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FI Engine Surge at Idle

Taildrgr

Well Known Member
I have searched and found threads dealing with rpm surging at full power settings, but my surging issue is at idle speeds.

I have a new Lyc IO-360-M1B, Hartzell CS C2YR-1BFP/F74972 prop, and MT P-860-4 Rev G governor from Van's, all with 32 hours time.

I set my prop pitch screw for max static rpm of 2690 rpm. I have not touched the governor.

The rpm is rock solid while flying.

At about 5 hours into phase I testing, my idle/taxi rpm started surging about 200 rpm after I stopped at the fuel pump after a flight. When I restarted up and taxied to my hangar, the engine was surging. I thought it was vapor lock (I only use 100LL). 90 degr day in Texas.

Days later, on the next flight, the engine was still surging on initial startup (cold) and did not stop when the engine warmed up or taxiing to the runup area. The prop cycled normally at 1700 rpm, so I took off and stayed over the airport, landed, and the surging was gone. It did not surge again for 27 hours so I thought it was just an initial break in issue.

Yesterday, at 32 hours total time, after a 2 hour flight, I landed, got fuel, and again, on startup it was surging. It was a cool day at 54 degr OAT but the engine was hot. Still 100LL. I have not restarted the engine since yesterday so I don't know if it will continue to surge when cold.

On the ground, changing rpm from 650 to 1200, changing the mixture from full rich to lean, and turning on the boost pump, all have no effect. I have not tried to move the prop control because I thought the governor has no effect at idle speeds. I also checked my prop governor control linkage and it is solid. The lever is on the stop screw with the prop control out 1/8 inch from the panel.

So, why would a fuel injected engine surge at idle? Hot or cold. Is it the prop or a fuel flow issue?

Any suggestions on what to investigate would be appreciated.
 
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Have you checked your idle mixture. A lean mixture can cause surging. Mixture knob cannot enrichen idle mixture beyond its setting, regardless of its position.
 
Have you checked your idle mixture. A lean mixture can cause surging. Mixture knob cannot enrichen idle mixture beyond its setting, regardless of its position.

I have adjusted the idle mixture to give a 50 rpm rise on shutdown.
I lean while taxiing, but the surging exists at full rich and my leaned taxi setting, too.

Thanks for the reply.
 
Mine does the same.

Just out of interest what sort of Mags do you have? My engine has dual Mags and before I installed a restrictor in the MAP line it idled quite rough, hunting up and down 50 to 100 RPM, when I tried to idle below 800 RPM. The restrictor improved this quite a bit. Even so when I restart the engine after a brief shutdown my fuel pressure sometimes fluctuates and unless I turn the boost pump on the idle is rough.
Cheers
Nigel
 
what kind of fuel injection system do you have? I am having the same issue and can't figure it out.
 
Many Lycoming engines with mechanical fuel injection (i.e. Bendix, Airflow Performance) do this when the engine is hot, as the fuel boils in the fuel injection lines above the engine when the fuel flow is low. This causes uneven fuel flow and rpm variations. The surging stops when you increase power, because the higher fuel flow requires higher pressure in the lines, and the boiling point of the fuel is now higher than the actual temperature.

Switching to slightly smaller injection nozzles (sold by Airflow Performance) increases the pressure in the fuel injection lines, which increases the boiling point of the fuel. But, the smaller nozzles pass less fuel at full rich, so there is a risk of being too lean at high power. I've chosen to live with the issue, as I don't want to risk being too lean at full power.
 
Normal behaviour.

Ours does it all the time after fuelling - the area around the engine gets hot very quickly after the taxi in and once you restart, there are vapour bubbles in the lines which take a while to work there way through.

If you have a full return fuel system, you can run the electric pump with the mixture in ICO - we used to do this on big Cessnas after fuelling when we were skydiving.

IO-360 180hp (ECI)
Silverhawk Injection
Dual P-Mags
MT prop and governor.
 
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Normal behaviour.

Ours does it all the time after fuelling - the area around the engine gets hot very quickly after the taxi in and once you restart, there are vapour bubbles in the lines which take a while to work there way through.

If you have a full return fuel system, you can run the electric pump with the mixture in ICO - we used to do this on big Cessnas after fuelling when we were skydiving.

IO-360 180hp (ECI)
Silverhawk Injection
Dual P-Mags
MT prop and governor.

It may be normal behavior or the controller may need recalibration by a service center. I had an intermittent problem with AFP at idle, sent it back to have it checked, they found nothing wrong but it has worked normal ever since.

Only explanation was it may have had some contamination within the unit that got cleared out during service. It was a new unit with very few hours.
 
Given that this is a new installation, I would try idling with the boost pump on to see if anything changes (rule out problem with mech pump). I would also be looking for a loose connection on the suction side (before mech. pump). Loose connections can introduce air bubbles, which would be more noticeable at idle. You may find slight leaks (blue staining) at the loose connections.

Larry
 
FYI - during testing of a 10 fuel system last week, we were pumping 12gph with a variable restrictor before the spider and saw bubbles in the clear discharge line. We added a clear section before the restrictor to see if it was suction. It was not. Ambient was 59F. 100LL. Boost pressure was 29 psi.

