Quantcast

EGT Response During Ignition System Check Question

HomeBuiltAirplanes.com

Help Support HomeBuiltAirplanes.com:

HomeBuilt101

Well-Known Member
Joined
Oct 20, 2015
Messages
325
Location
Aguila AZ
As you can tell it is a busy day running back and forth from the engine to the computer...repeat.

I am knocking out a number of items on the Olde Squawk List...another issue I am running is trying to find a bad spark plug or ignition coil.

QUESTION: When you turn off the GOOD ignition system and the engine running rough on the BAD ignition system...does the EGT in the poor running cylinder go up or down?

One (me) would think that if the spark plugs are not firing that well then there is a less hot fire and that would certainly be true in the cylinder itself however...

Since the EGT probe is 2 inches away from the valve during a poor/misfire the not completely burning gas would find its way farther down the exhaust pipe and indicate a hotter EGT...YES??? NO???

Specifically I have a 6 banger with dual Lightspeed Plasma III ignition systems and as designed one coil pack fires the opposing cylinders. When doing a "mag" check I turn off the IGN B the engine runs super smooth and of course following the procedure I turn IGN B back on and then when I turn off IGN A as the engine is running only on IGN B it runs very rough.

I was hoping to see one cylinder go cold and that would indicate a bad spark plug or wire(or wire connection) or if two opposing cylinders went cold that would indicate a bad coil pack or power wire to that coil pack...however his is what I get

..........EGT......EGT WITH......CHANGE
..........BOTH....IGN A OFF......IN EGT

#1 ......1180 .....1220 ...........+40
#2 ......1088 .....1106 ...........+18

#3 ......216 .......172 ............-44 Yes EGT #3 indicates cold today however CHT is normal
#4 .....1142 ......1180 ............+38

#5 .....1195 ......1190 .............-05
#6 .....1130 ......1160 .............+30

So my almost new EGT probe failed...great... !?!?WTF!?!? (Why The Failure!?!?!)

If all of the cylinders reacted in the same way (all getting hotter or all getting cooler) I could assume that the ignition timing/ vacuum advance/ power supply / or ignition module of IGN B was the problem...however not all of the cylinders reacted the same...some went up and some went down.

So whilst I drop a bunch of F Bombs while changing out the almost new EGT probe...can someone PLEASE provide some guidance as to where to look at my ignition system??? Yes...another $100.00 stack of pancakes is available at the next fly in...trailer in for me I guess

: ( + F F F Bombs = me

THANKS!!!
 
Last edited:

pictsidhe

Well-Known Member
Joined
Jul 15, 2014
Messages
8,606
Location
North Carolina
At work, 90% of thermocouples that read low are due to poor connections somewhere. Check that first. Around 200 suggests a short at the connector or in the cable which is the approximate temperature there. We get that a lot, few people can wire them up well...
It could be that no3 has been spitting flames at the thermocouple and fried it.
 

TFF

Well-Known Member
Joined
Apr 28, 2010
Messages
13,657
Location
Memphis, TN
Usually the EGT goes up. The why is unburned fuel is burning in the pipe instead of the cylinder because of less complete burning. It’s what you are balancing when you lean the engine.
Lean makes power and all the burning is in the cylinder. CHT is high. Rich and there is not enough time to burn in the cylinder and burning fuel not contributing to power is burning in the pipe and also carrying heat out. Same with timing. Advanced timing gets most of the burning in the cylinder; CHT goes up but more efficient. Retarded timing has ignition late and it’s out of sequence with the valves so it’s evacuating when it should have been contributing to power. You loose an ignition and the mixture in the cylinder does not have time to burn so it is relatively richer than with two ignitions contributing.
That’s why you don’t want a lot of mag drop. If they don’t share igniting, the first spark starts the fire and the flame keeps the other spark plug from doing anything because the mixture is now spent under that plug, but there is unspent fuel in the corner that is not lit in time, so it goes out the valve burning right on top of your probe.
 

Marc Zeitlin

Exalted Grand Poobah
Joined
Dec 11, 2015
Messages
726
Location
Tehachapi, CA
QUESTION: When you turn off the GOOD ignition system and the engine running rough on the BAD ignition system...does the EGT in the poor running cylinder go up or down?
If both spark plugs/coils/ignition wires are working correctly in a cylinder, then when you disable one of the plugs (mag check) the EGT's will go up.

