Unusual RPM drop on one mag at mag check (P-Mag)

[Westerhuis]

Hi,

I have noticed an increase in the RPM drop when doing the mag check before take-off. I have a fuel injected IO-360 with dual P-Mags installed from new and now flown app. 685 hours.

I used to get an RPM drop of app. 90 on the right mag and 110 on the left mag (from 1600 RPM.) However, the last couple of days I am seeing a drop of 140 - 160 RPM on the left mag only and the right mag continues to drop by app. 90.

I have very recently installed a new throttle cable and had to make slight adjustments to the mixture / throttle setting to get the required tick-over RPM. I also had a CHT reading going off the scale on warm-up, and resolved that issue by replacing the CHT probe.

I do have an EI commander and haven't noticed any unusual readings (but I must admit, I didn't pay particular close attention to it)

Any idea what might cause the increased drop in RPM on the one mag?

Thanks!

Rogier

 

[Dad's RV-10]

My experience with P-mags is limited but I have seen about as little as 100 RPM drop but as much as 180. The engine seems to run and perform fine (IO-375 w/dual P-Mags).

Based on the below information form the P-Mag manual, I haven't really obsessed over the amount of RPM drop:

The left/right Ramp Check rpm drops may be quite a bit different with E-MAGs than magnetos. We don’t apply the same rules to gauge ignition health.

Any rough or degraded behavior (before, during, or after each side’s Ramp Check) indicates a problem - not suitable for flight.

 

[Carl Froehlich]

The best way to verify pMag (or standard mag) operation during a mag check is to monitor EGTs. Turn off one ignition and all EGTs should go up. If not, then first look for bad wire or a bad plug.

Carl

 

[Toobuilder]

Also suggest you lean to highest RPM at the EOR ignition check. And as Carl suggests, watch the behavior of each EGT rather than focus on a RPM number on single ignition.

 

[lr172]

The RPM drop tells you several things, but one of the big ones is the timing difference. My first guess would be that the timing is drifting on one of the the Pmags. Could be several reasons. Pmags seem to have delicate bearings and a failure there could be the cause (they need to be pulled each year and checked due to this common failure), as could a leak in the vacuum source. Older s/w versions also were known to have the timing reset without human intervention.

 

[rv8ch]

My experience with P-mags is limited but I have seen about as little as 100 RPM drop but as much as 180. The engine seems to run and perform fine (IO-375 w/dual P-Mags).

Same for me with my IO-360, FM200, dual pmags. I tried changing the plugs to be 2 on top and 2 on the bottom, but it didn't change anything.

 

[Bavafa]

What is the timing divergence that the EIC is reporting? That is the screen that I monitor 99% of the time, both in the air or on the ground. But as others have said, EGT is what I look during the run ups.

However, returning to your possible issue, how are your sparkplugs wired? Does one PMAG go to all bottom plugs and one to the tops? If so, it is possible that your bottom plugs need attention. If timing divergence is not there, I would pull the plugs and have a look at those. Also, you can look at the vertical bars on EIC that checks your sparkplug wires and sparkplugs to see if there is a chance in behavior on those.

 

[Carl Froehlich]

SNIP Pmags seem to have delicate bearings and a failure there could be the cause (they need to be pulled each year and checked due to this common failure), as could a leak in the vacuum source.

This is a fairly broad brush comment….

I have been running pMags on three RVs for well over 2500 hours. Never had this issue. I do the annual bearing checks - simple enough and much less work overall to maintain a pMag then standard mag.

As with any ignition, if timing is not the same between the two you will see different RPM drops (the one with the more advance timing will show a larger RPM drop when it is turned off). This RPM difference is also very noticeable when running one pMag and one standard mag. At typical mag check RPM the standard mag is, as always, at base timing (25 degrees BTDC for most parrallel valve engines). The pMag will be at something less than base timing as engine RPM is not yet in normal operating range. So when the mag is turned off it will show more of an RPM drop then when the pMag is turned off. This is normal.

Carl

 

[Walt]

This is a fairly broad brush comment….

I have been running pMags on three RVs for well over 2500 hours. Never had this issue. I do the annual bearing checks - simple enough and much less work overall to maintain a pMag then standard mag.

Carl

Many of my customers have pmags.
On average they fly around 50hrs/year, some a little more others a little less.
That equates to 10 pmag R&R’s to 1 mag R&R at 500hrs.
I’ve sent a number of pmags in for bearing replacement.
Mags are still cheaper to operate.
PMAG service price is close to mag OH cost these days.

 

[lr172]

This is a fairly broad brush comment….

I have been running pMags on three RVs for well over 2500 hours. Never had this issue. I do the annual bearing checks - simple enough and much less work overall to maintain a pMag then standard mag.

As with any ignition, if timing is not the same between the two you will see different RPM drops (the one with the more advance timing will show a larger RPM drop when it is turned off). This RPM difference is also very noticeable when running one pMag and one standard mag. At typical mag check RPM the standard mag is, as always, at base timing (25 degrees BTDC for most parrallel valve engines). The pMag will be at something less than base timing as engine RPM is not yet in normal operating range. So when the mag is turned off it will show more of an RPM drop then when the pMag is turned off. This is normal.

