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Hot Idle Instability / Micro-Surging

T

TheNewGuy

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I have a Lycoming IO-360-B1F (180 hp) in my -8 and I’m noticing brief micro-surging at idle during taxi. The issue only occurs when the engine is hot. Cold starts and initial taxi/idle are normal. I tried using the electric fuel pump while taxiing to see if it would help with fuel vapor or pressure, but it didn’t make a noticeable difference. Mixture adjustments on the ground also don’t seem to significantly affect it.

CHTs are normal. I currently only have one EGT probe installed, so I don’t have full cylinder comparison data. I’ve had a small induction leak before on an O-360, but that case didn’t present with this same surging behavior, so I’m not sure if that’s likely here. A local pilot also commented that the idle sounded slightly unstable while taxiing.

Any ideas on likely causes, such as induction leak, idle mixture setting, fuel servo behavior, or vapor-related issues?

-Ian
 
Same problem with in RV8 with the IO-360-M1B engine. Other diagnosed it to the hot fuel line. By the time I taxi from parking to the end of the runway to perform run-up, the surging went away as if all the hot fuel was used up during the startup and taxi.
This unstable idle only occurred after a flight and shutdown when the engine was soaking hot. So instead of keeping idle at 800-900 rpm, I keep it at 1100-1200 rpm.

Edit: to expand on my experience with surging idle. The first time I experienced this phenomena was during the hot months in Socal the OAT was in the mid 90s. I did all my phase 1 testing during the cool weather month when the temperature rarely went past 70degF. This year I was starting to see a bit of surging when the local temperature edged past 80degF. The surging idle even only occurred if I restarted the engine after less than 1 hour after shutdown. If I let the plane sit in the hot sun for many hours, over 90degF, I didn't experience any surging. It only happened after the engine was thoroughly heat soak from the actual flying in hot weather and the engine restart occurred shortly after shutdown.
 
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TheNewGuy said:
I have a Lycoming IO-360-B1F (180 hp) in my -8 and I’m noticing brief micro-surging at idle during taxi. The issue only occurs when the engine is hot. Cold starts and initial taxi/idle are normal. I tried using the electric fuel pump while taxiing to see if it would help with fuel vapor or pressure, but it didn’t make a noticeable difference. Mixture adjustments on the ground also don’t seem to significantly affect it.

CHTs are normal. I currently only have one EGT probe installed, so I don’t have full cylinder comparison data. I’ve had a small induction leak before on an O-360, but that case didn’t present with this same surging behavior, so I’m not sure if that’s likely here. A local pilot also commented that the idle sounded slightly unstable while taxiing.

Any ideas on likely causes, such as induction leak, idle mixture setting, fuel servo behavior, or vapor-related issues?

-Ian
Click to expand...
There are three things needed for combustion -- fuel, air, spark. You didn't mention looking at the health of the ignition system (Magnetos/CDI/EI, Harness, Plugs)...
 
bjdecker said:
There are three things needed for combustion -- fuel, air, spark. You didn't mention looking at the health of the ignition system (Magnetos/CDI/EI, Harness, Plugs)...
Click to expand...
Mag drop (dual slicks) is around 100RPM and within 20RPM of each other. Ignition system looks fine overall but I could do another once over. The plugs were fouled from the prior owner, but we cleaned them up nicely
 
I have this on my IO-320-D1A. In my case, it is hot fuel.

Don't have it fully resolved in all situations, but mostly. Fuel line routing and heat shield made a big difference. Changing the spring in the spider to raise pressure in the servo-to-spider line made a small difference. Changing injector restrictors to raise spider-to-injector pressure made a big difference. Adding louvers to the cowl made the biggest difference.
 
noahhl said:
I have this on my IO-320-D1A. In my case, it is hot fuel.

Don't have it fully resolved in all situations, but mostly. Fuel line routing and heat shield made a big difference. Changing the spring in the spider to raise pressure in the servo-to-spider line made a small difference. Changing injector restrictors to raise spider-to-injector pressure made a big difference. Adding louvers to the cowl made the biggest difference.
Click to expand...
Me too. I'm told that it can be exacerbated by my composite prop.
 
TheNewGuy said:
Mag drop (dual slicks) is around 100RPM and within 20RPM of each other. Ignition system looks fine overall but I could do another once over. The plugs were fouled from the prior owner, but we cleaned them up nicely
Click to expand...
When you do the run up are you just looking at the RPM drop - What do the EGT’s do?

If you have a wonky ignition, you will see one or more EGTs fall when running on one ignition. Also, run up at higher RPM than “normal” to stress the system further.
 
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I have the same issue on my IO-320. The fuel line runs from the servo between the sump and the crossover exhaust before heading the the divider. I have a heat deflector on the exhaust but in the summer months, the engine gets fussy on the ground after it's been up in the air once. I keep the electric fuel pump on during these times and try and keep my RPMs at 1000 to make things better. It does make it run better but it still surges at times.

I have not added cowl vents yet, but that is an option with many stating it helped. I recall seeing another thread on placing fiberfrax between the heat shield and the fuel line but haven't tried that yet.
 
TheNewGuy said:
I have a Lycoming IO-360-B1F (180 hp) in my -8 and I’m noticing brief micro-surging at idle during taxi. The issue only occurs when the engine is hot. Cold starts and initial taxi/idle are normal. I tried using the electric fuel pump while taxiing to see if it would help with fuel vapor or pressure, but it didn’t make a noticeable difference. Mixture adjustments on the ground also don’t seem to significantly affect it.

CHTs are normal. I currently only have one EGT probe installed, so I don’t have full cylinder comparison data. I’ve had a small induction leak before on an O-360, but that case didn’t present with this same surging behavior, so I’m not sure if that’s likely here. A local pilot also commented that the idle sounded slightly unstable while taxiing.

