egt increase for no apparant reason

5 Replies

egt increase for no apparant reason

Posted by Dwayne Malsom on Jun 21, 2018 8:22 pm

I own a 2001 c172 sp. have owned for 14 years. have never had any issues like this. have had egt probe wire fail in past but has been repaired properly and no problems since. was flying recently, level flight for close to 30minutes, egt at 75 degrees rich of peak when suddenly, for no apparent reason, egt temp increased 50 degrees. no performance change, oil temp 195 deg like always, richened up mixture to re establish 75 deg rop, fuel burn increased around 1gl pr hr according to the fs450 fuel flow monitor. landed without incident. engine idle normal. forgot to redo mag check but did do the idle mixture check and increase of approximately 10 to 20 rpm  just prior to engine shut off as it is supposed to be according to lycoming bulletin.   was wondering if possible that I have a partially plugged fuel nozzle  anybody heard of issue like this or had similar problem ?.
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Re: egt increase for no apparant reason

Posted by James Ratichek on Jun 25, 2018 8:41 am

In my experience, Mag timing can also have a huge effect on egt. Clogged injectors obviously as you noted. Also, I think a valve sticking or something that changed in the combustion chamber could do it. I think you'll need more info and professional help to diagnose. A word of warning though: I've had a&ps (even good ones) misdiagnose those things and take remedial action that made the problem worse. So ask  questions and fly very carefully after a change is made. Get back on the ground Fast if you get odd indications. Doing that saved my engine after my timing was advanced too far by an otherwise good mechanic.

Re: egt increase for no apparant reason

Posted by James Blocker on Jun 25, 2018 9:41 am

Same exact thing happened to my 182, I was thinking the worst but the fix was pretty simple. Here's what the repair ticket from my A&P said:

Cleaned fuel injector nozzle. Replaced #2 intake gasket. Cleaned and retorqued EGT probe connections.

All has been well with my airplane for the last 5 months.

Re: egt increase for no apparant reason

Posted by James Carter on Jul 2, 2018 11:16 am

Was it a single cylinder reading or across the board? Was there any change in sound or feel at the time? Do you also measure CHTs and if so were there any changes there? Also, and probably most importantly - do you monitor all cylinders or just a single probe?

If you have a multi-point monitor and it was an across the board increase then the problem is probably in the connection to the monitor itself or is the monitor itself.

Assuming you have a multi-point monitor, if there was a change in sound or feel then you need to bore scope the individual cylinder very soon, as well as checking intake coupling and injector. I would do this on all cylinders if you only have a single-point monitor.

Timing would be my last suspicion unless this engine uses a common magneto drive gear.

As an aside; I switched from single-point analog EGT and CHT to a digital JPI EDM930 and was amazed at the imbalance induced by the induction system on an E-series TCM. According to the book, I was monitoring my hottest / leanest cylinder with the single-point, but that proved to be oh so wrong when I started seeing them all. I'll probably never rely on the single-point monitors again.


 

Re: egt increase for no apparant reason

Posted by Steven Boese on Jul 9, 2018 12:57 pm

It was found when a self powered multipoint EGT system was installed in a C172 that all cylinders then showed anomalously high EGTs.  This was due to the installation instructions of that STC requiring one lead of each thermocouple to be connected to a common point which then requires the switching of only the second lead of each thermocouple.  The thermocouples and gauge all individually performed within specification.  This behavior was confirmed with bench tests and also predicted by circuit simulations.  More importantly, when leaning the fuel air mixture with reference to the leanest cylinder, that cylinder will always be operating significantly leaner than intended when using this system in spite of following the engine manufacturer's recommended leaning procedure.  How much leaner than indicated by that EGT gauge the lean cylinder will be running depends on the inconsistency of the fuel air mixture between the cylinders of that particular engine and the conditions under which it is operating.

Re: egt increase for no apparant reason

Posted by Dwayne Malsom on Jul 9, 2018 1:20 pm

what I found was simply a lead fouled spark plug. during next start up after letting engine sit overnight, immediately ran mag check and left mag showed 200rpm drop. found which cylinder by simply feeling each one and noted temp difference. Pulled the lower plug as they are usually the first ones to foul  and sure enough, a hair like piece of carbon shorted the plug. used a tooth pic to remove, reinstalled plug and all problems gone.