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  #1  
Old 11-11-2017, 06:56 AM
gator88 gator88 is offline
 
Join Date: Jan 2015
Location: virginia
Posts: 5
Default No EGT after install of rebuilt mag

Hello everyone. I have an issue that I hope someone can shine some light on; I took my -10 into the maintenance shop at my local airport for the 500 hour Slick mag inspection and they told be it needed to be replaced. I have a Lycoming IO540 with one mag and an Electro Air electronic ignition. They got a rebuilt mag in, but the threads where the RPM sensor screws in were damaged, so they returned that one and finally got another one in after almost a week of being down. So here is where the issue starts; I do a run up after the install of the new mag. The engine is running rough and the number 6 cylinder is showing a higher than normal CHT, 302 degrees compared to the lowest of 224 degrees, and almost no EGT, 131 degrees. The mechanic rechecked the timing and the mag was timed correctly. He checked the CHT probe and found it working properly. He says that he only took off one lead on the cap, so there shouldn’t be any wire mixup issues. He seems to be a bit stumped about what could cause the CHT and EGT readings. Does anyone have any thoughts on this that could point me in the right direction? Thanks in advance.
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Old 11-11-2017, 12:02 PM
gasman gasman is offline
 
Join Date: Mar 2007
Location: Sonoma County
Posts: 3,341
Default

Test Run on the Electro Air only to isolate it from the issue.

Then check the lead on the cap all the way to the plug.
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  #3  
Old 11-13-2017, 04:07 AM
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RV10inOz RV10inOz is offline
 
Join Date: May 2008
Location: Brisbane Qld. Aust.
Posts: 2,050
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If that is on one cylinder only, you have a preignition event going on from some cross firing in the mag.

For the CHT at ground/run up power to have jumped that much, and have much lower EGT and run rough, that is a very likely cause.

It might be something else but without data it is hard to guess.

In any case DO NOT fly that thing. Do not even be tempted. Because if I am right, you will be very upset.

Find the root cause. Do not settle.
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  #4  
Old 11-13-2017, 07:59 AM
jrs14855 jrs14855 is offline
 
Join Date: Apr 2008
Location: Lake Havasu City AZ
Posts: 2,008
Default Mag

The most likely possibility is that one spark plug wire is not properly inserted into the distributor cap OR that the contact spring on the end of the wire was damaged. Pull off the cover that holds the plug wires and take a look.
I disagree with cross firing, most likely that would involve two cylinders and a lot of shaking/backfiring, as well as low EGT on two cylinders when the other ignition is switched off.
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Old 11-14-2017, 11:57 PM
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RV10inOz RV10inOz is offline
 
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Location: Brisbane Qld. Aust.
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Quote:
Originally Posted by jrs14855 View Post
The most likely possibility is that one spark plug wire is not properly inserted into the distributor cap OR that the contact spring on the end of the wire was damaged. Pull off the cover that holds the plug wires and take a look.
I disagree with cross firing, most likely that would involve two cylinders and a lot of shaking/backfiring, as well as low EGT on two cylinders when the other ignition is switched off.
I respectfully disagree. And so does the science/laws of physics.

If a plug was not connected properly the EGT on that cylinder will be much higher not lower. The CHT on that same cylinder will be lower and not much higher.

The OP has the exact opposite symptoms that you are talking about.

I have the data and the details including busted pieces of more than a couple of preignition events caused by things such as a cross firing.

Now it is possible there is something else going on......but this is too serious to not fully disprove it first.
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Old 11-20-2017, 10:02 AM
gator88 gator88 is offline
 
Join Date: Jan 2015
Location: virginia
Posts: 5
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Hey Guys, thanks for the replies. The roughness turned out to be a bad ground connection on the mag and the low EGT was just a bad probe. I don't first go with the "coincidence" theory - "it's just a coincidence that the EGT's are low after working on the mag", but that is what it turned out to be. We crimped a new terminal to the ground wire for the mag and I put a new probe in, and it is running like a champ with all EGT's looking normal.

Thanks again for the input!
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