Any ideas on likely causes, such as induction leak, idle mixture setting, fuel servo behavior, or vapor-related issues?

-Ian
Click to expand...
Ian,

A few months back the FAA sent out a reliability survey on this very issue. Focused on Injected engines. Since you mention a fuel servo in your post I am assuming yours in Injected.

It describes your symptoms: Start up operation and taxi are normal, with engine falter after power reduction to idle during post flight taxi. I have this same issue and from this thread seems there are quite a number of others. The focus is on the injection system, particularly Throttle Body function.

If I can relocate the survey I will post reference info.

Cheers!
 
Breezy said:
Ian,

A few months back the FAA sent out a reliability survey on this very issue. Focused on Injected engines. Since you mention a fuel servo in your post I am assuming yours in Injected.

It describes your symptoms: Start up operation and taxi are normal, with engine falter after power reduction to idle during post flight taxi. I have this same issue and from this thread seems there are quite a number of others. The focus is on the injection system, particularly Throttle Body function.

If I can relocate the survey I will post reference info.

Cheers!
Click to expand...
Thanks! I talked with the builder and he said he never had the issue I’m describing. The engine was IRAN’d after some corrosion was found. It’s possible they didn’t replace the hoses and intake gaskets and they are ready for replacement. Hopefully it’s something simple
 
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TheNewGuy said:
Thanks! I talked with the builder and he said he never had the issue I’m describing. The engine was IRAN’d after some corrosion was found. It’s possible they didn’t replace the hoses and intake gaskets and they are ready for replacement. Hopefully it’s something simple
Click to expand...
The symptoms you are describing are almost always the result of vapor lock. Or “hot fuel” or whatever other name is used. Especially in a tightly cowled non-certificated airplane like an RV. Fixing it could be simple or require multiple changes. Best to get someone involved that really understands this problem instead of going with suggestions from the internet. The problem goes away with a high power setting bc of increased fuel flow and the resulting cooling of that fuel. If it ever happens whilst airborne, you’ve entered an area of much greater concern. Good luck.
 
Same issue with my 180hp I0-360 M1B. Precision servo, Silck and Surefly, new plugs and wires. Starts great cold or hot. After a hot start if the oil temp is 190+ (haven't seen over 210) it begins to surge in about 20/30 sec. Better at around 1200 rpm but still not smooth. No surging or issues at high power or in cruise. Double fire sleeve and heat shields wherever possible FWF. CHT's are all ok, not above 360 in cruise. I have a gascolator on the firewall, which is insulated, as required in Canada. I have a 9 row oil cooler but still can't keep the oil temp to 185. It always rises to 195 ish as I slow down entering the cct and if hot out will be about 210 by the time I shut down.
 
Cold engines need richer mixture than hot engines. Cold ambients need more fuel than warm. Surging is more often caused by lean conditions, but can be caused by excessive richness. Because it is good cold and bad hot, i would take the time to properly set your idle mixture, on the chance it is too rich.

Rough running can come from hot fuel, but simple to distinguish. If it only happens after start up after a heat soak, that is likely hot fuel. If you just landed and the problem occurs on the taxi, that is not likely hot fuel.

Either way, properly adjusting idle mixture is always a worthwhile endeavor. Most engine guys crank the idle mixture wayyyy rich when they rebuild and test run. Very possible this happened on your iran and why the problem just appeared and the builder never experienced it.
 
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TheNewGuy said:
It is similar
Click to expand...
If it sounds like that, I would also look for any issues in any EI you have. changing ignition advance will strongly impact RPM at idle. If there are issues with the EI's MAP sensor, it could be swinging the advance up and down and that would create a very smooth surging likr in that video vs a more classic rougher surging seen with mixture issues. Very tough to help when we can't hear the issue.
 
lr172 said:
If it sounds like that, I would also look for any issues in any EI you have. changing ignition advance will strongly impact RPM at idle. If there are issues with the EI's MAP sensor, it could be swinging the advance up and down and that would create a very smooth surging likr in that video vs a more classic rougher surging seen with mixture issues. Very tough to help when we can't hear the issue.
Click to expand...
I keep forgetting to video it. I have conventional slick mags however. I spoke with an RV builder at the airport today and he shook his Head and said “normal. I wouldn’t worry about it. Go fly!”
 
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TheNewGuy said:
said “normal. I wouldn’t worry about it. Go fly!”
Click to expand...
🙄

I watched the video. it was helpful in giving an audio representation of the symptom but misleading in so many other ways. Especially in this world of learning by trial and error (when’s the last time a new purchase included an instructional manual?) Learning by trial and error and comments such as the one above by an amateur have no place in aviation.
 
rv8ch said:
Is this what you are seeing?
Or is it "rough" running?
Click to expand...
Not my briar patch but I have a hard time seeing the relevance. The surge in the video appears to be related to the fact that the structure supporting the intake/throttle plate isn't very rigid thus moves relative to the engine. (To a smaller degree?) the TP linkage position in 3D space moves relative to the frame from torque reactive loads. As the reactive torque moves the physical position of engine and engine relative to the intake, the associated linkage stays in one place. The increase in power -> reactive torque -> relative position change -> change in throttle plate position -> lather/rinse/repeat.

If the person in the video disconnected the linkage, would it still idle surge?

Likewise if the linkage was similarly mounted but on the other side of the engine, would it result in an RPM dip?

No way to prove but I'd bet the OPs reported idle surge is caused by a fuel quality issue (temperature).

If the root cause is ever determined, let us know. Go fly. Have fun. I'm logging zero hours these past several years.
 
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