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Hot Idle Instability / Micro-Surging

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TheNewGuy

Well Known Member
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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
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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.
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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
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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!
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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
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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.
 
TheNewGuy said:
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!”
Click to expand...
We did all the above thread suggestions, all the improvements we could find. In the end, we helped the situation but didn't fully resolve it. "normal, go fly" is pretty much where we wound up.

Edit: we sat on the flight line at KOSH and listened to a lot of RVs taxi by. Many of them had rough idles on the ground that sounded just like ours. I think in the end its a common outcome of our tightly cowled FI engines with a lot of variability under the cowl.
 
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It's comforting to know im not the only one. the builder said he never had any issues so I'll do some investigating. Ordered the remaining EGT probes for cylinder analysis. Will have a look at the induction system and see how the hoses are clamped or if anything is loose in general. If I find nothing that stands out, I'll live with it and enjoy every second
 
Not knowing your builder or tossing any shade, but I think a lot of people aren't in tune enough with their engine's operation to even notice the roughness.
 
@TheNewGuy What does a rough idle sound like to you? Here are some examples:

"mo-money, mo-money, mo-money, mo-money" --- Perfectly normal, healthy, well adjusted Lycoming/clone
"mo-mo-money, mo-mo-money, mo-mo-money, money, money, mo-mo-money" --- Lycoming with a Magneto/capacitor problem (happens when hot.)
"whizzzzzz, burp, whizzzzz, burp, whizzzzzz, burp" --- Perfectly normal Continental 6 Cylinder in a Cirrus at 2000 RPM "idle"
 
bjdecker said:
@TheNewGuy What does a rough idle sound like to you? Here are some examples:

"mo-money, mo-money, mo-money, mo-money" --- Perfectly normal, healthy, well adjusted Lycoming/clone
"mo-mo-money, mo-mo-money, mo-mo-money, money, money, mo-mo-money" --- Lycoming with a Magneto/capacitor problem (happens when hot.)
"whizzzzzz, burp, whizzzzz, burp, whizzzzzz, burp" --- Perfectly normal Continental 6 Cylinder in a Cirrus at 2000 RPM "idle"
Click to expand...
Ha! Haven’t heard of that. Mo-money
 
georgemohr said:
Not knowing your builder or tossing any shade, but I think a lot of people aren't in tune enough with their engine's operation to even notice the roughness.
Click to expand...
I am very in tune with my engine and don’t get significant -roughness or surging, at idle when my engine is hot, except for after a heat soak and that is just somewhat rough; no surging. Yes, things get a little rough sitting in line for 30 minutes at osh. It has been my experience that many engines have poorly adjusted idle mixture.
 
Breezy said:
Yes, that is the correct document.
The faltering in my engine didn't start until about 100 hrs ago, have just over 300 hrs now. Tried injector cleaning and Throttle Body adjustment with no change. Also, mixture leaning, boost pump to increase fuel pressure, no change. When I saw the above Doc, figured to wait and see what responses indicated.
Click to expand...
...Because it isn't fuel related...
 
Last year My new IO-360 powered Bearhawk Patrol had hot fuel issues that cropped up when the spring temps warmed up. I overcame those issues using temp data to guide remedies.

I outfitted my cowl and airframe with 9 thermocouples to understand what was happening and help to troubleshoot things.

Here is a thread I started last "Season" that might be worth reading. https://vansairforce.net/threads/hot-fuel-issue.234366/

Here are some things I found.
  • Cowl temps rise quit fast immediately after landing.
  • They peak at about 12 minutes after shutdown.
  • Fuel boils at about 140F.
  • The slower the fuel moves through a line the more time it has to heat up, so ...
  • Taxi: Fuel Heats during Taxi after landing; idle fuel flow slows fuel speed inside the fuel lines giving it time to heat up.
  • Thermocouples and a thermocouple reader are very inexpensive and builds a real picture using real data of whats going on.
  • my fuel heated up about 10F before ever entering the hot side of the firewall due to heat under the floor from the exhaust tunnel area.
  • Two heat boxes dumped twice as much heat into my cowl and may have cylced around in the aft area raising the cowl temps in the area of the fuel lines.
  • Extracting that hot air with intent helped..meaning that hot air now gets ducted out of the cowl exit now.
  • a cowl lip improved cowl pressure differential by 30%,
  • Shortening all fuel lines, and inuslating them with double wall of fire sleeve made a large improvement,
  • a fuel line with the higher quality integral fire sleeve is a poor insulator but the Non-Integrated fire sleeve is a very good insulator.
  • the engine driven fuel pump is a large source of heat for fuel, and so I like my fuel pump cooling shroud.
 
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@TheNewGuy - I recently had a similar problem in my RV-8 with a IO-360. It was idling rough and the RPM would go up and down constantly (surging) by 50-75 RPM. Runup @1700 was normal with RPM drops and EGT rise as expected. However, in Idle - its just wasn't normal - as the engine had always run super smooth previously.

My A&P and I chased this issue for a week+ thinking the Idle adjustment was off or some other cable was not set right. We tried everything and nothing seemed to fix it. Finally, we started chasing fuel lines thinking there might be an air leak somwhere - and sure enough - the gasket in the Gascolator bowl was found to be severed and we were getting a little bit of a leak there. We replaced the gasket and the engine has been running as smooth as can be. The theory was it was sucking in air causing the roughness at idle and the micro surging.

So the point is, if you're still having issues - check some obvious places where there might be a fuel or air leak in the lines, fittings, etc. It might not be heat-related.
 
