High CHT then EGT/RPM fluctuates on the ground

[Draker]

Hey, engine masters,

Could use a little help diagnosing. Lycoming YIO-360-M1B. 37 hours total time.

I was in the middle of a phase 1 test flight today. I took the -7A up to 16.5K indicated, 18.5K density altitude to check high altitude performance. CHTs got a little hot but not past my "personal redline" of 425F. At some point during the climb, CHT4 suddenly broke away from the pack [see pic 1] and remained 10-15 degrees hotter than the rest of them. I decided not to continue testing and did a descent to cool everything back off.

Once I was back on the ground, CHT4 was noticeable cooler than the rest. Then, during taxi, at idle-ish low throttle settings, RPM was all over the place [see pic 2]. Every 8 seconds or so, it would drop by about 200RPM and EGT 2 and 4 would also drop like a brick. Engine stumbled, then recovered, only to repeat that pattern over and over until I taxied all the way back. The periodic stumbling was less severe taxiing at higher RPM.

Where do you suggest I start looking? I plan to check the basics, look for leaks, look at the plugs, stick a borescope inside #4 and so on, but any helpful insights? This is the first kind of scary squawk I've had so far.

 

[blaplante]

The cyclic EGT variation of #4 is really interesting.
The only thing that comes to mind is the Savvy "Exhaust Valve Pattern", where there's a cyclic pattern of EGT up/down as the exhaust valve rotates indicating a leaking spot on an exhaust valve.

Borescope #4 for sure, with particular attention to exhaust valve.

Get 4 glass jars and flow test the 4 injectors to confirm equal flow. I sort of suspect some partial blockage of the #4 injector.

 

[bjdecker]

Ryan,

Did the spark plug wire harness come loose off one of the mags?

B

 

[wcalvert]

New airplane, new problems, lots of variables available to cause the issue.

For sure take a look at the easiest things first: Ignition wires, plugs, connections on all your sensors and mags. Touch every wire and terminal looking for the unexpected.

Then move on to fuel: filters, injector flow, injector screens, boost pump affect on the missing etc. On the ground testing might show the culprit but a flight (if safe) might be required.

Take a look at the air flow path: filters and intake screens, servo linkages and air leak paths, etc.

You might then move on to intake/exhaust valve operation, although it seems a little young to be showing sticky valve issues.

I'm would really like to know if you used the boost pump during your taxi roughness? Leaning procedure? Fuel? Temps that day? The scope might be enlightening, how do all the plugs look?

You're on the right path, just be sure to keep an open mind with an eye to the unexpected.

 

[Draker]

Pulled the cowl off and had a look around, with an eye toward anything different about cyl.4.

Plugs look fine


Intake valve


Exhaust valve


The only thing a little questionable was there was a little bit of dark green fluid down on the lower part of the #4 intake pipe, where the others were pretty much dry


Maybe the early beginnings of an intake leak?

Anyway, I put everything back together, fired up the engine, and everything is normal. Smooth running, rock solid RPMs at all taxi power settings, at idle. No outlier EGT. No problems at any mixture setting. Fuel pump on or off. Air temp is a little cooler today (80F) than when the problem occurred (90F), but nothing else changed.

Very unsatisfying--ya never like to see things fix themselves. I don't see much more for me to do besides fly and monitor for further problems.

 

[blaplante]

Did you get a borescope look at the face of the #4 exhaust valve?

See https://airplaneownermaintenance.com/191-a-tale-of-two-exhaust-valves/
you want a symmetrical pattern, not something with a different color off on one side.

Highly unlikely on a low time engine, but easy to check.

 

[Tim Lewis]

Once I was back on the ground, CHT4 was noticeable cooler than the rest. Then, during taxi, at idle-ish low throttle settings, RPM was all over the place [see pic 2]. Every 8 seconds or so, it would drop by about 200RPM and EGT 2 and 4 would also drop like a brick. Engine stumbled, then recovered, only to repeat that pattern over and over until I taxied all the way back. The periodic stumbling was less severe taxiing at higher RPM.

Where do you suggest I start looking?

Those symptoms sound similar to how my IO-540 idled when it was hot: Surging RPM at low fuel flow (during taxi after flight, especially after brief engine shutdown), which could be remediated by increased throttle/RPM. Problem absent when engine was cool.

Reason for the problem: the fuel vaporized in the hot (post-flight) fuel lines FWF, causing uneven fuel flow to the cylinders and uneven RPM. I tried a fuel pump shroud (didn't seem to help). What did help was replacing the fuel injection spider spring (4lb vs 2lb) per Lycoming SI 1539 and SI 1489C. The heavier spring increases the fuel pressure in the fuel line before the spider, helping suppress fuel vapor formation.

I bought my spring from Airflow Performance.

I still get some surging when the engine is hot and it is hot outside, but much less than before.

 

[Draker]

Did you get a borescope look at the face of the #4 exhaust valve?

See https://airplaneownermaintenance.com/191-a-tale-of-two-exhaust-valves/
you want a symmetrical pattern, not something with a different color off on one side.

Highly unlikely on a low time engine, but easy to check.

Unfortunately my bargain bin borescope doesn’t have a 90 deg camera so I was only able to get a look at the valves in side profile. Pro tip: when you get a borescope for engine analysis, get the kind with two cameras!

Test flew today and did a mag check in-flight, aggressively leaned, to see if there were any wild EGT behavior. EGT 4 went a little hotter that the rest while the left mags were off. That plug may be gapped a little to much. I checked the plug gaps during my inspection but might have missed this one. I also compared EGTs at 25”MP vs 15” holding everything else constant. EGT rise is consistent across all cylinders, so I’m doubting there’s an intake leak.

