MAGS (again)

[sailon]

Been having an issue with mag drops that doesn't quite fit other topics in the forum.  Here is what is going on:  Aircraft is a 68 F, I0360 A1A.  830 hrs TSMO.  Engine starts immediately, runs smooth on both mags.  MAG check results at 2000 RPM: 120 RPM drop on left mag, right mag 400 RPM with roughess.  Number 4 cylinder EGT totally cold when on right mag, could not be cleared when problem discovered.  Switch on both, now no 4 is hotter than other cylinders.  Plug in no. 4 bottom soaked in gas.  Pulled mag, had it tested at mag shop, and found mag was excellent.  (firing on all 4 with hot spark at 250RPM in test jig).  Diode and capacitor all tested perfect.  Rotated plugs, retested, still issue with cylinder no 4.  Replaced harness, retimed mags, now here is where it starts to get interesting.  First runup test, issue still there.  However, after 3 runup attempts, engine smoothed out, all EGTs equal at all times, 120 RPM drop each side.  Shut down, let cool for 10 minutes.  After restart, saw no 4 running hotter than rest at idle, runup failed with 4 going cold again, then no 4 started to fire and followed with perfect mag test again.  (120 each side).  No 4 bottom plug not soaked, but slightly damp with gas.  Mechanic and I are getting stumped.  Oil consumption acceptable.  (quart per 10 hours), boroscoped cylinders during pre-purchase and found valves and cylinder walls like new.  Potential issues could be:

Fuel Spider with diaphragm partially blocking fuel to no4?

Not leaning enough prior to runup?

Not getting engine warm enough prior to runup?

Cracked cylinder?

Other ideas?

Mechanic says to fly for a few hours, then take another look at no4.  Thoughts?

[Bob - S50]

When you say you rotated the plugs, do you mean you swapped top and bottom or did you move plugs from one cylinder to a different cylinder?

[sailon]

One cylinder to a different cylinder.

 

[Bob - S50]

Guess that eliminates the spark plug then.

[Bob - S50]

Just had a thought.  If it is always the right mag on #4, could it be a grounding issue at the plug?  That is, shorting out the plug somehow so there is no electricity to create the spark?

[ArtVandelay]

On,off problems like that are usually electrical,since its one cylinder and you replace the harness, did you check the plug resistance? I know you swapped them but it's an easy test. Might want to swap EGT probes while your at it to eliminate a bad probe.

[N601RX]

Check plug resistance.  You may also have an intermittent exhaust valve sticking on #4.

[ArtVandelay]

Check plug resistance.  You may also have an intermittent exhaust valve sticking on #4.

I thought of the valve sticking too, but he said cylinders looked clean and only 800 hrs since overhaul, seems a little early for sticking valves, no?

[sailon]

I have been giving some thought to the sticky valve issue.  Perhaps a compression test with engine cold, followed by warm?

 

[sailon]

1 hour ago, Bob - S50 said:

Just had a thought.  If it is always the right mag on #4, could it be a grounding issue at the plug?  That is, shorting out the plug somehow so there is no electricity to create the spark?

Even with new harness?

 

[N201MKTurbo]

I had the same madness about two years ago. Did all that you did. New plugs finally fixed it. Go figure...

[N601RX]

The best way to eliminate a sticking valve is to remove the spring and do the wobble test. Even if you don't have the indicator and holder if it is even remotely snug it will stick. They can stick on low time engines also.  If you have Champion plugs measure the resistance 1st.

 

[Yetti]

Hand spin the prop when it is cold. See if there is a soft cylinder usually 3 or 4.   Do an injector flow test with the 4 mason jars instead of injectors.  Injectors go into Hoppes number 9 for a nice soak.

 

[jlunseth]

I have one thought for you.  I see you rotated the plugs and also replaced the spark plug harness, so I assume you have a new harness.  However, it is possible you had an issue with the old harness.  The conductor goes bad inside, and/or you start to get carbon build up in the socket that the spark plug wire plugs into.  I assume that if that happened, the worst of it was spotted and cleaned up and/or corrected when the new harness went it.  The problem, though, is that an issue like that can create tracking across the cap, and once a track has built up you have a place for the current to go other than into the plug.  I would make sure the sockets and the distributor cap are thoroughly checked for arcing/tracking/deposits.

If "replaced harness" just means you put the old harness back on, then there is your problem.  Conductor is bad in one of the wires.

Since you have gas on the plug I would guess your fuel flow is ok, but it is possible there is some kind of intermittent obstruction in the injector or the fuel distribution system that leans the mixture out to the point where the cylinder won't fire.. 

