Help Diagnose Engine Issue - Mag Drop

[Skid]

So I'll preface this with saying I'm not an engine expert by any means so hopefully I don't sound too stupid in my description of things or guess as to whats causing it. I'll try to keep this short:

There's nice 2008 Cessna 172SP at the place I rent, but seems to always be right on the limits when checking the mags (140-180rpm drop, limit is 150 max). The plane is in great shape inside and out so I really don't want to write it off if I can figure out a way to correct the problem I'm seeing. Here's some background:

1. Density altitudes have been a consistent 1600-2100ft at the airport (sea level, but very hot and humid)
2. I always try to aggressively lean after start (1200 rpm, lean until RPM drop, etc)
3. On the first go at the mag check, I run it up to 1800 (mixture rich), and both mags seem to fall at the same rate down to around 150, sometimes greater, sometimes less. Never have I seen it less than 100.
4. Since the plane is equipped with a G1000 I am able to see the EGT's for each cylinder, and they all seem to rise somewhat in unison for both the L & R mags. The #2 cylinder lags a little bit, but not by much. Makes me believe all the plugs are working ok.
5. I try the clearing procedure of running it up to 2000, attempt to get on the lean side of peak and leave it there for 30 seconds. When re-checking there really is no difference, maybe 10 rpm less (ie 140), if that. At that point the CHT's are getting pretty high, so I never feel comfortable doing it more than once or twice, esp since I see no real improvement.
6. During the times where the RPM drop was say 150-140 and barely within limits, the actual RPM on takeoff was a sluggish 2290-2310ish.

So all that said, people continue to fly the plane regularly and seems to never miss a beat. About a month ago the thing died on me when doing the idle check which I scrubbed the flight and wrote it up for maintenance. It took them quite a few attempts to recreate the problem apparently, but on my last flight it seemed to be ok.

Part of me thinks the is just running overly rich. Since the engine isn't stumbling or losing some excessive 200-300 rpm when isolating the mags it makes me think its not merely a fouled plug. I just came across service instruction 1132 B, but wanted to get some outside opinions just to get my bearings straight as to the likely cause before I put too much faith in anything else.

The most logical thing I suppose would be to lean it out on run up (I assume to peak rpm) and check the drop then. If that checks out ok, is there any general rule of thumb as to then how to operate the mixture thereafter? Living at sea level my entire flying career there hasn't been much need for mixture manipulation on the ground. Its also frustrating that all the other Cessna's run just fine and the mag drops are within range even during the hottest part of the day.

 

[Steven Untet]

What engine?


Sent from my iPhone using Tapatalk

 

[Skid]

What engine?

IO-360-L2A

 

[hotprops]

when were the injectors cleaned last ,sparkplugs replaced,mags 500 hour service, as pic you should know these things and go through the logs a few years back .you are responsible for the airworthiness of the aircraft so start reading.

 

[danhagan]

when were the injectors cleaned last ,sparkplugs replaced,mags 500 hour service, as pic you should know these things and go through the logs a few years back .you are responsible for the airworthiness of the aircraft so start reading.

Previous annuals with different mechanics and my mags were apparently never checked. I got exactly 2 mag drops of 175 over a 2 month period that appeared to clear after running a "fouled plug lean" run up. A short time later in the runup it failed completely. Put two brand new mags on it last November.

 

[CMongoose]

I had a similar issue recently, if you can get hold of the service manual run through the shutdown procedure to test the mixture setting. On my IO-360 it says to allow for +25 rpm at min throttle as you close the mixture to ICO, more than that and you're running too rich.

 

[Skip Miller]

1. During the times where the RPM drop was say 150-140 and barely within limits, the actual RPM on takeoff was a sluggish 2290-2310ish.

Why do you think this is sluggish?

The POH should list a range of max static rpm, brakes locked. This is throttle WOT, leaned as specified for takeoff altitude. Going from memory on a Lycoming 360 powered Archer II, 2300 is in the normal range. Maybe I mis-remember?

