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Finding Bad Ignition System or Cylinder

Vans101

Well Known Member
Can someone please help me understand the EGT changes one would see with a poor spark or cylinder?

I am trying to find the bad spark plug or wire set or possible even the cylinder itself by watching the EGT changes when one ignition system is turned off.

The engine is a IO-540 with dual Lightspeed III ignition systems.

I assume that when one of the two ignition systems is turned off then the mixture is not burning as quickly and cleanly so I would suspect that the EGT in the cylinders will all rise because the mixture is still burning as the exhaust valve is opening so then since my EGT sensors are exactly 2 inches from the exhaust flange then the EGT indications would be hotter. Is that accurate.

If the spark plug in a certain cylinder is not sparking at all then naturally that cylinder will show as a cold EGT indication.

But what is the spark is intermittent?



At a 1000 RPM idle with both ignition systems on I see 11.7 MAP and the following EGTs:

1026
980
1026
1019
1095
1060


I turn off the A ignition system and when operating only on the B ignition system the engine is smooth and I see 12.1 MAP and all of the EGTs rise by this amount F:

33
21
40
13
8
73 (so #6 showed a much higher EGT rise than the others.


I then run both ignition systems to let everything stabilize and then turn off the B ignition system and the engine has a slight popping sound and I see a MAP of 12.5 with all of the cylinders getting an EGT rise except #6 and it runs cooler that before:

15
12
9
23
10
MINUS 9

So cylinder #6 has the highest EGT rise when the A system is turned off and then it is also the only cylinder to get cooler when the B system is turned off.

So does the cyber collective agree that perhaps the B system spark plug of the wires to that one cylinder are lazy?

I want to go up and do the complete SAVVY engine LOP test and the ignition stress test plus the intake manifold test the next flight but I want to see if I can fix it first.

THANKS for your help!!!
 
You are correct that when one plug / ignition is off that cylinder(s) will show higher EGT. Should see the same thing in a single cylinder if one but not both of the plugs isn't firing.

For the intermittent thing... sounds like you're hooked into Savvy? Have you uploaded the flight(s) and looked on the graph? Free to do that and it would probably simplify diagnosing the problem. Although someone smarter or less lazy than me will probably be able to look at your numbers and tells you what's going on. :D
 
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Swap the #6 plugs and repeat the test. If the problem now moves to the other ignition system, the issue is the plug. If it stays the same, look at the wires.
 
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Consider repeating the test at altitude with the engine as lean as you can run it. A lean mag check at altitude will show issues you won’t find on the ground.
 
First of all, the fuels not burning when the valve opens. Not true. It is the lower cylinder pressure being vented to atmosphere that gives less cooling of the gases. PV=nRT

Having said that you need to do this test at some degree of power and best at high power LOP.

The RPM changes at idle will scramble the EGT value so no meaningful data is coming from that.

Go up to 1500', WOT/2450RPM and set 14GPH. Then do a mag check.
 
First of all, the fuels not burning when the valve opens. Not true. It is the lower cylinder pressure being vented to atmosphere that gives less cooling of the gases. PV=nRT

Having said that you need to do this test at some degree of power and best at high power LOP.

The RPM changes at idle will scramble the EGT value so no meaningful data is coming from that.

Go up to 1500', WOT/2450RPM and set 14GPH. Then do a mag check.


+1
CHT falls and EGT rises when running on one plug due to the retarded timing and early peak pressure.

You really can't read much into these observations at idle. Record the same details at Runup RPMs or higher. Even beter, do the same test at cruise power settings. I routinely do mag checks in cruise.

The different MAP readings on the two individual ignition systems points to an issue, but it could easily be caused by different advance settings on the two ignitions.

Larry
 
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Good information everyone...THANKS!!!

OK what I will do is swap out the spark plug with a new plug and then I will go up and do the full Monty Savvy Flight Test protocol including the ignition LOP stress test and the intake manifold leak test and report back.

Again THANKS for the advice
 
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