High EGT on one cylinder (it's not the probe)

[Super Dave]

Background: IO-360 A1A with roller tappets, 650hrs since factory overhaul. Peak EGTs are typically high 1400s-low 1500s (20°BTDC timing). Recently, #2 EGT has increased, and is now 100°-150° higher than it has run in the past and now peaks at over 1600°.

Here's what we've tried so far:

Swapped probes, but high EGT stayed with #2.

GAMI spread is still .2-.3GPH, and it will still run deeply LOP... but just to rule it out, we cleaned the injectors.

Borescoped #2- exhaust valve shows no asymitry and looks to be seating normally.

Compression check - 78/80.

Tried an inflight induction leak test by recording EGTs at full throttle-full rich at 5000'. Then, reduced MP to 18" and noted delta in each cylinder's EGT. #2 EGT actually got a little cooler with the reduction, which is the opposite of what would be expected with an induction leak. Also, I don't think an induction leak could cause EGT to peak in the 1600s.

I guess we're leaning toward exhaust valve issues, but like I said, borescope looked normal and compression was good.

Is it possible for an exhaust valve to seat properly while static, idling, and at run up power, but then to somehow not seat completely while at full power?

Any other thoughts on diagnostics? remedies?

[carusoam]

WAG...

Could the exhaust valve sticking cause the raise in EGT?

Can the exhaust valve be easily tested for carbon build-up?

If this were the case... Cleaning the valve guide would remove the carbon, restore the valve's motion, and reset the situation...

Nice collection of data for somebody to be able to help.

Best regards,

-a-

[Shadrach]

Did you do a mag check in flight? What kind of plugs and how old. Before going crazy chasing possible mechanical issues, make sure you don't have a bad plug that goes off line in high power cruise. This happened to a pro that flies my plane. It passed a standard full rich mag check of the ground. When I leaned it out the bottom plug dropped of line at ~2000 RPM.

Try swapping plugs between #2 and #4.

[N201MKTurbo]

Ross is correct, if a plug stops fireing your EGT will rise about what you are seeing. If you do a full power mag check you will identify the bad plug quickly.

[Super Dave]

Sory, forgot to mention the spark plugs; cleaned, gaped, rotated. Inflight LOP mag check- normal EGT rise on all cylinders, before and after rotation. Tempest massives with about 150hrs.

[Awful_Charlie]

Are you sure #2 has got hotter, or have #1,#3 and #4 got cooler

 

You seem to have done the obvious, one thing left would be the wiring between the probe and the gauge (or the instrument itself) - the instrument itself is only measuring millivolts, so have you run/moved a high current (alternator?) wire anywhere near the #2 lead? You could test this by swapping the wiring on a pair of probes (probably with #4 I guess)  or by moving some pins in the connector (likely to be more difficult!)

 

Exhaust valve does look the favourite though - and yes, from the stories I've read, the "morning sickness" that goes away shortly after start has often then failed when at full or cruise power. Thinking about this, it is the valve stem taking heat from the valve head and conducting it to the valve guide - if the valve is sticking causing premature exhaust, then as the stem gets hotter it expands, causing the exhaust to leak more, making the stem get hotter.....

 

Worth removing the rocker cover and the opinion of your A&P I suggest

[Shadrach]

How are you getting normal rise and a peak of over 1600? This really reads like it's ignition related. Failing exhaust valves typically cause EGT oscillations of 30-50df with mixture contestant. High EGTs are typically the result of a combustion event so slow that it is still taking place through the exhaust stroke and past the EGT probe (causing elevated EGTs). An engine timed @ 20BTDC is naturally going to run higher EGTs than a 25degree engine. The fact that yours happened all at once indicates ignition. How far past peak can you lean on the bad cylinder?

Try leaning to peak on the cylinder in question and then doing a mag check. Is it possible that in rotation the plugs just swapped top and bottom positions? Could also be a plug wire issue. Do report back, I'm really curious.

[Super Dave]

How are you getting normal rise and a peak of over 1600? This really reads like it's ignition related. Failing exhaust valves typically cause EGT oscillations of 30-50df with mixture contestant. High EGTs are typically the result of a combustion event so slow that it is still taking place through the exhaust stroke and past the EGT probe (causing elevated EGTs). An engine timed @ 20BTDC is naturally going to run higher EGTs than a 25degree engine. The fact that yours happened all at once indicates ignition. How far past peak can you lean on the bad cylinder?