-Skip

 

[James331]

Well most student pilot / renters run rich, especially on the ground

Two I'd want to know how the plugs look

Three I'd want to know what mags and how many hours since they were OHed

 

[RyanB]

Three I'd want to know what mags and how many hours since they were OHed

I was thinking this as well. Worn electrodes are an important element of the plug that needs to be looked at during the plug's 100-hour. Most of these plugs have a lifespan around 400 hours or so. If it's been longer than that, I'd be concerned.

Anyway, from what you describe, the mag drop falls within POH limits, so there's no need to concern yourself, even if they are 'close' to the outer limits. 2300 RPM doesn't sound too far out of line either. Does this 172 have a climb or cruise prop? A higher pitch cruise prop can drag the engine, taking a bit longer to spin up on initial power up during takeoff.

If leaning the engine at higher RPM to clear any potential build up doesn't change performance and the mag drops are within limits, than just be happy and fly it.

 

[Skid]

Why do you think this is sluggish?

The POH should list a range of max static rpm, brakes locked. This is throttle WOT, leaned as specified for takeoff altitude. Going from memory on a Lycoming 360 powered Archer II, 2300 is in the normal range. Maybe I mis-remember?

-Skip

I don't have the poh in front of me but if I recall it says it should run smoothly between 2300-2400 rpm on takeoff. Sluggish isn't the right word it just barely wants to touch 2300.

 

[GRG55]

Ryanb; I think you meant a "...higher pitch cruise prop..."?

 

[Skid]

I was thinking this as well. Worn electrodes are an important element of the plug that needs to be looked at during the plug's 100-hour. Most of these plugs have a lifespan around 400 hours or so. If it's been longer than that, I'd be concerned.

Anyway, from what you describe, the mag drop falls within POH limits, so there's no need to concern yourself, even if they are 'close' to the outer limits. 2300 RPM doesn't sound too far out of line either. Does this 172 have a climb or cruise prop? A higher pitch climb prop can drag the engine, taking a bit longer to spin up on initial power up during takeoff.

If leaning the engine at higher RPM to clear any potential build up doesn't change performance and the mag drops are within limits, than just be happy and fly it.

Well the drop is relatively rapid to about 150 rpm, then continues to drop very slowly if I just sit there like that for 10 seconds or so reaching about 180. So technically out of range from the poh's max 150.

If I recall it just came out of 100 hr a week or two ago, but I'll have to check about the rest of the stuff.

 

[RyanB]

Ryanb; I think you meant a "...higher pitch cruise prop..."?

Thank you. Edited.

Well the drop is relatively rapid to about 150 rpm, then continues to drop very slowly if I just sit there like that for 10 seconds or so reaching about 180. So technically out of range from the poh's max 150.

If I recall it just came out of 100 hr a week or two ago, but I'll have to check about the rest of the stuff.

If it gradually declines beyond the limits and doesn't clear, than that's an issue and it shouldn't be available to rent. Originally I read the post to say that it's 'right on the limits'. I'd be curious to know the maintenance history of the mags and plugs.

 

[GRG55]

Well most student pilot / renters run rich, especially on the ground

Two I'd want to know how the plugs look

Three I'd want to know what mags and how many hours since they were OHed

Most common problems, in that order:
- lead fouled plugs that need to be cleaned and re-gapped, or replaced if electrodes worn;
- mags not properly serviced
- ignition harness problems.

If both mags are exhibiting the exact same symptoms, unlikely #3 however.

 

[texasag93]

Do you put the mixture at full rich for your mag check?

When I forget on our airplanes to go full rich after leaning for taxi, the mag check always goes higher than normal.

At 1600 to 2100 density altitude, I run full mixture on takeoff. Homedrome is 642 MSL and density altitude during the summer is over 2000.

In the mountains, I lean for performance before takeoff.

