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Intermittent power loss - high RPM

RedRaider94

Active Member
Hoping someone might have an idea as this is driving me absolutely crazy.

RV7 - 1547 hours
Superior IO360
Slick mag
Light speed II EI
Silverhawk Bendix EX FI
Constant Speed prop

Problem: when doing touch and goes, the second takeoff and beyond sometimes has a feeling off power loss for a second or two then comes back and then a loss again, and then comes back. This is during climb out at max throttle.

*** it does not happen on the first takeoff of the day ***

The couple second feeling would be light switching off the Lightspeed and running just on the mag.

Fuel flow is in the 18 gph range when the issue is happening (that is normal fuel flow for me at takeoff) and RPM’s don’t go down while this happens - they stay steady.

The engine is not stopping, it’s just a slight loss of power for a second or two, comes back for a second or two, then loss again and so on.

This doesn’t happen on the first takeoff of the day and I am just stumped. Any initial ideas or how in the world would you go about troubleshooting something like this?

Thanks in advance for any thoughts!!
 
Just some questions - Does the manifold pressure show a drop? And do any of the cylinder's EGT readings show a drop at that time?

Dave
 
MP stays in the 27’s when it happen (RPM’s stay around 2650) so nothing noticeable with my eyes. I meant to record the takeoff in the GRT today but forgot to hit the record button. Will do that tomorrow to see if anything looks weird. If either are happening I am going to need data to really see it as it isn’t perceptible to me eyes.
 
I might suggest that you are getting some generation of fuel vapor in the line between the servo and the flow divider. I had this happen several years ago on a FI RV-6. It will only happen when the engine is hot. I typically would see this at its worse when doing laps in the pattern. A high power takeoff and climb, followed by a pattern at low speeds which doesn’t cool the engine well.
 
Hadn’t thought about that Dan. This started happening about 6-8 months ago best I can remember. Any idea if something specific might be causing it (outside of high heat generated with touch and goes) or anything that i could try to do to fix it)?

If I do a normal climb up to 4-5k ft with CHT’s running warm around 400, it doesn’t happen if it is the first takeoff of the day (best I can recall it hasn’t happened in that situation).
 
When on downwind/base/final, are you still running full rich? Possible fouling of plugs/deposits from running too rich at low power settings. Might consider leaning after leveling out and turning crosswind/downwind. I know the reason we run full rich is if we have to go around/missed, you want full power. per the POH. Just remember to advance to full rich again, after touch down, if going back up again (or if going around).
 
Avgas or Mogas? Fuel boost pump running or not? You mention 18 gpm, but what is the fuel pressure at the servo?
 
I am running 100LL and full rich the entire time. Electronic fuel boost is on when it happens (happens without it on also - checked that last weekend).

Don’t know about fuel pressure but always run between 26 - 28. I am going to record everything tomorrow so should have good data on egt, flow, fp, etc if I can get it to do it again.
 
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Hadn’t thought about that Dan. This started happening about 6-8 months ago best I can remember. Any idea if something specific might be causing it (outside of high heat generated with touch and goes) or anything that i could try to do to fix it)?

If I do a normal climb up to 4-5k ft with CHT’s running warm around 400, it doesn’t happen if it is the first takeoff of the day (best I can recall it hasn’t happened in that situation).
When the fuel leaves the servo it is under low pressure and when you look at the boiling temp for Avgas, you can see it coild boil creating vapor in the line. The flow divider will normally hold a back pressure on the line from the servo which then raises the boiling temp of the fuel. If the flow divider is not holding the pressure on the line, it can induce vapor issues.
 
Sounds like.....

Sure sounds like the fuel is getting hot. When the engine is warm, like during touch and goes, and the fuel flow drops, like during landing, the fuel will be flowing slower in the fuel lines and get warmer. I would make sure all the fuel lines are firesleeved and away from heat sources. But that is just me
 
Best I can recall, all fuel lines firewall forward have firesleeves installed with the only exception being the 4 stainless lines going from the fuel divider to the injectors. These are sitting just a inch or two above the heads. Are these lines likely where the vaporizing could be taking place and if so, should these have firesleeves installed over them?

I would have to get sleeves large enough to fit over the brass nuts on either end which are quite a bit larger than the lines themselves.
 