I kind of like Tim’s theory. It *was* hot as damnation during my taxi on Friday. Probably the hottest OAT I’ve ever operated in so far. I wish I would have thought to turn on the boost pump while the problem was happening.

 

[blaplante]

Given that the problem seemed localized to certain cylinders I'm doubting that the boost pump would have helped. That would pressurize fuel on the path to the engine pump but not any different *after* the engine pump. Which means you only up the pressure in a common line for all cylinders.

Anything is possible, but hard to figure how that would manifest on particular cylinders.

Since you have an IO (like me), and it was HOT. I agree that the under cowl temps might have had a lot to do with the problem. The fuel gets hotter as it spends time under cowl and the pressures are reduced after the servo and after the flow divider. Making vapor bubbles more likely. Assuming you have fire sleeve already, look into something like DEI HEAT SLEEVE to add an additional barrier. It helps some. Not a cure in my experience, but a help.

 

[Draker]

Update in case anyone's following along. This happened once more. Identical conditions: 1. After a flight, 2. After refueling and taxiing back to hangar, 3. Very hot weather

I tried a few more diagnosis steps this time. Boost pump on/off did not help. Cycled through the entire mixture range from full rich to cutoff, did not help. Left mag only: no better or worse, right mag only: no better or worse. Exact same symptoms as before: when taxiing under 1000RPM or so, every 8-10 seconds, EGT2 and EGT4 fell to the floor, RPM went down, MAP went up. CHT2 was consistently 30 degrees cooler than CHT1 and CHT4 was consistently 40 degrees cooler than CHT3 during the taxi.

Plugs are all gapped correctly. I see no evidence of an induction leak. Very strange. Might be time for a call to Lycoming.

 

[sailvi767]

I would place a 100 dollar bet it’s fuel boiling in the lines. I had the same issue on the ground with a 360 on hot days. Push the power up to 1700 RPM and see if it goes away. Once you flow fuel faster it stays cooler in the lines and the problem goes away.

 

[ERJDriver]

Update in case anyone's following along. This happened once more. Identical conditions: 1. After a flight, 2. After refueling and taxiing back to hangar, 3. Very hot weather

I tried a few more diagnosis steps this time. Boost pump on/off did not help. Cycled through the entire mixture range from full rich to cutoff, did not help. Left mag only: no better or worse, right mag only: no better or worse. Exact same symptoms as before: when taxiing under 1000RPM or so, every 8-10 seconds, EGT2 and EGT4 fell to the floor, RPM went down, MAP went up. CHT2 was consistently 30 degrees cooler than CHT1 and CHT4 was consistently 40 degrees cooler than CHT3 during the taxi.

Plugs are all gapped correctly. I see no evidence of an induction leak. Very strange. Might be time for a call to Lycoming.

Angle valve 360 has done this since I’ve had it, 350 hrs and on. It’s either fuel or mags but anything off idle and it’s fine. Just hot.

 

[blaplante]

Post photos of your firewall forward fuel line runs.

 

[pjc]

I would place a 100 dollar bet it’s fuel boiling in the lines. I had the same issue on the ground with a 360 on hot days. Push the power up to 1700 RPM and see if it goes away. Once you flow fuel faster it stays cooler in the lines and the problem goes away.

Agreed. The heat soak while refueling is telling. If it doesn’t do it when you land and taxi (and have some cooling air flow above the cylinders) I think it’s likely you have inconsistent fuel delivery from the spider to the cylinders due to vaporization in the lines. Compounded by high OAT. Install the spring swap for the spider and retest to be sure.

Peter

 

[Draker]

I like where this is going. You guys (and Tim) seem to be onto it. Heat soaked under the cowl and perhaps it’s hotter on the 2-4 side to the point where it is boiling in the lines post-spider. I’ll have to try the spring swap SB. Wonder if there is a good way to heat insulate those thin lines, too.

I happen to have a thermometer under the cowl, RTV’ed to my red cube (which itself hooks directly to the fuel divider). Yea, it was hot: over 180 deg F after sitting on the apron refueling, dropping to about 150F during taxi.

 

[birddog486]

very common of a hot Injected engine when you have no fuel return line.

Either fill the plane before first flight of the day or get to the runway ASAP after a hot fill-up.

Your also far enough into the engine break in to start running lean of peak and balancing your injectors which will keep engine temps under control at any altitude

 

[Jetmart]

I have the exact same problem with my RV-14 with a 390. Has happened when engine is hot and outside air is hot since day 1. Always on taxi back or 2nd start.
It was suggested in earlier in this thread by Tim to try going from a 2 psi spring in the fuel divider to a 4 PSI. My Avstar set up already has a 4 PSI. Airflow Performance suggested instead that I go from my current .028 to .024 injector restrictors to put a bit more back pressure on the system.
I have not installed & tested the new yet.

 

[lr172]

I have the exact same problem with my RV-14 with a 390. Has happened when engine is hot and outside air is hot since day 1. Always on taxi back or 2nd start.
It was suggested in earlier in this thread by Tim to try going from a 2 psi spring in the fuel divider to a 4 PSI. My Avstar set up already has a 4 PSI. Airflow Performance suggested instead that I go from my current .028 to .024 injector restrictors to put a bit more back pressure on the system.
I have not installed & tested the new yet.

I have .024's on my 320. This will help off idle, but won't do much at idle. Unfortunately the backpressure is not created by the injectors at low fuel flows (i.e. idle). That backpressure is being produced by the restriction at the bottom of the V shaped slot in the spider, which is smaller than the injector and there just for that purpose.