It just sounds to me like you had an issue with a cracked plug, or a cracked connector in the wiring harness, but it caused some problem elsewhere, like in the cap, that you have not caught up with yet. 

Edited May 10, 2016 by jlunseth

[Shadrach]

I can't say what your problem is, though my hunch is that it is plug related. Given the symptoms you describe, I would eliminate every ignition related possibility first as it does not read like it is fuel or valve related. EGTs don't run high on a cylinder with a stuck valve, EGTs run high on a cylinder that only has a single point of flame front propagation rather than two. For whatever reason, the plug for right mag, cyl #3 is dropping out. Did your shop actually disassemble the mag. Spinning the mag up in the shop environment is not the same as having it mounted on a vibrating engine. I've had intermittent coil failure that only manifest itself  at certain RPMs. I can't tell what's wrong, but I'd bet against anything external to the ignition.

[sailon]

23 minutes ago, Shadrach said:

I can't say what your problem is, though my hunch is that it is plug related. Given the symptoms you describe, I would eliminate every ignition related possibility first as it does not read like it is fuel or valve related. EGTs don't run high on a cylinder with a stuck valve, EGTs run high on a cylinder that only has a single point of flame front propagation rather than two. For whatever reason, the plug for right mag, cyl #3 is dropping out. Did your shop actually disassemble the mag. Spinning the mag up in the shop environment is not the same as having it mounted on a vibrating engine. I've had intermittent coil failure that only manifest itself  at certain RPMs. I can't tell what's wrong, but I'd bet against anything external to the ignition.

Good insights.  Mag shop totally disassembled the mag, checked the inside, cleaned all of the distributor contacts, checked condenser and diode, and re-lubed it.  (based at KFIN, highly recommend shop by the way).  Good point regarding EGTs and stuck valve. 

 

[sailon]

37 minutes ago, jlunseth said:

I have one thought for you.  I see you rotated the plugs and also replaced the spark plug harness, so I assume you have a new harness.  However, it is possible you had an issue with the old harness.  The conductor goes bad inside, and/or you start to get carbon build up in the socket that the spark plug wire plugs into.  I assume that if that happened, the worst of it was spotted and cleaned up and/or corrected when the new harness went it.  The problem, though, is that an issue like that can create tracking across the cap, and once a track has built up you have a place for the current to go other than into the plug.  I would make sure the sockets and the distributor cap are thoroughly checked for arcing/tracking/deposits.

If "replaced harness" just means you put the old harness back on, then there is your problem.  Conductor is bad in one of the wires.

Since you have gas on the plug I would guess your fuel flow is ok, but it is possible there is some kind of intermittent obstruction in the injector or the fuel distribution system that leans the mixture out to the point where the cylinder won't fire.. 

It just sounds to me like you had an issue with a cracked plug, or a cracked connector in the wiring harness, but it caused some problem elsewhere, like in the cap, that you have not caught up with yet. 

Replaced harness refers to new distributor caps, wires, etc.  Could not find anyone who "rebuilds" harnesses anymore.  Seems the only way to replace the harness is to buy the whole ball of wax from someone like Spruce.

 

[Guest]

I didn't see if you tested the P leads to be sure left is really left and right is really right.  It would such to be troubleshooting the wrong mag.

Clarence

[Smitty]

Checking the P-leads is a good idea. Never hurts to make sure you're working on the correct magneto.

I think your problem is in the magneto. I know you said that a shop looked at it but a coil can test fine on the bench and then start acting up once the engine warms up. By swapping the spark plugs (assuming you switched the plugs with plugs associated with the other magneto), you eliminated them because the issue stayed with the same cylinder. By changing out the wire harness you eliminated a spark plug wire breaking down. A sticking valve would show up on both mag checks, not specific to one side. Also a sticking valve would be extremely noticeable during taxi and in flight. 

Good luck  

[sailon]

14 hours ago, Smitty said:

Checking the P-leads is a good idea. Never hurts to make sure you're working on the correct magneto.

I think your problem is in the magneto. I know you said that a shop looked at it but a coil can test fine on the bench and then start acting up once the engine warms up. By swapping the spark plugs (assuming you switched the plugs with plugs associated with the other magneto), you eliminated them because the issue stayed with the same cylinder. By changing out the wire harness you eliminated a spark plug wire breaking down. A sticking valve would show up on both mag checks, not specific to one side. Also a sticking valve would be extremely noticeable during taxi and in flight. 