GRT

MP stays in the 27’s when it happen (RPM’s stay around 2650) so nothing noticeable with my eyes. I meant to record the takeoff in the GRT today but forgot to hit the record button. Will do that tomorrow to see if anything looks weird. If either are happening I am going to need data to really see it as it isn’t perceptible to me eyes.
Two things - I recommend recording all the time and just leaving a small USB key in there for cases like this. You can put the data on savvyaviation.com and get some interesting information.

You may also want to set the recording speed for this issue at the highest frequency, which I believe is 200 milliseconds. 1000 milliseconds (1 second) is probably not going to catch this issue.
 
Does this happen at the moment that you shove in the throttle or does it come a bit later?

Dropping an ignition does create a distinct sound, but usually only a slight power reduction. You could intenionally drop one ignition to get a sense for the power reduction in this case to confirm or eliminate. Sounds like a fuel issue to me.

Larry
 
If this is a new issue, I would first suspect the Lightspeed ignition. If a coil problem (not unusual) it will show up with two EGTs (1&2 or 3&4) going a little higher.

Do the plug wire and spark plug resistance checks as well (Lightspeed install manually). High resistance in either will end up killing the associated coil.

Carl
 
If it’s happening on rollout I can’t tell but start to feel it for sure about 500 ft AGL and then throughout my climb to pattern altitude. I changed EI plugs yesterday before I went up to recreate the issue and same thing so I know it isn’t the plugs.

If the coil or wires were bad, I would think it would do it on the 1st takeoff of the day but it doesn’t miss a beat.

Hoping to get up late this evening and do some recording at 2000 milliseconds to see if I can identify anything. Will try and post a picture of the results.
 
200 - 5Hz

...Hoping to get up late this evening and do some recording at 2000 milliseconds to see if I can identify anything. Will try and post a picture of the results.
Probably a typo, but I recommend going for 200 milliseconds (5 times per second).
 
If it’s happening on rollout I can’t tell but start to feel it for sure about 500 ft AGL and then throughout my climb to pattern altitude. I changed EI plugs yesterday before I went up to recreate the issue and same thing so I know it isn’t the plugs.

If the coil or wires were bad, I would think it would do it on the 1st takeoff of the day but it doesn’t miss a beat.

Hoping to get up late this evening and do some recording at 2000 milliseconds to see if I can identify anything. Will try and post a picture of the results.

Ignition doesn't really fit the symptoms, though failing coils often mis-behave when hot, but not when cold. My money is on the fuel getting too hot. On the ground, the F/F fuel system is heat soaking and that hot fuel gets to the post-servo, low pressure lines (where the pressure drops as does the boiling point) right after take off. This also explains why you don't get it on the first flight - no heat soak. You aren't using auto gas, are you? I suspect they have transitioned to the winter blend and it's lower vapor pressure.

Not certain on the diagnosis, but have a decent confidence that it is heat related. Problem right after a ground-based, heat soak and abates 30 seconds later when things cool down due to increased airflow. boiling fuel can create the hiccup symptom you describe, when it is limited in nature.

Larry
 
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I was able to get up and do some recording although I forgot to change the settings to 200 MS - this is recording at 1000MS or every second. It is still a bit interesting and my initial thought that the tach wasn't changing on the second flight isn't true.

1st picture is the 1st takeoff. Top graph is EGT and Fuel Pressure. Bottom graph is CHT and Fuel Flow. FP is running between 27 and 29 which it always has and I think is normal. EGT's seem to be extremely consistent. I don't see anything that jumps off the page as abnormal here.

2nd picture is the 2nd takeoff. Same graphs but the only thing that jumps out is how much more erratic the tach is. It's not crazy swings but a high of 2623 down to 2587 but definitely not smooth like the first takeoff.

It's never a situation where i feel like the engine is going to quit but I can feel the back and forth of power changes in my body enough to feel that something is going on.

Any thoughts on these or what it could be? Could it be something with the Constant Speed prop or governor even (I have a slight leak on one side where the base of the whirlwind prop connects to the hub - there is a little grease coming from the seal).

I am just at a loss.....
 

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I was able to get up and do some recording although I forgot to change the settings to 200 MS - this is recording at 1000MS or every second. It is still a bit interesting and my initial thought that the tach wasn't changing on the second flight isn't true.

1st picture is the 1st takeoff. Top graph is EGT and Fuel Pressure. Bottom graph is CHT and Fuel Flow. FP is running between 27 and 29 which it always has and I think is normal. EGT's seem to be extremely consistent. I don't see anything that jumps off the page as abnormal here.

