old C/S prop had to be wondering what that idiot pilot was trying to do!

Brad suggested the probable cause to be either a bad connection on the wiring between the main circuit board and the coil packs or a bad circuit board. He also suggested that a pair [(1 & 2) or (3 & 4)] of plugs not firing is almost never a bad coil.

I provide this info solely in the hope it may help you isolate faults in your installation.
 
The CHT dropping is a sign of one plug not firing. You should also see an elevated EGT from normal (whatever normal would be for that probe) of something like an extra 60-90dF.

As others have noted, corrosion wire faults or just a flakey plug from new. It happens.
 
Did you gap and ohm the new plugs, or just open the box and install?

I always gap and ohm, but have yet to find a NGK that's out of spec right out of the box.
 
I had a rough running Pmag a month or two ago... was a kinda weird deal. In September I sent my old 1200+ hour 113 series units in for check up and updates. Reinstalled with all new plug wires. Ops check okay at first...

Then, on a local hop, right one started running rough intermittently, but not in a manner that changed EGT/CHTs. Weird. Land, check wires and timing... fly again, all is well.

Next up is a long trip from Houston to Palm Springs; initially they ran fine for 3 hours, then the right one became intermittently rough again. Pecos to El Paso is a long stretch with a rough mag! Landed Santa Theresa, again checked timing, wires, etc. Couldn't find anything. Runup okay. Launch. 2.5 hours later over Blythe, it started running rough again! Ugh. Land, let it sit for 15 minutes. Cranked up, runup okay, fly the last 85 miles to Bermuda Dunes. This was a strange failure mode in that it seemed to fix itself after a "reboot"...

The next morning I had a loaner replacement unit on the way via overnight Fedex. The loaner unit ran as smooth as ever on the way home, roughly 8 hours with the same wires and plugs. So, my unit had a glitch; I never heard from the shop as to the actual failure, but whatever it was, it needed a new circuit board for $425. Ouch.
 
Mag drop about 200 RPM and emgine pop on bad mag.

Changed plugs twice. Made sure plug wires were not touching any metal without insulation. Checked resistance of plug wires all OK. Changed plug wires. Sent P-mag back for checkout it was fine. They said they could change the electronic modules I said go ahead. Installed P-mag after return and it worked great. This took about 5 months to get it fixed. They were always very helpful and sent new plug wires and a high voltage module. I never saw a EGT drop on the bad mag.
 
Same here. 113 Pmag, ran rough. Changed plugs, still rough. Sent unit and plug wires in for testing, they said all was fine. Reinstalled, still rough. Sent back again, replaced the board for $$$$. Immediately sold the unit and haven't looked back since.
 
So here is what I've tried to far.

1 - Replaced all 8 plugs with new ones.
2 - Checked timing on both mags
3 - Checked resistance of plug wires on #2 cylinder (.5 ohms)
4 - Made sure all of the spark plug wires are properly seated

Any other suggestions?

I'm thinking next step is to pull the mags and send them back for service?

-Dan
 
One other thing, that may or may not be related.

On the two flights that I've done since I got the plane back from paint, my headset is breaking squelch. Not all of the time, but enough to be annoying. Not sure if/how it could be related, but thought it was worth mentioning.

-Dan
 
So here is what I've tried to far.

1 - Replaced all 8 plugs with new ones.
2 - Checked timing on both mags
3 - Checked resistance of plug wires on #2 cylinder (.5 ohms)
4 - Made sure all of the spark plug wires are properly seated

Any other suggestions?

I'm thinking next step is to pull the mags and send them back for service?

-Dan

You might check those plug wires again - the P-Mag manual calls for 180 Ohms per foot of plug wire - so if you really only have .5 ohms on each of the cylinder #2 wires, you?ve got something anomalous going on.
 
I agree with Paul

Check those wires again. Your comment about "my headset is breaking squelch" is exactly what happened to me a couple yrs back. One of my plug wires had just decided it had done enough work. The resistance ohm'd out higher than expected. I replaced that one wire and all was quiet again. I then replaced all 4 ... then the other 4 this past annual. They had about 600 hrs of operation on them.
 
One other thing, that may or may not be related.

On the two flights that I've done since I got the plane back from paint, my headset is breaking squelch. Not all of the time, but enough to be annoying. Not sure if/how it could be related, but thought it was worth mentioning.

-Dan

That is symptomatic of a bad ignition wire or plug, amongst other things. Make sure that your plug wires are fully seated on the Pmag. I have seen several posts here, where they have backed off. There is not positive lock and they can creep off.

Larry
 
MAYBE THE COIL??

i have dual Pmags i bought in the early stages of my build (no warranty), but had only 50 hours of run time on them. during runup one of them dropped 200rpm rather than the normal 75 to 100 rpm. i changed the plugs just because but i didnt check the wires as i didnt think they would go bad in just 50 hours. talked with Brad and he had me check the EGTs. normal for mine was 1000 for each cylinder. when i switched to 1 mag #3 dropped to 900. i swapped the plug wires with #3 & #4 at the coil and now #4 dropped to 900. this indicated to me that the coil was bad and i was operating on 7 plugs not 8. confirmed this with Brad, and ordered 2 new coils, 1 to install and 1 for the shelf, at $110 each. PLUS shipping of course. seemed reasonable to me. i like spares, and Brad's service was excellent. got them 2 days later and had it installed in about an hour, and im no A&P. EGTs now normal for all 4 cylinders, both Pmags. if i never need the spare i'll be selling it here at a discount. so try the wire swap on the same bank of plugs (see the manual) and see if yours does the same.
 
Tried a few more things this morning.

