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IO-540-C4B5 RPMs Hunting

mikelupo

Active Member
Patron
Dear Abby,

I've been chasing this problem for 4 months now. My RPMs are hunting. At idle, in cruise...basically any time the engine is running, the RPMs are hunting. The problem presents itself as such: It will basically stay where it's supposed to (i.e. 2350) for a moment, then it usually surges up, sometimes 100 RPMs before the prop governor corrects for it, then the problem resolves and the RPMs go lower than my 2350, then the governor corrects for it again. Rinse/repeat.

800 SMOH

In troubleshooting I've done this:
* Sent the MT Governor out for overhaul.
* I have a Lycoming test plate for the governor attach point on the engine. Oil pressure at the main bearing is steady at 80. The prop when using differential pressure tester, injecting air, moves smoothly stop to stop. When we used air to set a random pitch, it seemingly holds it without bleeding down.
* Ran the engine without the governor (it's now a fixed pitch prop right?). Still hunting.
* Removed/Cleaned injectors. They sparkle. I even put them back in. :)
* Checked for induction air leaks at the cylinders and where the induction tubes mate to the oil sump. No blue. All hoses and bolts are fine. Bolt torques are still spot-on.
* Does it on L/R/both mags. Mag timing is spot on. (Left Mag is a Surefly E-mag).
* I have 12 Iridium Plugs. Starts like a late model car.
* Does it when the electric fuel pump is on/off. Does not make any difference.
* Manifold pressure behaves normal
* CHTs don't seem beyond the ordinary. EGTs expectedly move +/- a few degrees...b/c the engine is surging.
* I've explained this to a few Hartzell prop shops and they ALL insist that the prop is not the root cause in this case. I feel safe with this consensus to look elsewhere...

Where do I go next? Fuel servo/Flow divider?
Mechanical Fuel pump?
Blinker fluid?

I'm starting to feel that this experience is going to make me the master someday, but for now, I'm the pupil and I'm ready to take notes. Any takers?

Thanks in advance.

Signed,
Frustrated in Massachusetts
 
Good question.
We have used an optical tach to verify my electronic tach. So that’s been ruled out.
 
Dear Abby,

Where do I go next? Fuel servo/Flow divider?

Frustrated in Massachusetts

Dear Frustrated in MA,

Carefully pull the air side of the servo regulator and look for excessive blue staining/goo and clean up if found. In updraft installs of RSA servos there are a number of service bulletins that address contamination of the servos due to fuel making its way down the intake system from flooding and/or fuel leaking at shutdown.
 
My 540-c4b5 hunts approximately 50 rpms at WOT, 2700 rpms and full rich. It get worse as I climb if I don’t lean. It is just enough to notice in the seat of the pants. I have found that leaning it back a little smooths it right out. I suspect my servo is running a bit rich. I ran into the same problem on a very rich running fixed pitch 0360 last year. Leaning it out smoothed it out nicely. It required a jetting change.
 
Bubbles in fuel?

Mike

Is fuel pressure and flow steady when this occurs or are they fluctuating. There are many instances where people have changed the location of the fuel flow meter (red cube or other) out of the tunnel and solved similar problems.
Others have put a clear line in to test for the presence of bubbles in the fuel lines which could be indicative of a leak allowing air in.
A leak before the fuel pumps might not be obvious and actually allow air in rather than fuel out. Worth checking all connections.

Regards

Peter
 
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Dear Frustrated in MA,

Carefully pull the air side of the servo regulator and look for excessive blue staining/goo and clean up if found. In updraft installs of RSA servos there are a number of service bulletins that address contamination of the servos due to fuel making its way down the intake system from flooding and/or fuel leaking at shutdown.

Thank you Bob for taking the time to read all that and reply. If you have any references to this SB, I'd love to see it. I'll also search on my own for the same.
I'll post back what the resolution is so we can all learn from it.

Mike
 
Mike

Is fuel pressure and flow steady when this occurs or are they fluctuating. There are many instances where people have changed the location of the fuel flow meter (red cube or other) out of the tunnel and solved similar problems.
Others have put a clear line in to test for the presence of bubbles in the fuel lines which could be indicative of a leak allowing air in.
A leak before the fuel pumps might not be obvious and actually allow air in rather than fuel out. Worth checking all connections.