2nd picture is the 2nd takeoff. Same graphs but the only thing that jumps out is how much more erratic the tach is. It's not crazy swings but a high of 2623 down to 2587 but definitely not smooth like the first takeoff.

It's never a situation where i feel like the engine is going to quit but I can feel the back and forth of power changes in my body enough to feel that something is going on.

Any thoughts on these or what it could be? Could it be something with the Constant Speed prop or governor even (I have a slight leak on one side where the base of the whirlwind prop connects to the hub - there is a little grease coming from the seal).

I am just at a loss.....

Seems pretty improbable that the governor would work unreliably in ONLY the very limited situations that you outlined in your first post. I would expect fuel boiling in the low pressure lines could easily cause the tach variations seen, as different cylinders see rapid swings across the mixture range, including too rich to fire and too lean to fire. While a 1 PSI difference is too small to be meaningful, changes in FP could point to fuel boiling as well. Would be wise to watch for consistency in FP reduction when symptoms occur. Notice in your charts that the 1 Psi reduction only occurs during the period of unstable RPM; That is a clue. Vapor lock (also boiling fuel) often results in lower FP.

Larry
 
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When you say low pressure lines, how do we define low and high pressure lines in the fuel system. Is high pressure up to the fuel divider on the top of the engine where the thin stainless lines take fuel to the cylinders or do they start at some point before that?
 
I also just read another post by someone who had a valve that was sticking sometimes. Would I definitely see a sticking valve in the data set with my EGT and CHT’s at my sample rate of 1000 ms (once every second)? Can I rule that out given the data I posted?
 
I also just read another post by someone who had a valve that was sticking sometimes. Would I definitely see a sticking valve in the data set with my EGT and CHT’s at my sample rate of 1000 ms (once every second)? Can I rule that out given the data I posted?

Not likely. At full speed each valve is cycling open and closed at 22.5 Hz on a 4 cylinder engine. The valve would have to be sticking for quite a number of cycles and you'd see the EGTs drop and feel a lot of power loss, I would think.
 
When you say low pressure lines, how do we define low and high pressure lines in the fuel system. Is high pressure up to the fuel divider on the top of the engine where the thin stainless lines take fuel to the cylinders or do they start at some point before that?

Fuel to the servo is 25 psi. Fuel after the servo is a much lower pressure. It varies based upon the flow rate commanded by the servo. Lower pressure fuel boils at a lower temp.

You want to protect from heat absorption at all points. However, the actual boiling is taking place in the low pressure lines. Goal is to keep the fuel below that boiling point by reducing heat absorption in all areas.

Larry
 
I also just read another post by someone who had a valve that was sticking sometimes. Would I definitely see a sticking valve in the data set with my EGT and CHT’s at my sample rate of 1000 ms (once every second)? Can I rule that out given the data I posted?


Sticking valves rarely present hot but not cold. Typically the opposite. They are also not limited to high power regimes. The symptoms appear (rough running in varying degrees from barely noticeable to shaking like a dog, depending upon the amount of sticking), typically at cold start, and continue for some period of time (minutes, not seconds) and then disappear as the engine warms up and not seen again until the next cold start. They do not just appear only during climb for a few seconds.

Larry
 
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Basic stuff, when was the last time the Magneto was serviced? A bad condenser in the mag (they often fail when hot) can/will cause problems.
 
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You mentioned grease coming from one of the prop seals?

This could be indicative of a very serious problem and may explain your RPM/power surge problem if your prop is binding and not adjusting pitch smoothly.

Losing a blade off your prop in flight would be catastrophic!
 
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Walt, the Mag went through a 500 hour inspection and rebuild about 150 hours ago. It was sent off to Aircraft Magneto Service in Montana. I'm only a 5-10 minute plane ride from you so need to bring it by to let you take a look someday (I'm at Aero Country).

The grease is very slight to where you can just see little lines of grease slung outward from the seal (it's not leaking real bad and A&P who did inspection a couple months ago wasn't concerned - I was more than him I believe).

When I fire it up everytime it sounds perfect and doesn't miss a beat so I guess between that and nothing indicating a problem in the data dump we can rule out stuck valves.

I am going to try and fly this afternoon and will record startup and then takeoffs again at 200ms just to see if anything else comes up at higher recording rates and if it does it again will start taking pictures of every fuel line to get everyone's thoughts on where the fuel might be getting too hot.
 
I had something similar. I checked plug,wires and changed the coil. It was still there. So I changed the plug and it went away!
 