Carl

I didn't make this up Carl; It is a required annual inspection according to eMag. If it was robust and not prone to early failure, it wouldn't have an annual inspection requirement.

 

[Dan 57]

Many of my customers have pmags.
On average they fly around 50hrs/year, some a little more others a little less.

Exactly, and that is another reason I would not install Pmags on a new build… is calendar time possibly ruining them bearings? If not, that would be running time. And there are vast differences in the amount one flies a year. As an example I almost hit 500 hours last year, some other RVs on my field barely hit 20 hours.

Maybe some other factor I don’t get? I also thought the bearing check was every 100 hours, which would make sense (to me), but it seems it is now during the condition inspection only

 

[Tankerpilot75]

Recently dual PMags were installed when my engine was rebuilt from a prop strike. First, I’m not a great engine person, but after about ten hours on the rebuilt engine, it started having a 200 rpm drop during the mag check. Troubleshooting the problem, I discovered one PMag wouldn’t enter “program mode.” I subsequently sent the PMag in for warranty service where they said a bearing was too tight. They replaced it and returned it to me within 48 hours!
After reinstalling the offending PMag both mags now indicate about a 140 rpm drop during the mag check. So I guess there was a bearing issue.

Now I’m working on getting CHTs (#3) to stay below 400*. Last week following several VAF suggestions I moved timing to about 6* ATDC and moved the one larger injector tip (0.0285 vs 0.0280) to #3 cylinder. Hopefully these two changes will address the #3 CHT issue and cool things down a bit. I’ll find out as soon as things cool down a bit. With daytime temperatures above 100, and taking an hour to drive to the airport - it’s no fun to fly - so I’m waiting for cooler temperatures to check out my work.

As others have said, bearing failure is a possibility, and in my case did cause rpm’s to have increased deviation during the mag check. I suspect EMag is working to increase bearing reliability and stability.

 

[Carl Froehlich]

SNIP
Now I’m working on getting CHTs (#3) to stay below 400*. Last week following several VAF suggestions I moved timing to about 6* ATDC and moved the one larger injector tip (0.0285 vs 0.0280) to #3 cylinder. Hopefully these two changes will address the #3 CHT issue and cool things down a bit. I’ll find out as soon as things cool down a bit. With daytime temperatures above 100, and taking an hour to drive to the airport - it’s no fun to fly - so I’m waiting for cooler temperatures to check out my work.

First verify you do not have another problem - like a clogged injector or spider port. Do the “shot glass” test. Also verify both plugs working in that cylinder. Do a simple check for an air leak by taking data at WOT at altitude (if there is an air leak on that cylinder, at WOT the air leak will have minimal effect - so if the CHT comes in line with the other cylinders an air leak is indicated).

I suggest you do not guess when changing injector sizes. Do a full GAMI run, then change the most offending cylinder. I go after “the last to peak” first and install a smaller injector, then do another run and adjust the new “last to peak”) cylinder. It typically takes three data runs to get the GAMI spread to ~0.1GPH. Don at AirFlow Performance will look at your data and recommend what injectors to change and to what size. At ~$28 each for a replacement injector nozzle this is a bargain.

Keep in mind this is an interactive process - changing one cylinder will affect the others.

I typically end up with the largest injector at 0.028” and the rest smaller.

The injector size is a not a big consideration for total fuel flow (within practical reason). A smaller injector will still flow far more than the engine can consume.

In other words, I’d not change timing for one hot cylinder. Save that tool for after your other checks are completed and the GAMI run done and fixes applied.

Carl
IO-360-M1B
Dual pMags (set at 25 degrees BTDC with jumper in and very well behaved CHTs)
AFP FM-150C Fuel Manager (a very nice upgrade from the stock Precision FM - and recommended for those buying new engines)

 

[sabswbc]

Back to his original post. When I had dual p-mags, I think the manifold pressure was feed to one p-mag and the other received information electronically. Turning off the one with the hose will cause the other one to go back to 25 degrees resulting in rpm drop.
Sam

 

[Bavafa]

I didn't make this up Carl; It is a required annual inspection according to eMag. If it was robust and not prone to early failure, it wouldn't have an annual inspection requirement.

So, can we conclude that Lycoming engines are prone to early failure since they require annual inspection of the oil screen?

 

[Bavafa]

Back to his original post. When I had dual p-mags, I think the manifold pressure was feed to one p-mag and the other received information electronically. Turning off the one with the hose will cause the other one to go back to 25 degrees resulting in rpm drop.
Sam

Sam,
I am pretty sure there is no electronic communication or synchronization between to the EMAGs. If you do not have the manifold plumbed to your second EMAG, then you ought to fix that.

 

[lr172]

So, can we conclude that Lycoming engines are prone to early failure since they require annual inspection of the oil screen?

Come on now, I didnt say anything negative about the Pmags, only reiterated the guidance from their maker and made a logical assumption as to why it exists.

Yes, the Lyc engine IS prone to complete failure (i.e. seizing) IF the screen becomes fully clogged, which can occur in various different abnormal conditions, most of which you likely will not know about without looking. Lyc knows this and is why they recommend the inspection.

Larry