Regards

Peter

Thank you Peter.
Fuel pressure is steady, but it's lower than I think it should be. 7-8 PSI in cruise. Isn't it supposed to be 25-35 PSI?
I'll go through the fuel connections and ensure nothing is loose.
Hmmm...Now you have me thinking about my fuel selector valve. I don't have the nice one, I have the original one shipped by Vans.
Where do you find clear fuel line?

Thanks
Mike
 
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Sounds like an issue with your prop governor itself.

Thanks for you reply. But the governor has been eliminated. When we tested with the governor off, (Lycoming test plate installed in its place), the engine continued to hunt for RPMs even at idle. In this test situation without the governor installed, the prop is set against the stops. For all practical purposes, it's a fixed pitch. RPMs still hunting means it's something else.

Thanks,
Mike
 
You didn't mention the period of the surging. Every 10 seconds? 60 seconds?

I agree with the others that it sounds like a fuel delivery problem.

Another idea - have you tried running with your alternator turned off? I could see a scenario where the power requirements from the alternator fluctuate, pulling a bit of power from the engine, and then backing off.

Perhaps an easy way to check is to look at current draw and voltage out of the alternator, if you are logging this in your EFIS.
 
Mike

I’ve read about where people have used a clear piece of hose to check for bubbles. My guess is they’ve made a a fitting onto a clear hose. Absolutely for ground running only and best to record with a GoPro or such rather than being anywhere near a running propeller.

Regards

Peter
 
Frustrated in MA,

Not saying Bob, isn't right, but what prop do you have? Does this excursion happen suddenly then resolve or last for a few seconds? Do you have any panel data you could up load to Savvy?
 
No one has picked up on your fuel pressure report yet. When they do, you will get the advice you need, I think.

I would not fly with a FI system that was not making at least 20 psi with the boost pump on. I know from experience with mogas vapor lock in a fuel system with suboptimal aftermarket filters and elbows ahead of the boost pump (now remedied) that the engine will not run smoothly at fuel pressure below about 9 psi in a way that will demand your attention.
 
Thank you Peter.
Fuel pressure is steady, but it's lower than I think it should be. 7-8 PSI in cruise. Isn't it supposed to be 25-35 PSI?
I'll go through the fuel connections and ensure nothing is loose.
Hmmm...Now you have me thinking about my fuel selector valve. I don't have the nice one, I have the original one shipped by Vans.
Where do you find clear fuel line?

Thanks
Mike

I'd be surprised if was able to develop power if it really had that low a pressure, I believe 18psi in minimum for operation.
And yes standard FP is >25 psi.
 
Thank you Peter.
Fuel pressure is steady, but it's lower than I think it should be. 7-8 PSI in cruise. Isn't it supposed to be 25-35 PSI?

Thanks
Mike

This is way too low if it is an accurate reading. Pressure is supposed to be 25 PSI. From my research, the bendix servo can effectively deal with pressures down to about 12-13 PSI (quoted from Don back in the days he was active here). If you are truly at 8 PSI, I am confident that is your problem. That needs immediate verification and rectification before next flight.

I am guessing this is an indication error, as I don't believe the servo would reliably make full power at 7 PSI, but have no experience to back that up.

Also, have you observed the hunting via your tactile senses? You should be able to hear and feel a 100 RPM change and manifold pressure should not be stable/unchanged during that RPM fluctuation. IOW, are your sure the engine RPM is actually hunting and not just the indicated RPM hunting (i.e. problems with the indicators electronics)?

Larry
 
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They wont run on 7psi, but you have to get the low pressure indication ironed out first. Unlikely both pumps are not putting out pressure. Look for a loose fitting upstream of the boost pump, could be an air leak also. Blockage unlikely but possible.
 
Dear Frustrated in MA,

Carefully pull the air side of the servo regulator and look for excessive blue staining/goo and clean up if found. In updraft installs of RSA servos there are a number of service bulletins that address contamination of the servos due to fuel making its way down the intake system from flooding and/or fuel leaking at shutdown.

Hi Bob, Sorry to bother you for a clarification. I'm revisiting your post as I'm about to start digging into this today. When you said, "Pull the air side of the servo regulator", I'm not sure what that in particular means. I can surely remove the air filter assembly and look up inside the up-draft and venturi impact tubes. Are you meaning for me to remove some fasteners and take something apart on the fuel servo to perform this inspection?