I had this problem

The symptoms you describe were the same for my plane. Not sure if the problem i discovered is similar to yours though...

I had changed engines to a fuel injected motor with a new unfiltered airbox. The plane would surge, losing 100-200 rpm for 2 or three seconds, then develop full power again. After months of trial and error, Don at AFP suggested I send in my new airbox to be flow tested. He called a few days later to say the turbulence created by my airbox shook the room it was being tested in!

I changed the shape of the intake. Problem solved. Any chance the inlet air is developing turbulence in your bird?
 
Walt, the Mag went through a 500 hour inspection and rebuild about 150 hours ago. It was sent off to Aircraft Magneto Service in Montana. I'm only a 5-10 minute plane ride from you so need to bring it by to let you take a look someday (I'm at Aero Country).

I probably have a mag laying around if you want to swap it out. Personally I don't think it would be hot fuel, under high power FF is high enough usually to avoid problems with fuel temps. One of the Ign systems IMO.
 
Shawn,

Some thoughts from previous experience, that may help you eliminate bogeys:

My plane originally had a LS Plasma 1 and 1 mag. While that was installed, I had half a coil fail once, with a single errant EGT indication. Talked to Klaus, and he confirmed a coil can fail halfway like that, but it's rare.

I later had a brain box failure, which was intermittent, and only happened when cold. It was also an errant EGT. As the engine, and the box, which was on the cold side of the FW, warmed up, it went away. I changed plugs, wires, coils throughout, and finally sent Klaus the brain box. Klaus tested the box, and it was similar on the test bench. As soon as the box and all the transistors and gizmos inside warmed up electrically, it went away. He could also press on the box itself and it would go away before warming up. I know this is opposite of your situation, and you don't have EGT variances...just describing so you can either eliminate, or look at the box, if you develop other indications.

In a racing Glasair III, I had a slow degradation of a Romec mechanical fuel pump, which was masked by a boost pump also starting to go (yes...not a good situation, eh!). Changing the boost pump fixed FF issues at high power for a season, but they returned during the ensuing year. The manifestation of this was fuel pressure that dropped a bit and a hi EGT past a certain power setting. It was a turbo, so I'm talking very high power (greater than 50" MP). In the normal MP ranges the issue was not apparent, but I've been told that it would have been seen at lower power as the pump degraded further. New mechanical pump fixed it all. Your FF appears solid, so you may be able to eliminate this. However, its a parameter worth keeping an eye on, and perhaps it's worth testing to see if you see any RPM fluctuations at cruise power settings when hot.

I also had governor issues with that TIO-580 on the Glasair. Indications were RPM fluctuations, slightly higher in magnitude than yours, but not limited to when hot. an overhauled governor solved that one. The governor is an oil pump, so could hot oil unmask an internal governor issue...perhaps. Might be worth having it looked at.

Infant mortality on the overhauled mag may be unlikely, but not out of the question. Walt's swap offer isn't a bad idea. Just had similar happen on a Stearman mag. Mag bench tested good, so I troubleshot every other potential cause for 2 weeks, and we finally had a new mag built up...boom, fixed it.

Also agree that, while vapor issues could be present, if the configuration of fuel lines and injector lines hasn't changed, and its a new issue at 1500ish hours, this seems like a less likely cause. Have you looked for indications of cracks in the injector lines? I'd think this would cause other indications than just RPM flux (EGT primarily), but trying to help you rule out causes.

Good luck...these things can be hair-pulling...AMHIK! ;)

Cheers,
Bob
 
Thanks for all of the thoughts so far. I tried again last night and I feel like I have something going on with Cylinder #3. I have attached three recordings:

*** Note - the number 3 cylinder was replaced around 500-600 hours due to an SB on the superior jug. It was replaced with a superior head. The other three heads are the original Superior heads.

#1) After initial start and warming up. The plane was leaned aggressively for the warm up and I noticed on the monitor that #3 CHT was lower than the others but registering and the EGT's were significantly lower until it warmed up a bit. Don't know if this is typical as I have never noticed it before.

#2) 1st lap around the pattern. I did have some slight hesitations at full throttle so it did happen a little on the this flight but not a lot. What got my attention was how erratic the #3 EGT is around when I pull back on the throttle at the end of downwind. I have never seen this before.