 

[James331]

I was thinking this as well. Worn electrodes are an important element of the plug that needs to be looked at during the plug's 100-hour. Most of these plugs have a lifespan around 400 hours or so. If it's been longer than that, I'd be concerned.

Anyway, from what you describe, the mag drop falls within POH limits, so there's no need to concern yourself, even if they are 'close' to the outer limits. 2300 RPM doesn't sound too far out of line either. Does this 172 have a climb or cruise prop? A higher pitch cruise prop can drag the engine, taking a bit longer to spin up on initial power up during takeoff.

If leaning the engine at higher RPM to clear any potential build up doesn't change performance and the mag drops are within limits, than just be happy and fly it.

Meh, that's like the "in the green" thinking, I'd personally want to know why the plane isn't behaving like it normally does. Sometimes things don't just get worse in a nice predictable linear scale.

 

[Dan Thomas]

Well the drop is relatively rapid to about 150 rpm, then continues to drop very slowly if I just sit there like that for 10 seconds or so reaching about 180.

A continued slow drop like that is often indicative of a too-rich mixture. Try leaning it some in the runup. The injection system is likely out of whack.

That said, ignition is the major factor in most engine performance problems.

 

[Stewartb]

I'd want to see what the leaning authority is at a reasonable low altitude. That is, at level cruise at say 2000' what's the EGT range between full rich and peak? That may help identify a rich mixture. Aside from that? I'd try to do the mag check while leaned for best power. See how it changes.

 

[Skid]

A continued slow drop like that is often indicative of a too-rich mixture. Try leaning it some in the runup. The injection system is likely out of whack.

That said, ignition is the major factor in most engine performance problems.

I'd want to see what the leaning authority is at a reasonable low altitude. That is, at level cruise at say 2000' what's the EGT range between full rich and peak? That may help identify a rich mixture. Aside from that? I'd try to do the mag check while leaned for best power. See how it changes.

I think I'll give a leaned run up a go on the next flight and see what changes. I meant to post the link earlier, but here is the service instruction 1132 B from 2010 which looks to apply to all Lycoming engines: https://www.lycoming.com/sites/default/files/Magneto Drop-Off.pdf

Here are a few things to note:

1) "Although this basic drop-off check, based on just engine speed, is satisfactory under most normal conditions, it does not take into account unusual circumstances of temperature, humidity, and engine wear." (Temperature and humidity being the two extremes at the airport recently)

2) "Drop-off must not exceed 175 RPM and must not exceed 50 RPM between magnetos. Smooth operation of the engine but with a drop-off that exceeds the normal specification of 175 RPM is usually a sign of propeller load condition at a rich mixture. Proceed to step E.(1)(d)." (Looks like the drop off limit is now 175 instead of 150)

3) "If the RPM drop exceeds 175 RPM, slowly lean the mixture until the RPM peaks. Then retard the throttle to the RPM specified in step E.(1)(a) or E.(1)(b) for the magneto drop-off check and repeat the check. If the drop-off does not exceed 175 RPM, the difference between the drop-off values for both magnetos does not exceed 50 RPM, and the engine is running smoothly, then the ignition system is operating properly. Return the mixture to full rich."

Seems pretty straight forward and I have a feeling this will clear it up. What throws me off a little bit is it says at the end to return the mixture back to rich. This whole rundown seems to imply that the mixture is too rich from the get-go so returning it to full rich on takeoff seems like it would inhibit the engine some. Since the airport is at sea level and DA's below the advised 3000 ft 'leaning altitude' in the POH, it makes me wonder if I should be doing a leaned takeoff or just leaving it rich and accepting a lower rpm.

 

[Skip Miller]

I don't have the poh in front of me but if I recall it says it should run smoothly between 2300-2400 rpm on takeoff. Sluggish isn't the right word it just barely wants to touch 2300.

Be sure to check the calibration of your tachometer. As they age they tend to indicate slower than the actual. Doing so is cheap and simple....