What does this mean?...

[LR2001]

Looking for ideas: I have an Aerosport O-360 on my -7A. I'm pretty convinced the EGTs on 1 & 2 are 'different' than they were say six months ago. Hard to quantify, but I often see them running about 100 deg hotter than 3 & 4, and I don't THINK they used to do that. So today I did an in-flight mag check and saw this:

Cruising along with everything looking as stable and even as I could get it, 24 squared, EGTs hovering 'pretty' even around 1350 deg, ROP.

- Switch from Both to Right (Lightspeed): No change, in anything. Rock solid. (actually very impressive).

- Switch from Both to Left (Mag): EGTs on 1 & 2 rise immediately by over 100 deg, within about 3 seconds. EGTs on 3 & 4 stay fixed. Engine sounds and runs totally fine - just the EGTs on those two cylinders shoots up fast. They stabilize around 125 deg hotter than 3 & 4. Everything cools to normal when I switch back to Lightspeed or Both.

Any ideas what that means? If all four EGTs went up I would think nothing of it, but it is very clearly just affecting 1 & 2. Only about 120 hours on the engine, always run ROP.

Thanks -
Matt

 

[Phlyan Pan]

I'll preface this by saying I know NOTHING about aircraft engines...

so here's a theory; what if those 2 cylinders were seeing the timing retarded a bit when switching to the left mag? Could that allow some of the fuel to still be burning as the exhaust valve opens and thus raise the egt?

 

[D-Dubya]

What are your CHTs doing? There was a good article in one of the magazines recently about the relationship between EGTs & CHTs, as it talked about what was acceptable, along with the causes for variations. But I can't find it! It would be really good reading right about now.

In any case, it certainly looks like something has changed with either those two plug wires or plugs coming from the mag. Since it runs fine on the Lightspeed, it would seem to eliminate that as a cause. Hopefully somebody with more knowledge on this will chime in, but that's where it seems to point to me.

 

[Walt]

It's normal and expected that the egt will rise 100d or more when switching off one ignition system. So it not normal to be able to turn off the mag and have no change in egt unless the mag is not doing it's job and you're actually just running off the EI (or when you turn the mag off it not actually off). You should definately feel the difference when turning off one system if both systems are operating normally and all egt's should rise. If I was to guess I would first look at the mag and plugs, check mag timing etc. and make sure the mag is not "hot" when it should be off.

 

[pierre smith]

I just went through this...

...on my -10. It turned out that the opposite pair of cylinders share the same coil on a Lightspeed system. I replaced the coil that drove 3 and 4 and my problem was solved. What happens is that the mag doesn't provide enough spark for complete combustion and the remaining fuel burns in the exhaust, causing the higher CHT reading!

On another coil, the wire that attaches to the coil from the amplifier, is simply 'crimped' at the spade connector and a very light tug pulled it loose. Check all of the wires going to the coils for security and preferably solder them in the crimp.

Best.


BTW, Klaus said to check the ignition at cruise, not only at 1700 RPM, where the problem wouldn't reveal itself!

 

[N427EF]

Exactly what Pierre said.

It may be that your lightspeed coil that controls #1 and #2 has intermittent service.
When it cuts out on you, you'll see the temperature increase.
It can be a little hard trouble shoot, on mine it turned out to be a corroded spade connector that had been arcing for a while and caused intermittent firing especially on low RPMs.

 

[Rick_A]

Also, double and triple check the crimps on the spade connectors going to the coils.

I did a pull test on the connectors when I first installed them and I was certain that I had good crimps. After flying for about 200 hours I had to remove my #4 baffle. I hadn't planned properly for baffle removal so I had to remove the spade connectors from one coil. Too my great surprise, one connector pulled off without much trouble

I had had some intermittent LSE problems prior to this and I feel certain that the inadequate crimp was part of the problem.

 

[LR2001]

Thanks, guys. I'm a bit confused though... A number of you are suggesting it could be a problem with the Lightspeed. But when I switch from Both to Right (Lightspeed) everything is perfectly fine. When I switch from Both to Left (mag) that's when the EGTs on 1 & 2 shoot up. I assume when the ignition is on Mag, the Lightspeed is basically out of the equation. Is there a way the Lightspeed can cause trouble when the ignition is on the Mag only?

 

[Kevin Horton]

Mags fire the spark at the same crank angle under all conditions once the engine is running. Electronic ignition (EI) fires the spark at about the same crank angle as a mag when the engine is running at high power. The EI will advance the spark so it fires well before the mag at lower power.

At lower power conditions the EI fires its spark plugs early enough that the flame front has already passed the other spark plugs before the mag fires them. So, the mag is essentially doing nothing, and you'll see no change in rpm or EGT if you select the mag OFF. On the other hand, if you select the EI OFF at low power, combustion starts later, and the exhaust gas is hotter when the exhaust valves open, so the EGT increases.

Normally you should see all four EGTs increase by about the same amount when you select EI OFF. In your case, only two EGTs are increasing. It could be that some intermittent issue with the EI on the other two cylinders.

 

[RV10inOz]

I bet in 120 hours you have not regapped your plugs.

Pull all the plugs, clean and gap them carefully. I assume you have the correct gap data and for massive electrodes that is between 16-20 thou and 18-22 being the upper limit.

Make sure you use the wire spike gap testers not feeler gauges.

Let us know how that goes.

DB