However, if one of the spark plugs/ignition wires/coils is NOT working, meaning that only one of the spark plugs is firing, then when the GOOD spark plug is turned off during the mag check, there will be no combustion in that cylinder and EGT's will fall precipitously.
 

wsimpso1

Super Moderator
Staff member
Log Member
Joined
Oct 18, 2003
Messages
7,140
Location
Saline Michigan
QUESTION: When you turn off the GOOD ignition system and the engine running rough on the BAD ignition system...does the EGT in the poor running cylinder go up or down?
Some of the answers supplied here and in common press are approximately correct in describing raised EGT on a cylinder running with one plug not sparking. They are correct in the outcome - EGT goes up on a cylinder with one plug not firing.

On an engine that needs both spark plugs to fire for good combustion - almost all traditional airplane engines have big pistons and do need both firing for full output - losing spark at one plug means a majority fraction of the fuel charge is ignited later than it would normally be lit. Later as in the piston is past TDC and on its way down. This piston has less stroke left to convert expanding gases to power, so less power is extracted during the stroke. That unconverted energy is higher temps and pressures remaining in the gases when the exhaust valve opens. Higher EGT results on that cylinder. All of the fuel-air mix burns, and pretty close to the top too, it just does it later in the piston stroke than it is supposed to and so cannot recover as much of the energy to run the engine.

I was hoping to see one cylinder go cold and that would indicate a bad spark plug or wire(or wire connection) or if two opposing cylinders went cold that would indicate a bad coil pack or power wire to that coil pack...however his is what I get

..........EGT......EGT WITH......CHANGE
..........BOTH....IGN A OFF......IN EGT

#1 ......1180 .....1220 ...........+40
#2 ......1088 .....1106 ...........+18

#3 ......216 .......172 ............-44 Yes EGT #3 indicates cold today however CHT is normal
#4 .....1142 ......1180 ............+38

#5 .....1195 ......1190 .............-05
#6 .....1130 ......1160 .............+30

So my almost new EGT probe failed...great...
Now, knowing that a missing spark raises the the EGT, let's evaluate your data:
  • On Ign A only it is smooth because all six are firing the same - suboptimally, but still all the same - with each cylinder making about the same output, we would expect smooth;
  • On both, one or more cylinders is running differently from the rest. Four or five cylinders are firing both plugs and making good power, while one or two are making less power, so it runs with more vibration;
  • On Ign B only, you did not say, but we imagine that it runs awful, with one or maybe two cylinders not firing;
  • 1,2,4, & 6 went up, we expect that;
  • 3 clearly has something wrong that must be fixed. If the engine is running smoothly, I would run your EGT circuit carefully before replacing the thermocouple.
  • 5 is no change (5 degrees is essentially zero) and it should change.
What cylinder is 5's B coil paired with?
  • If it is paired with 3, it still might be 5B coil - Tough to say what is going on in 3 with the EGT malf, and the data from that cylinder must be viewed as garbage until fixed.
  • If the 5's B coil is paired with 6 - which is behaving like it should - I would be going through the wires and plug on 5B.
Now let's try to remember something about scientific process - When you test a theory and the results run backwards from expectations, there is a good chance that your theory is not junk, just backwards, like it was here.

Billski
 

HomeBuilt101

Well-Known Member
Joined
Oct 20, 2015
Messages
325
Location
Aguila AZ
Hmmm...this airplane never ceases to piss me off!!!

So the engine was running great...then it was not when doing a "mag" check.

In this post I stated that #3 EGT probe was inaccurate so I swapped it out and the next run up indicated that #3 cylinder was running very cold when operating on one ignition...so I checked the wires and they were good, checked connections...good. Removed spark plugs and they looked good...yikes before digging deeper I decided to swap plugs from 3 to 1 and the misfire followed.

The spark plug was bad!!!!!!!!!!!!!!!!!!!!!!!!!

It was a NGK BR9ECM and it went bad after only about TEN HOURS.

I will post my spark plug question on a different thread.

THANKS again for your help!!!
 
Top