Good luck  

Thanks Smitty, issue is that the problem currently shows up with the engine cold. During recent ops test with the new harness, the right side failed initially, then passed after system warmed up and a few more tries with all even EGTs.  What I am starting to conclude is that there was a harness issue, (there was evidence of burn thru at one of the clamps which was so tight the leads could not be moved within it) as well as plugs that are old.  (16 years and 830 hours on plugs, electrodes are becoming oval).  I have ordered 8 new plugs, and hope to install them this weekend.  My theory (note MK201 Turbo comment above) is that the resistance of the plugs is higher than spec, with the proximity of the no 4 and no 3 cylinders to the engine mount and resulting vibration increasing plug resistance to disrupt the spark.  I will advise results after this weekend.  Mags only have 270 hrs since overhaul by the way. Certain that it is the right mag side, as it is the system that feeds cylinder 4 bottom which is the plug that was initially soaked in gas and is now damp.

[carusoam]

I only read through this quickly, once...

1) The run-up indicated something is not right.  150 and 400 rpm drop? When a number like 50 is expected...

2) your mechanic? said fly it for a while...?

3) something goes completely cold.  That is a less than subtle hint of where the problem is.

You have me convinced that I could actually be a mechanic one day...

To me, this is a don't fly until you get a resolution.  Flying it isn't magically going to make it better. Determining what isn't working is normal mechanic activities.

I have the experience of flying with a stuck valve (on departure), as does MooneyGirl. My experience was better than Jolie's.  My flight had less trees involved than hers...

Unfortunately, I am still not a mechanic.  I am just a PP.

Best regards,

-a-

 

 

[Shadrach]

3 hours ago, carusoam said:

I only read through this quickly, once...

1) The run-up indicated something is not right.  150 and 400 rpm drop? When a number like 50 is expected...

2) your mechanic? said fly it for a while...?

3) something goes completely cold.  That is a less than subtle hint of where the problem is.

You have me convinced that I could actually be a mechanic one day...

To me, this is a don't fly until you get a resolution.  Flying it isn't magically going to make it better. Determining what isn't working is normal mechanic activities.

I have the experience of flying with a stuck valve (on departure), as does MooneyGirl. My experience was better than Jolie's.  My flight had less trees involved than hers...

Unfortunately, I am still not a mechanic.  I am just a PP.

Best regards,

-a-

 

 

Did your stuck valve reveal itself in the EGT data?

[sailon]

1 hour ago, Shadrach said:

Did your stuck valve reveal itself in the EGT data?

Not sure I have a stuck valve, this forum is leading towards an electrical issue.  As you said, hot EGT indicates a single spark.  Replacing all plugs this weekend.  We will see what happens.  Agree I don't want to fly it until issue sorts itself out.

[Shadrach]

1 hour ago, sailon said:

Not sure I have a stuck valve, this forum is leading towards an electrical issue.  As you said, hot EGT indicates a single spark.  Replacing all plugs this weekend.  We will see what happens.  Agree I don't want to fly it until issue sorts itself out.

I don't think you do either; I was asking Anthony (Carusoam) about his incident. 

[carusoam]

My O360 stuck It's valve in my first 10 hours of ownership.  

There were three devices I wish I had installed in my C.  A JPI, FF and carb temp...

There were no signs of impending problems.  Mostly because I had such limited experience with the plane and it's engine.  I had no instruments to properly show the health of the engine.

When the valve stuck, it gave tremendous signs of lack of power and terrible vibration. Upon having a mechanic take the cylinder off, the valve marks on the piston top were pretty obvious of what was happening.  The bent valve stem was another clear hint.  

How some logic works...

It was a weekend, no mechanics were available.  After borrowing some tools from another pilot and checking a few spark plugs, we performed a run-up. Found no issues, so flew it home.  Flying with an unknown bent valve.  We were in the middle of nowhere with a plan B that included multiple runways, a million fields and an insurance card.  Flying it back it had some new vibrations that were more subtle.

It is tough to be a young CB without MooneySpace.

With MS, you realize when something is wrong.  You get help diagnosing it.  You get help with how logic works.  Crummy logic can be overpowering.

Jolie has posted pictures of the last flight of her ill fated plane. (Maggie 1)?

Valves are usually independent of Magnetos. Testing mags probably won't reveal a valve sticking. Oddly enough you might hear the valves after start-up. If one is misbehaving, you may hear click, click, click, clack,  repeating.  For other sticking valve ideas look up morning sickness.

Sorry to go so long on this topic.  Just looking out for someone in the event they were using crummy logic the way I did back in the day.

There isn't anything good going to come out of taking a plane into the sky that has failed the basic run-up.  There are times where a plug has fouled and caused a mag test failure.  Clear the fouling, retest the mags.  When all is good , go fly.

PP logic, not a CFI or mechanic.

Best regards,

-a-