What's going on here?

Jay Honeck wrote:
: 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?

It can't, you've got a bad plug or wire. Pull the plug and clean it. Spray
the oily plug wires with contact cleaner and clean the inside of the plug
where the wire goes. Put on silicone rocker cover gaskets to prevent future
oil leaks.
--
Aaron Coolidge

"Jay Honeck" wrote in message news:cI9od.66570$V41.11973@attbi_s52...
Our JPI EDM-700 engine analyzer may have paid for itself today.

Here's the scenario:

snip

Your tests have pretty well narrowed it down to a plug that isn't
firing (the 1st thing I look for when EGT shoots up on one cyl.).
It's unlikely that the mag is firing well on all but one cyl. The
most likely causes are a plug wire or the plug itself. If your mech
is equipped with good testing gear, he should be able to isolate the
problem pretty quickly.

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

Bad ignition lead? Excessive plug gap? Bad plug? Flashover inside
the mag?

Somehow you have a marginal ignition situation on that one plug.
These things aren't supposed to happen with autogas.

Post us with what you find. THX

"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)

It won't. What I'm saying is that if a plug doesn't drop out when
leaned on one mag, that's not your problem. Look elsewhere.

On Mon, 22 Nov 2004 13:00:28 GMT, "Jay Honeck"
wrote:

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?

No fuel, no fire. No fire, no big temps.

On Mon, 22 Nov 2004 15:58:22 GMT, "Dude" wrote:

With apologees for hijacking...

I have the opposite problem, high CHT with normal EGT. Strangely enough,
its also my #2 cylinder. I was thinking it was a fuel injection problem.

(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

A dropped spark plug can crack the insulator enough to be almost
undetectable to the eye but can misfire under compression, making it hard to
find otherwise.
Dan

"Jay Honeck" wrote in message
news:cI9od.66570$V41.11973@attbi_s52...
Our JPI EDM-700 engine analyzer may have paid for itself today.

Here's the scenario:

Mary is PIC. During her run-up, the left mag dropped a bit more than the
right. She leaned it back (the usual fouled-plug procedure) and it
smoothed out to within specs. Still not as smooth at the right, but with
less than a 75 rpm drop.

Climb out was normal, with that O-540 pulling like a tractor at over 1000
feet per minute. Life was good, especially after surviving the
Wisconsin-versus-Iowa Big Ten football game...

Upon leveling out, and getting everything cleaned up, I was messing around
with our new CO detector, which is mounted on the pilot's-side at elbow
height. (Recommendation: Don't reach across your pilot like this unless
you're married to her! :-)

As I was tinkering with it, my eyes were drawn to the EDM-700, which was
clearly showing something amiss with the #2 cylinder. EGT was over 1630
degrees (we try to keep them under 1500), and the CHT was way low -- like
in the low 200s.

We tried leaning, and enrichening, and different throttle settings, but
that cylinder stayed way out of line with all the others. Strangely,
nothing sounded amiss at all, and the engine seemed to be performing
normally.

I was about to suspect something wrong with the JPI, when I suggested that
we try switching to one mag, then the other, in flight. On the right mag,
all was well -- but on the left mag, the engine started shaking, and the
#2 EGT shot off the scale!

Switching back to "both" everything smoothed out. In fact, we still could
not detect anything amiss "by ear" -- but the #2 cylinder was still "out
of whack" with the others.

Since we were closer to our destination than home, we elected to continue
on and land. On the ramp Mary did a run-up, and the #2 cylinder was still
clearly not firing on the left mag. We then ran it up on "both", and
leaned it severely -- again, the usual procedure for a fouled plug.

After this it was better, but not perfect -- so we de-cowled the engine,
and checked everything thoroughly. Other than the #2 cylinder's bottom
spark plug wire being oily (actually, all the bottom wires are), I
couldn't find anything amiss. I wiggled all the wires, made sure they
were not bent over too sharply, checked all connections at the mags -- and
then went to dinner.

Of course, this bothered me throughout dinner, and I really couldn't enjoy
myself.

On the return flight, with me acting as PIC, I did a VERY thorough run-up,
with a long, brutally leaned 2000 RPM test. Everything tested and ran
fine -- and remained fine all the way home, at all throttle, prop and
mixture settings.

What's going on here? We burn unleaded car gas, so it seems unlikely
that we had a lead-fouled spark plug (we haven't bought avgas in several
weeks) -- could it be a bad plug? Or a bad spark plug wire?

Can a mag fail in such a way as to only hinder a single spark plug from
firing?

I'm going to call my A&P tomorrow to have him check it over, but anyone
got any ideas?
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

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"