Fluctuating EGT?

[SpamPilot]

Hello Forum,

 

I just bought my first Mooney (a '68 M20F with J windshield, cowl closure, and other mods).  What a great plane.  But on the 15 hour flight back home, I noticed EGT readings I didn't understand.  I was wondering if anyone on the forum had any thoughts on the issue.

 

The plane has an Insight GEM 602 for EGT and CHT on all cylinders, plus an analog EGT meter in the panel.  After engine start, EGT's and CHT's come up evenly across all four cylinders, and come up further during the runup.  In climb running full rich, they remain fairly even across the board, and CHTs remain around 375F.  But once I configure for cruise and lean conservatively (10% richer than book ROP fuel flow for my chosen power setting, measured with a Shadin fuel flow meter), the EGT for cylinder #3 starts acting up.  It oscillates, plus or minus 50F at first, and then gradually the oscillations increase until after an hour of flight they are going from bottom to top to bottom of the EGT scale.  The temperature will drop off the bottom of the scale for maybe 5 seconds, and then will shoot to the top of the scale and stay there for another 5 seconds.  After 2 hours, the reading biases towards the bottom of the scale, and eventually the EGT drops off the bottom of the scale and stays there.

 

The analog EGT meter in the panel does not quite follow the same pattern, but it's similar.  It reads around 1200F through the runup, then flaps around rapidly in flight so much it is impossible to get a solid read on it.  It never goes above 1200-1300F.  Then it starts reading colder and colder and eventually it, too, falls to the bottom of the scale (<800F) and stays there for the remainder of the flight.  I do not know which cylinder this EGT is reading.  I assume it is cylinder #3.

 

The EGTs on the other 3 cylinders are all steady and respond as expected according to the GEM.

 

CHTs on all cylinders, including cylinder 3, are always steady in the 375F range.  Cylinder #3 CHT is no different than the other cylinders.

 

The engine otherwise seems to run like a clock.  It only has about 650 hours on it SMOH (Penn Yan).  The aircraft cruises at 150-155 KTAS at around 65% power and 10000', so it seems to be putting out full rated power.  There are no unusual sounds or smells or rough running.

 

Any ideas?

[KSMooniac]

It sounds like an electrical issue, but it could be mechanical as well.  If the oscillations were consistent, I'd say it is a wobbly valve that really needs attention, but since the readings change over the course of the flight it is likely electrical.  You should start with the probe connections and see if there is any corrosion or loose connections.  If not, then switch #1 and #3 and see if the flaky behavior follows the probe.

 

Welcome to Mooneyspace, too!

[Marauder]

I have a 602 and have seen what you are describing from that unit. It was a problem with the exhaust stack attachment. The fact you are seeing it from the factory EGT is probably confirmation.

 

On the GEM, if the probe fails, you get an inversion display (the CHT bar glows and the EGT is blank). With both the factory EGT and GEM doing the same thing, I tend to think it probably is something in the stack. The solid and consistent CHT temp supports this and suggests that something down stream in the exhaust is the problem.

 

Have you looked at the exhaust system closely? I had a failure of one of the #2 cylinder exhaust mounting studs. It sheared off in flight. I saw fluctuating EGTs because it was occassionally opening up at the flange and letting the exhaust out before the probe. After a while the second nut vibrated free and the entire stack dropped into the cowling. The GEM inverted and the exhaust EGT went away and CHT went orange.

 

Look over the exhaust system for signs of exhaust gasses coming out in places it shouldn't...

[SpamPilot]

Great suggestions.  I inspected the engine carefully and found no evidence of any exhaust leak anywhere.  The stacks are on tight and the studs and nuts appear to be in good shape.  The #3 exhaust gasket is in good shape.  Actually the entire engine looks quite clean.  The analog CHT meter is on cylinder #3 with its own probe located just above the GEM 602 EGT probe.  Switching probes 1 and 3 is going to be a problem as the leads for the probe on 3 is not long enough to reach to 1, but I suppose I could put probe 1 into cylinder 3, cover the hole, and leave probe 3 tied up just to see if probe 1 reads the same way.  The probes themselves all appear to be in good shape.  I have not checked connections at the 602.

[Marauder]

Definitely a strange one. If the factory EGT was on 3 as well, it sure a weird coincidence that both are acting up at the same time. Checking connections and moving probes is the next logical step. Keep us advised. Sure would like to hear what you find.

[FloridaMan]

You may also have a spark plug that's not firing. Do a mag check in flight while this is happening and see if it runs rough on one of them. My EGTs went sky-high when I had a mag fail. 

[takair]

Could simply be the probes. They do burn up eventually and the results can be inconsistent readings.. Pull the probes and take a look.. It is fairly obvious when they are burned up, the outer case has a hole in it.

[SpamPilot]

I had my A&P/IA take a quick look at the probe from the GEM.  He says it looks fine.  While he agrees it would be odd to have the GEM probe and the factory probe fail at the same time, he is confident it is either a probe or wiring issue.

 

I did just notice that cylinder #3 (the one with the EGT fluctuation) had the lowest compression on the last two checks.  The compressions (hot) were 74/78/71/75 three weeks ago, and 75/75/70/76 four months ago.  The cylinders were borescoped during the prebuy inspection and nothing unusual was noted.

 

All the spark plugs were replaced 15 hours ago.

[SpamPilot]

Hm...   The engine has a tendency to pop every few seconds at idle after startup.  I assumed this was occasional backfire out the exhaust due to far excessively rich mixture, since I can pretty much eliminate it with a mixture pull (and going full rich audibly bogs the motor).  On the other hand, if #3 has a sticky intake valve, then maybe it's backfire out the intake that I am hearing.  A sticky intake valve could maybe explain both the lower compression reading and the low EGT reading as well as the backfire.

[SpamPilot]

Resolved.  There were two main problems and a couple ancillary issues.

 

The first problem was that whichever numbskull did the wiring harness in the engine compartment ran the EGT probe wires alongside the spark plug wires.  Crosstalk from the spark plug wires coupled into the EGT probe wires, causing a fluctuating reading on both the factory and GEM EGT displays.  By simply spacing the wires away from the ignition harness, the fluctuations went away and the readings are now nice and steady.

 

The second problem was that the GEM EGT probe had an intermittent failure and needed to be replaced.

 

Additionally, although not a contributing factor to the original problem, I discovered that the GEM EGT display had not been calibrated correctly.  It was reading far too high, giving misleading information about actual EGT.

 

Finally, the backfire/popping sound on startup has completely disappeared now that I have figured out how to start and idle the motor without choking it with fuel.

[N601RX]

The JPI STC doesn't really give any direction on how to route the wires.  I didn't want mine anywhere near the plug wires and didn't like the idea of running them down by the exhaust like I've saw many others.  I kept them up high and have not had any problems.  I also figured they would be a lot less likely to get oil soaked up high.