Thanks!
Mike
 
This is way too low if it is an accurate reading. Pressure is supposed to be 25 PSI. From my research, the bendix servo can effectively deal with pressures down to about 12-13 PSI (quoted from Don back in the days he was active here). If you are truly at 8 PSI, I am confident that is your problem. That needs immediate verification and rectification before next flight.

I am guessing this is an indication error, as I don't believe the servo would reliably make full power at 7 PSI, but have no experience to back that up.

Also, have you observed the hunting via your tactile senses? You should be able to hear and feel a 100 RPM change and manifold pressure should not be stable/unchanged during that RPM fluctuation. IOW, are your sure the engine RPM is actually hunting and not just the indicated RPM hunting (i.e. problems with the indicators electronics)?

Larry

Hi Larry, The RPM hunting has been verified with an optical sensor. The sensor readings coming from my mag is very accurate. Despite that, I don't notice (feel/hear) surging.
 
No one has picked up on your fuel pressure report yet. When they do, you will get the advice you need, I think.

I would not fly with a FI system that was not making at least 20 psi with the boost pump on. I know from experience with mogas vapor lock in a fuel system with suboptimal aftermarket filters and elbows ahead of the boost pump (now remedied) that the engine will not run smoothly at fuel pressure below about 9 psi in a way that will demand your attention.

Thank you Bill. I will focus on troubleshooting here next. I will advise this forum topic with the outcome.
 
Frustrated in MA,

Not saying Bob, isn't right, but what prop do you have? Does this excursion happen suddenly then resolve or last for a few seconds? Do you have any panel data you could up load to Savvy?

I have never heard of Savvy, but I do have some video of the problem.
I have a hartzell CS prop.
 
You didn't mention the period of the surging. Every 10 seconds? 60 seconds?

I agree with the others that it sounds like a fuel delivery problem.

Another idea - have you tried running with your alternator turned off? I could see a scenario where the power requirements from the alternator fluctuate, pulling a bit of power from the engine, and then backing off.

Perhaps an easy way to check is to look at current draw and voltage out of the alternator, if you are logging this in your EFIS.

Good question Re: alternator! I'll check that. I don't think it is that because current draw and voltage are steady, but very easy to check.
My elderly TruTrak EFIS does not have a recording function.
 
Have you validated the RPM with an optical tach device?

Hello! Yes. Two different ones in fact. A battery operated portable optical tach and secondly the one that comes with our prop-balance kit.
All are in agreement.

Thank you
Mike
 
Mike

Is fuel pressure and flow steady when this occurs or are they fluctuating. There are many instances where people have changed the location of the fuel flow meter (red cube or other) out of the tunnel and solved similar problems.
Others have put a clear line in to test for the presence of bubbles in the fuel lines which could be indicative of a leak allowing air in.
A leak before the fuel pumps might not be obvious and actually allow air in rather than fuel out. Worth checking all connections.

Regards

Peter

Hi Peter, over this past weekend, I disconnected the fuel servo hose at the distribution spider end. I put a clear plastic tube in-line and ran the engine. The air that was introduced by my breaking the system apart was immediately sucked down and distributed just by running the boost pump. Now there are 0 bubbles in the line at all. Then I started the engine. I ran for a good two minutes at varying power settings. There were no new bubbles. I was sure hoping for bubbles as this would be an easy fix.
Now with the low FP pressure readings that I'm seeing, and by others posting about how it seems rather "off", I will focus more sharply on that.

Thank you!
Mike
 
I have never heard of Savvy, but I do have some video of the problem.
I have a hartzell CS prop.

Savvy Aviation is a service founded by Mike Busch. You can upload (free) your EFIS data files and plot for analysis and share with others for help. I am pretty good with Excel, but highly recommend this (free) as it is very fast and does everything you want. You could plot your fuel pressure against RPM to see what is really happening. Others on this site are pretty good, the best if I dare say, at doing analysis of this data and would increase the quality of assistance 100 fold.

Just a suggestion.
 
Now with the low FP pressure readings that I'm seeing, and by others posting about how it seems rather "off", I will focus more sharply on that.

Thank you!
Mike

You definitely need to start there, as it is not just "off," It is well below the minimum required. Best bet is to buy a cheap pressure gauge and the hardware store and plumb it into the fuel pressure line temporarily with the sensor and confirm your ACTUAL fuel pressure. That will quickly rule in or out FP as a source of your hunting.

Larry
 
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