What's going on here?

"Jay Honeck" wrote in message news:Mplod.135318$R05.23672@attbi_s53...
Okay, after looking at all the replies, you need to stop and analyze
the info given to you on the instrument. If there was a bad plug,
when he switched mags, (he should have leaned it out), the temps would
drop because there was no fire. Same with any other ignition part.
If there was no fuel getting to the chamber, same thing. What you're
seeing is a bad valve, probably exhaust.

How could a bad valve on #2 work okay on "both" mags -- but not on the left
mag?

I think there's some confusion about your troubleshooting. In the
original post, I (possibly by mistake) assumed that when you switched
to the rough mag, the #2 EGT "shot off the scale" to the low end (i.e.
cold). If you're saying it shot off the high end of the scale, this
would not be the kind of reading you'd expect from a cylinder that is
not firing.

On the other hand, like you, I have a hard time imagining how a
stuck valve would behave differently by switching the mags. If an
exhaust valve is stuck open, your EGT should shoot upward off the
scale, but it shouldn't go back to normal by switching mags.

If the symptom is truly that the #2 EGT shoots up when switching to
the rough mag, then something isn't adding up.

John Galban=====N4BQ (PA28-180)

(John Galban) wrote:

I think there's some confusion about your troubleshooting. In the
original post, I (possibly by mistake) assumed that when you switched
to the rough mag, the #2 EGT "shot off the scale" to the low end (i.e.
cold). If you're saying it shot off the high end of the scale, this
would not be the kind of reading you'd expect from a cylinder that is
not firing.

A weak spark rather than no spark maybe? If it's firing but less
efficiently?
My understanding is that this effect also causes EGTs go up when
running on only one mag.

Andrew Rowley wrote:

(John Galban) wrote:


I think there's some confusion about your troubleshooting. In the
original post, I (possibly by mistake) assumed that when you switched
to the rough mag, the #2 EGT "shot off the scale" to the low end (i.e.
cold). If you're saying it shot off the high end of the scale, this
would not be the kind of reading you'd expect from a cylinder that is
not firing.


A weak spark rather than no spark maybe? If it's firing but less
efficiently?
My understanding is that this effect also causes EGTs go up when
running on only one mag.

I believe this is possible due to the fact that having both sparks
allows the mixture to burn more quickly and thoroughly prior to opening
of the exhaust valve. If the valve opens while the mixture is only
partially combusted, a jet of flame will exit the exhaust port. This
can greatly increase the EGT, while at the same time lowering the CHT as
the same amount of heat is produced from the charge, but more of it is
going out the exhaust pipe rather than being used to perform work in the
cylinder.


Matt

I believe this is possible due to the fact that having both sparks allows
the mixture to burn more quickly and thoroughly prior to opening of the
exhaust valve. If the valve opens while the mixture is only partially
combusted, a jet of flame will exit the exhaust port. This can greatly
increase the EGT, while at the same time lowering the CHT as the same
amount of heat is produced from the charge, but more of it is going out
the exhaust pipe rather than being used to perform work in the cylinder.

Thanks, Matt -- I believe that's exactly what was happening when the EGTs
went off the scale (hotter), and the CHTs dropped down into the 200s.

I didn't have time to work on the plane today, but I'm gonna yank that plug
and have my mechanic test it in his brand new spark plug tester -- the kind
that actually pressurizes (or does it lower the pressure?) it while testing.

If that's not it, I'm betting on the wiring harness.

Thanks to all who chipped in -- man, that EDM-700 is an awesome
trouble-shooting tool.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

Jay Honeck wrote:

I didn't have time to work on the plane today, but I'm gonna yank that plug
and have my mechanic test it in his brand new spark plug tester -- the kind
that actually pressurizes (or does it lower the pressure?) it while testing.

Increases...no matter what the ambient pressure in, the cylinder is many
times that inside.

Now if you're testing the ignition leads....

Of course you could bite the bullet and spend the $25 on a new plug.

On Tue, 23 Nov 2004 04:30:36 GMT, "Jay Honeck"
wrote:

snip

I didn't have time to work on the plane today, but I'm gonna yank that plug
and have my mechanic test it in his brand new spark plug tester -- the kind
that actually pressurizes (or does it lower the pressure?) it while testing.

If that's not it, I'm betting on the wiring harness.

Swap all the plugs top-to-bottom. If it is a bad spark plug (which may
pass the bomb test just fine) the next time the problem rears it's
head it will be on the other magneto.

If your local guy has a high-tension lead tester, testing the leads is
simple and fast. You could check just the one with the "issue", or
when swapping the plugs top-to-bottom, you could check them all.

As someone else indicated, oil on the lower plug wires is a bad thing,
and not what I would consider to be typical (or acceptable).

I'm curious, what type of mags are installed? How much time is on the
engine/mags since you bought the airplane/had the engine major'd?

TC

A couple of days ago I posted about a problem with my #2 cylinder. Watching
my JPI EDM-700 in flight, the EGTs on #2 shot to over 1600 degrees, and the
CHT sank to 200 degrees. After trouble-shooting, it became apparent that the
condition became much worse when set on the left mag only.

In other words, the #2 cylinder was not firing -- or, at least, not firing
consistently -- when on the left mag. On "both", the only symptom was the
bar graph on the EDM -- the engine sounded perfectly normal.

Yesterday I removed BOTH spark plugs from the #2 cylinder (I figured "why
not?" as long as I was at it) and had my A&P test them in his pressurized
spark plug tester. Both were clean, and checked out fine.

He re-gapped them, and I reinstalled them. They were off, according to my
A&P, so I believe my next step will be to re-gap ALL the spark plugs.

I also sprayed the oil-saturated wiring harness down with contact cleaner,
which did a fine job of "de-greasing" them.

All connections appeared fine, although I didn't take the mag connections
off. I figure I'll take her around the patch when I get time, to see if all
is now well. (It WAS well on our way home, if you recall, after leaning
severely during a long run-up.)

My mechanic said what I did MAY fix it, if it was just a chunk of crud on
the spark plug causing it to mis-fire. But he also said it could be a bad
lead wire, or a bad distributor inside the mag.

Thanks to all who have responded -- is there anything else to check? I
find it hard to believe that I could have had a fouled plug, given the fact
that we usually burn unleaded mogas, but perhaps it's a plug that only fails
when hot?
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

Jay Honeck wrote:

He re-gapped them, and I reinstalled them. They were off, according to my
A&P, so I believe my next step will be to re-gap ALL the spark plugs.

disclaimer: I'm no mechanic, but I've cleaned and gapped a few spark plugs in my
life, both automotive and aircraft.

It's possible you just got a piece-o-junk in the gap and it was dislodged when
the plug was removed, and it will be just fine forever onward.

You could also have an invisible crack in the insulator that only shows up when
hot or under combustion chamber pressure, in which case the problem will reappear.

Some people intentionally make the gap wider than spec, particularly on a plug
that has a tendency to foul.

My understanding of the rationale is that the wider gap gives a more reliable
spark, but causes higher voltages in the high tension wiring, which might cause
other symptons, like crossfiring. Apparently you can routinely get away with a
gap that's wider than spec.

Dave