Replaced the coil on the left mag. Engine still runs rough, and now the EGT,s on#3 drop off as soon as I run on just the left mag.

Is it time to send them back to EMag and have them look them over?

-Dan
 
Keep in mind this is a wasted spark system, one coil fires two plugs so if it were a coil problem you?d most likely see in on both cylinders served by a coil, 1&2 and 3&4 are your companion cylinders and will share a coil.
I?d study your EGT?s more closely and see if you can pin it down to a cylinder or pair of cylinders, then move plugs and or leads around until you pin it down or eliminate these high tension parts.
Good luck
Tim Andres
 
Tim - in conversation last week with Brad Dement of EMag he provided information which contrasts starkly with the suggestion you have made.

To summarize Brad:
- if ONE cylinder is not firing it is likely a bad plug, plug wire or, worst case, coil
- if TWO cylinders are not firing it is likely a loose connection on one of the two outboard wires in the 3-wire connector running from the circuit board to the coils, or a bad circuit board
- Brad was quite clear that a bad coil rarely causes two cylinders not to fire - bad coils typically show up as a single cylinder not firing.

In our case we had Cyl's 1 & 2 not firing (in-flight mag check resulted in terribly rough running and #1 & #2 EGT falling like rocks). Today I replaced the circuit board. Hopefully tomorrow the weather will be good enough for a test flight to confirm whether this fixes it. If not I'm going to be a bit steamed as the circuit board is not cheap at $425 plus shipping.
 
I'm officially out of ideas.

To recap. When running on the left mag (top spark plugs). My engine runs rough, and the EGT's on #3 drop from about 1400, to about 900.

I have tried.

1 - Pull through test. All four plugs are making sparks
2 - OHM testing the wires.
3 - Replacing all spark plugs
4 - Replacing the coil pack on the left P-Mag
5 - Replacing all spark plug wires on the left P-Mag

The left mag is running rev 40, and has about 340 hours on it in a little less than three years.

I guess I will pull the mag and send it to Emag Air in the morning. Anyone have any other ideas?

-Dan
 
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Left = Left ???

To state the simple and the obvious > You?re positive the left switch operates the left mag and the right switch operates the right mag ??
 
Figured it out!

What are the odds that after you have two bad spark plugs in a row?

Greater than 0 apparently.

I was ready to pull the P-Mags and send them back, when my neighbor suggested swapping the spark plugs just to see what would happen. We did it, and the problem moved from #3 to #1.

Replaced the spark plug with another new one, and it runs great now.

Thanks for everyone's help.

-Dan
 
Glad to hear you found the source of trouble, Dan.

As a follow-up to my earlier posts, today I flew with the new circuit board in our P-Mag. Engine ran like a top. Failure to fire #1 & #2 plugs was indeed a problem with the circuit board.
 
Figured it out!

What are the odds that after you have two bad spark plugs in a row?

Greater than 0 apparently.

I was ready to pull the P-Mags and send them back, when my neighbor suggested swapping the spark plugs just to see what would happen. We did it, and the problem moved from #3 to #1.

Replaced the spark plug with another new one, and it runs great now.

Thanks for everyone's help.

-Dan

Did you ohm check the last batch of new plugs, and if not, did you ohm the most recent bad one? If so, what was the reading?
 
Original post DATA
Might be unrelated, but going back and looking at #2 CHT's they usually run at about 340 in cruise, the last few flights they have been mid 320's.


In Instrument flying you learn to trust your instruments. The EMS is no different. The data was telling the story......

The CHT dropping is a sign of one plug not firing. You should also see an elevated EGT from normal (whatever normal would be for that probe) of something like an extra 60-90dF.

As others have noted, corrosion wire faults or just a flakey plug from new. It happens.


We have seen this on magneto's, what is the chances of two identical faults in overhauled mags? Not =0 either it seems.

Glad you found a cheap problem.
 
EI Commander is worth its weight when it comes to diagnosing Pmags. My pmags have been back to Brad twice in 4 years...both times because the EI Commander noticed something going south before it affected the engine enough to cause me any real issues. Both times Brad found and fixed the issue quickly because I could tell him exactly what the symptom was and which cylinder it was affecting rather than "the engine is running rough".
 
EI Commander is worth its weight when it comes to diagnosing Pmags. My pmags have been back to Brad twice in 4 years...both times because the EI Commander noticed something going south before it affected the engine enough to cause me any real issues. Both times Brad found and fixed the issue quickly because I could tell him exactly what the symptom was and which cylinder it was affecting rather than "the engine is running rough".

Thanks, that is nice to hear.
 
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What I find is really worrying is the original subject said P-Mag issues when it was clearly not, really unfair to any manufacturer when an issue is not with their product - it's about time some of us think before 'pointing the finger'.
 
Concerning plugs - I was just installing a new set in the Rocket and grabbed a few boxes of the NGKs for pre installation prep. First step was to gap them all, and then do the ohm check. First 4 were the typical 4500 that I've come to reliably expect from the many years I've done this. Started on another box and the first was 5500, as was the second, third and fourth. Strange. Set that box aside and grabbed another box. They were all back to 4500, as was the final box of 4. Same part numbers, all. Dont think its an issue, but just unusual to see this level of change, (and consistent across the whole box) from NGK.
 
Figured it out!

What are the odds that after you have two bad spark plugs in a row?

Greater than 0 apparently.

I was ready to pull the P-Mags and send them back, when my neighbor suggested swapping the spark plugs just to see what would happen. We did it, and the problem moved from #3 to #1.

Replaced the spark plug with another new one, and it runs great now.

Thanks for everyone's help.

-Dan
This makes me wonder if the box of sparkplugs was dropped sometime before they got to you.
 
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