Notso said:
@TheNewGuy - I recently had a similar problem in my RV-8 with a IO-360. It was idling rough and the RPM would go up and down constantly (surging) by 50-75 RPM. Runup @1700 was normal with RPM drops and EGT rise as expected. However, in Idle - its just wasn't normal - as the engine had always run super smooth previously.

My A&P and I chased this issue for a week+ thinking the Idle adjustment was off or some other cable was not set right. We tried everything and nothing seemed to fix it. Finally, we started chasing fuel lines thinking there might be an air leak somwhere - and sure enough - the gasket in the Gascolator bowl was found to be severed and we were getting a little bit of a leak there. We replaced the gasket and the engine has been running as smooth as can be. The theory was it was sucking in air causing the roughness at idle and the micro surging.

So the point is, if you're still having issues - check some obvious places where there might be a fuel or air leak in the lines, fittings, etc. It might not be heat-related.
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Thanks, Notso. Did the surge begin only when the engine was warm/hot in your case or at idle most of the time? I’ll have a look. I’m replacing the intake hoses and gaskets tomorrow but I’m not sure that will fix it in my case
 
TheNewGuy said:
Thanks, Notso. Did the surge begin only when the engine was warm/hot in your case or at idle most of the time? I’ll have a look. I’m replacing the intake hoses and gaskets tomorrow but I’m not sure that will fix it in my case
Click to expand...
No, it was present right at start/cold. And persisted through a warmed up engine.
 
I was also told by my A&P that potentially if you have an air inlet leak in your air inlet pipes where they meet the engine case, that could potentially cause the problem too.
 
TheNewGuy said:
Thanks, Notso. Did the surge begin only when the engine was warm/hot in your case or at idle most of the time? I’ll have a look. I’m replacing the intake hoses and gaskets tomorrow but I’m not sure that will fix it in my case
Click to expand...
things that lean the idle mixture, such as intake leaks and air introduction in the fuel system, can cause surging from the lean condition, but generally present at all times, hot or cold, and usually a bit worse when cold. your symptoms are only when hot. Not saying they can't be the problem, but you are generally looking for things that affect idle when hot. in that case vapor in fuel can be an issue, but more likely from boiling than being pulled in via a leak. you are looking for things that will affect fuel mixture or ignition timing only when hot; those are usually tougher to find. Surging is most typically from a lean mixture, but that is NOT universal.
 
lr172 said:
things that lean the idle mixture, such as intake leaks and air introduction in the fuel system, can cause surging from the lean condition, but generally present at all times, hot or cold, and usually a bit worse when cold. your symptoms are only when hot. Not saying they can't be the problem, but you are generally looking for things that affect idle when hot. in that case vapor in fuel can be an issue, but more likely from boiling than being pulled in via a leak. you are looking for things that will affect fuel mixture or ignition timing only when hot; those are usually tougher to find. Surging is most typically from a lean mixture, but that is NOT universal.
Click to expand...
Lean mixture (incl induction leaks) tends to create a rythmic surging. Bubbles in the fuel tend to create surging that may be constant, but the frequency is more random or hap hazard. the bubbles reduce the amount of fuel going to a cyl, but their quantity is always changing.
 
Like some have said, I could end up chasing my tail forever. I’m replacing my unstable gaskets and hoses today. Perhaps that’s contributing although I wouldn’t place too much stock on it under these circumstances. Either way they need to be replaced. Also adding EGT probes today.
 
In post #1 you suspect its a heat issue. I think your right as I had the same symptoms. When you are ready to investigate heat related data you will want to put something like digital meat thermometers all over your engine compartment.....A meat thermometer is a thermocouple.

Amazon has 5 thermocouples 3M long for $14.
A good K type thermocouple reader with four channels is $27

The thermocouples wires are fradgil at the tips, so I use Aluminum tape to fix them in place. Consider placing them on fuel line fittings at strategic locations like the firewall fitting, the fuel pump outlet, the servo outlet, and the flow divider inlet. I ran the wires out of the cowl and taped them with packaging tape to my boot cowl. You might consider running them through your heat box too.

Ugly is OK In fact Ugly and quick install is an industry best practice...you just need data. Thats all. Spend 45 minutes installing them. Move them around after flying and keep getting data. I even used them to test the insulation value of Firesleeve material...put a line in my turkey roaster at 250F and watched to see how fast the temperature rose.
 
You've already gotten some good advice here, I'll add my 2 cents. The leak down check with soapy water is easy, When I got my 8 last year (IO-360A) I ended up replacing a few intake gaskets, and then did the rest at condition inspection. I also had to set up the idle mixture, I got it very close and have been flying it, it's probably 1 click rich, but I just lean aggressively on the ground.

It idles very nicely once warmed up, and even on taxi in it's great. But if you shut down and then restart, it surges and doesn't like to idle smoothly, it's worse below 1000 RPM.

My educated guess is that the fuel spider lines on top of the cylinders get heat soaked and they are not going to cool down until there's significant fuel flow. If your cylinders are 300 degrees and fuel boils at 140 (something new I learned from an above post) the spider line is right on top of the cylinders.

My airplane tends to run cool with cylinder temps barely over 300 no matter what I seem to do, so my issue may not be as bad as someone who's at 400. And that's in South Florida where we're in the 90s every day (temperature wise, no Miami Vice jokes please).

When you park and you know you have to startup within 30 min, try popping the oil dipstick door and parking into the wind if you can. You'll be surprised at how much hot air comes out of that little oven we call a cowl! Seems to help.

My brother just got a cirrus, that thing will simply not run without the boost pump once the cylinders are up to temp, unless you are in cruise. Scared us a few times, especially during our 15 mile straight in GPS aproaches to non towered airports with 5 guys in the pattern. (Just kidding).
 
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