Try leaning to peak on the cylinder in question and then doing a mag check. Is it possible that in rotation the plugs just swapped top and bottom positions? Could also be a plug wire issue. Do report back, I'm really curious.

I bet you're not as curious as I am!

By normal rise, I mean that all cylinders have a roughly equal increase in EGT when doing an inflight LOP mag check. #2 is actually rising more than the other cylinders during the mag check, but it does so equally on either mag. EGT on that cylinder rises to near 1800° degrees during the inflight check.

I can go nearly 100°LOP before roughness. All cylinders still peak almost simultaneously, it's just that the EGT values on #2 are way higher than the other values.

Spark plug rotation was typical top to bottom, left mag to right mag. So, #2 plugs went to #3, and vice versa.

I agree that it doesn't act like a burned/warped exhaust valve, and it doesn't look like one through the borescope. I was thinking more along the lines of the exhaust valve is somehow not completely closing when the engine is making power? Something like a broken/damaged valve spring?

[Super Dave]

Are you sure #2 has got hotter, or have #1,#3 and #4 got cooler

You seem to have done the obvious, one thing left would be the wiring between the probe and the gauge (or the instrument itself) - the instrument itself is only measuring millivolts, so have you run/moved a high current (alternator?) wire anywhere near the #2 lead? You could test this by swapping the wiring on a pair of probes (probably with #4 I guess) or by moving some pins in the connector (likely to be more difficult!)

Exhaust valve does look the favourite though - and yes, from the stories I've read, the "morning sickness" that goes away shortly after start has often then failed when at full or cruise power. Thinking about this, it is the valve stem taking heat from the valve head and conducting it to the valve guide - if the valve is sticking causing premature exhaust, then as the stem gets hotter it expands, causing the exhaust to leak more, making the stem get hotter.....

Worth removing the rocker cover and the opinion of your A&P I suggest

The probes were left wired to their original channel when the swap was made. This way, we could be sure that it wasn't a bad probe, wire, or connection. So, when we had the probes swapped we were actually reading #2 cylinder as #4 on the JPI.

Will investigate sticking valve. When valves stick, do they stick open, closed, or in between?

[Oldguy]

Sticking valve would typically be sticking open.

 

I have seen weak valve springs cause a valve "float" at higher RPM and lack of oil cause a valve to get sticky, but this one just sounds odd.

 

Haven't seen any mention of a CHT change on the cylinder in question. Has there been any change in it's temps?

 

Also, is there any possibility one of the mags might be off just a bit?

 

John

[Shadrach]

Valves stick open, but you would likely not have that perfectly symmetrical collection of red combustion byproducts (what you saw through the boroscope) on the valve if you had a leak. It would be asymmetrical mix of grey and red.

[Shadrach]

I was thinking more along the lines of the exhaust valve is somehow not completely closing when the engine is making power? Something like a broken/damaged valve spring?

I don't think you'd be able to lean to 100df LOP if you had an open exhaust valve or an ignition issue. I would think it would get rough way before that.

Has your MX suggested reaming the exhaust valve guide? That's an easy 2hr procedure that does not require removal of the jug.

[HRM]

My #2 runs hot, always ran hot.

Same engine as you.

Now it runs "real hot".

I have pulled hair out trying everything.

I noticed when I slow down on climb, it does not get as hot.

Solved the problem, but I liked taking off like a jet.

I am now suspecting engine mounts.

[Shadrach]

We're talking EGT, I think you're referring to CHT.

[Super Dave]

First of all, thanks to all the responders so far, I appriciate the time and thought you are putting in to help another out.

Sticking valve would typically be sticking open.

 

I have seen weak valve springs cause a valve "float" at higher RPM and lack of oil cause a valve to get sticky, but this one just sounds odd.

 

Haven't seen any mention of a CHT change on the cylinder in question. Has there been any change in it's temps?

 

Also, is there any possibility one of the mags might be off just a bit?

 

John

CHT has been unaffected.

Coincidentally, we were due for a 500hr inspection on the left mag and pulled it off just after the trouble started. Mag was sent out, replaced and both mags timed to the engine. So, that rules out timing.

[Shadrach]

Are you sure CHT has not changed? Often times CHT go down when we're talking about NA engines with EGT's in this range.