#3) 2nd lap around (I taxied back to the runway with Fuel boost on the whole time to try and keep pressure high). I did feel the slight feeling of drop in power for a couple seconds followed by a slight surge in power then slight loss and slight surge a few times. The data though looks pretty clean on this run.

I am now pulling the last amount of hair I have out of my head. Worried this might be a valve problem on #3.

Again, I don't feel noticeable misses on startup. It has a good rhythm to it and doesn't shake or feel like anything is off.

I am getting concerned to fly it thinking something is really wrong with the #3 cylinder and it is going to throw a rod in flight or am I just overthinking this?
 

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Thanks for all of the thoughts so far. I tried again last night and I feel like I have something going on with Cylinder #3. I have attached three recordings:

*** Note - the number 3 cylinder was replaced around 500-600 hours due to an SB on the superior jug. It was replaced with a superior head. The other three heads are the original Superior heads.

#1) After initial start and warming up. The plane was leaned aggressively for the warm up and I noticed on the monitor that #3 CHT was lower than the others but registering and the EGT's were significantly lower until it warmed up a bit. Don't know if this is typical as I have never noticed it before.

#2) 1st lap around the pattern. I did have some slight hesitations at full throttle so it did happen a little on the this flight but not a lot. What got my attention was how erratic the #3 EGT is around when I pull back on the throttle at the end of downwind. I have never seen this before.

#3) 2nd lap around (I taxied back to the runway with Fuel boost on the whole time to try and keep pressure high). I did feel the slight feeling of drop in power for a couple seconds followed by a slight surge in power then slight loss and slight surge a few times. The data though looks pretty clean on this run.

I am now pulling the last amount of hair I have out of my head. Worried this might be a valve problem on #3.

Again, I don't feel noticeable misses on startup. It has a good rhythm to it and doesn't shake or feel like anything is off.

I am getting concerned to fly it thinking something is really wrong with the #3 cylinder and it is going to throw a rod in flight or am I just overthinking this?

#3 EGT behavior points to an intake leak (goes lean with resulting higher EGT as the MAP goes down). However, that you should not have a significant impact at full power, only at low MAP regiments, such as idle to 1500. Quite unlikely this is causing your high power stumble. The leaking intake also accounts for the low EGT at idle. When the leak is moderate, as MAP starts to drop, the EGT rises. As the leak increase while you move towards idle, the EGTs will plummet.

Quite unlikely for a valve to stick for only 2 seconds in only highly repeatable, full power situations.

Larry
 
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Fuel starvation due to dirty gascolator (or other fuel filters you may have)?

I've fried a Ferrari engine that way :-(
 
Thanks for all of the thoughts so far. I tried again last night and I feel like I have something going on with Cylinder #3. I have attached three recordings:

*** Note - the number 3 cylinder was replaced around 500-600 hours due to an SB on the superior jug. It was replaced with a superior head. The other three heads are the original Superior heads.

#1) After initial start and warming up. The plane was leaned aggressively for the warm up and I noticed on the monitor that #3 CHT was lower than the others but registering and the EGT's were significantly lower until it warmed up a bit. Don't know if this is typical as I have never noticed it before.

#2) 1st lap around the pattern. I did have some slight hesitations at full throttle so it did happen a little on the this flight but not a lot. What got my attention was how erratic the #3 EGT is around when I pull back on the throttle at the end of downwind. I have never seen this before.

#3) 2nd lap around (I taxied back to the runway with Fuel boost on the whole time to try and keep pressure high). I did feel the slight feeling of drop in power for a couple seconds followed by a slight surge in power then slight loss and slight surge a few times. The data though looks pretty clean on this run.

I am now pulling the last amount of hair I have out of my head. Worried this might be a valve problem on #3.

Again, I don't feel noticeable misses on startup. It has a good rhythm to it and doesn't shake or feel like anything is off.

I am getting concerned to fly it thinking something is really wrong with the #3 cylinder and it is going to throw a rod in flight or am I just overthinking this?

Have you checked injectors? Any change on fuel burn rate? If you swapped 3 and 4, does the problem chase the swap? Injector could be fine at low power, but starting dumping fuel when pushed to full. Just a thought.

apologies if I'm repeating something.
 
Possible Cause?

... Try this as you lift off on second go round! Switch off your lightspeed ign.
and just see what happens, do offs and ons and see how it acts. Thanks, Allan
 
Check the ignition. Been there, done that. Symptoms appeared after engine was warm and had run a while. Also, one intake valve problem affects all 4 cylinders. Been there, done that too.
 
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