So far we know/think we know the following:

1) all mags timed correctly.

2) all spark plugs firing as they should.

3) exhaust valve looks normal under the scope.

4) no symptoms leading up to the high EGT condition (e.g. oscillating EGT's)

Have I missed anything?

[Super Dave]

Valves stick open, but you would likely not have that perfectly symmetrical collection of red combustion byproducts (what you saw through the boroscope) on the valve if you had a leak. It would be asymmetrical mix of grey and red.

  

I need to understand more about sticking valves. I thought burned valves were a result of poor fit between the valve and the seat

leading to a hot spot on one particular position on the valve face. The hot spot continues to warp the valve face in a vicious cycle until part or all of the valve face separates.

I thougt sticking valves were something completely different where the valve does not open or close properly, but there is not necessarily anything wrong with the valve to seat fit, i.e. no asymmetry.

I don't think you'd be able to lean to 100df LOP if you had an open exhaust valve or an ignition issue. I would think it would get rough way before that.

Has your MX suggested reaming the exhaust valve guide? That's an easy 2hr procedure that does not require removal of the jug.

Ditto on the ability to still run LOP, this whole thing is a head scratcher. if it were not for the engine monitor I don't think we would suspect anything amiss. Good CHTs, engine still makes good power and runs smoothly ROP and LOP.

As far as consulting MX; one of the co-owners is an AP/IA, but other than doing the work on our Mooney for a little over a year now, he works exclusively on turbin equipment. Obviously, he's got good basic knowledge of piston equipment from A&P school, but his expertise is in kerosine burners.

[Super Dave]

My #2 runs hot, always ran hot.

Same engine as you.

Now it runs "real hot".

I have pulled hair out trying everything.

I noticed when I slow down on climb, it does not get as hot.

Solved the problem, but I liked taking off like a jet.

I am now suspecting engine mounts.

#2 has always been our hottest also, but not by much. We typically see 360s climbing out. Low 300s in cruise.

[Bartman]

What would a dirty injector do to EGT ?

I think it would make the cylinder run cooler, but the EGT might get hotter ?

[Super Dave]

What would a dirty injector do to EGT ?

I think it would make the cylinder run cooler, but the EGT might get hotter ?

This would be true ROP, but a dirty injector would peak before the rest and have a lower EGT when LOP... Not what we are seeing. Also, a dirty injector would mess up the GAMI spread, and ours is still .2-.3 GPH

[Super Dave]

Are you sure CHT has not changed? Often times CHT go down when we're talking about NA engines with EGT's in this range.

So far we know/think we know the following:

1) all mags timed correctly.

2) all spark plugs firing as they should.

3) exhaust valve looks normal under the scope.

4) no symptoms leading up to the high EGT condition (e.g. oscillating EGT's)

Have I missed anything?

You got it. If we had not been so deliberate about swapping the probes, I would be convinced it's an indication error.

[Super Dave]

The one thing you recently changed around was the plugs ... ? Might be worth a switch back ... and a run ... just to see?

If it were an ignition issue, I would expect to see the offending cylinder to show no EGT rise when one mag is switched off, and cold EGT/roughness (dead cylinder) when the other mag is switched off. But instead we see a normal EGT rise when either mag is switched off.

[Cruiser]

let's see the JPI graphs. You are missing something.

[carusoam]

Second WAG...

Champion spark plugs at all?

High resistance limiting the usefulness of a single spark plug may result in unburned fuel being delivered into the exhaust causing higher EGT...

The resistance of the plugs can be easily measured when removed...

To be clear...WAG = wild a** guess, I'm still not a mechanic...

Best regards,

-a-

[Super Dave]

let's see the JPI graphs. You are missing something.

Maybe. We've never downloaded data from the JPI before, but we're looking into it now.

Second WAG...

Champion spark plugs at all?

High resistance limiting the usefulness of a single spark plug may result in unburned fuel being delivered into the exhaust causing higher EGT...

The resistance of the plugs can be easily measured when removed...

To be clear...WAG = wild a** guess, I'm still not a mechanic...

Best regards,

-a-

Tempest massives; I haven't checked resistance since when they were new 150 hrs ago, but I've never heard of a Tempest failing the resistance test. Also, the mag checks sure make it seem like ignition is healthy.