I can now see that a restriction dropping the fuel pressure rapidly can induce cavitation bubbles even in cold fuel.

If my (new) M1B did this . . . after reviewing VAF, and the manual, I would exercise my fingers and call Vans/Lycoming Support to discuss the possibilities with a list of the conditions and things checked/tried. I have gotten very good response from them on other questions so far. YMMV
 
Given that this is a new installation, I would try idling with the boost pump on to see if anything changes (rule out problem with mech pump). I would also be looking for a loose connection on the suction side (before mech. pump). Loose connections can introduce air bubbles, which would be more noticeable at idle. You may find slight leaks (blue staining) at the loose connections.

Larry

There are a lot of fuel line connections on the suction side of an FI system. Any leak will introduce air bubbles into the system. Usually, this is not noticeable at high power/high fuel flows but is at idle power. These air bubbles expand rapidly when heated, so a hot engine makes it worse. This is in addition to fuel vapour bubbles caused at idle in a hot engine.

You won't see large blue fuel stains, but if you wipe all the fittings with a shop towel and notice any stains on the towel, you have a leak.

In Canada, we must have gascolators, even in FI systems, and I have found that they can be (are always) an air leak problem.

It can take several hours of operation to find the leaks, so good luck.
 
FYI - during testing of a 10 fuel system last week, we were pumping 12gph with a variable restrictor before the spider and saw bubbles in the clear discharge line. We added a clear section before the restrictor to see if it was suction. It was not. Ambient was 59F. 100LL. Boost pressure was 29 psi.

I can now see that a restriction dropping the fuel pressure rapidly can induce cavitation bubbles even in cold fuel.

Bill,

I am curious what your resolution was to this? Or is it just an accepted flaw of the mechanical FI system.

Larry
 
Normal behaviour. Ours does it all the time after fuelling - the area around the engine gets hot very quickly after the taxi in and once you restart, there are vapour bubbles in the lines which take a while to work there way through.

NOT normal behavior. Original post says engine was still surging on intial start up (cold) and did not stop after warmed up or on taxing to run up area. Fuel will not be vaporizing in injection lines under these conditions. Sounds more like there is a leak somewhere letting air into the fuel system.

erich
 
Bill,

I am curious what your resolution was to this? Or is it just an accepted flaw of the mechanical FI system.

Larry

Larry, there is no problem here.

This was a test to measure unusable fuel in each tank, and at different attitudes. The servo was bypassed with a coupling, the spider inlet hose was removed and connected to a tube with a ball valve and then a length of clear vinyl tubing to allow it to flow into a 1 gallon jug.

The boost pump was pumping fuel through the full system, i.e. the mechanical fuel pump and the red cube (between the servo and spider).

The test was conducted at 59F. The boost pump was turned on, the valve opened until it ran clear, then the valve shut down to either cruise or TO fuel flow to check pressure at flow conditions.

We noticed that the clear tube was discharging bubbles when restricted to 12 gph. The EFIS was reading 29 psi. The entire system had already been pressure tested with air at 30 psi and being left overnight to see if there was a drop. Issues were repaired until it held w/o drop overnight. Something could still have surprised us, so the tubing upstream of the ball valve was separated and a 6" length of clear vinyl was installed. 8 more test conditions were run and each time after the bubbles cleared from upstream with full flow, the flow dropped to 12 or 24 gph, the bubbles were still being generated through the restriction of the ball valve.

So, there was not problem, just learned that if the pressure is dropped from 30 to zero via a turbulent orifice, then some cavitation bubbles are generated. I don't know what the pressure drops are though the servo, spider, and injectors, but one would have to think that gas is generated somewhere. It would be best if it were after the spider or the metering function could be compromised.

It probably has nothing to do with the issue here unless the spider is having an issue, or there is some leak in the system sucking air. In order to ensure that air is not being pulled in between the tank and the mechanical pump, a systems test (as described above) would be in order. The OP did not indicate any test (like above) was performed on the system prior to first flight. I will be running a full fuel systems test on my 7 before first flight.

NOT normal behavior. Original post says engine was still surging on intial start up (cold) and did not stop after warmed up or on taxing to run up area. Fuel will not be vaporizing in injection lines under these conditions. Sounds more like there is a leak somewhere letting air into the fuel system.

erich
Agree.
 
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Thanks for some ideas guys. I will check again for leaks. I has put a wrench on all the fittings at my last inspection 7 hours ago but I will inspect the lines and connections for leaks too. Have not seen stains anywhere.

Another thing I will check is the inlet fuel strainer on the servo. I have checked the filter at the boost pump but it is always clean.

BUT one thing is interesting, when the plane sits after a flight, I get about a 3 inch diameter fuel stain on the ground under my sniffle valve drain after about 24 hours. I thought it was normal but apparently some people don't get any leaks from the sniffle valve. Could this be a related issue?

FYI, my red cube is hard mounted to the firewall at a low point between the mechanical fuel pump and the servo.

The servo is an Avstar AVX-5VA1 unit that came with the engine from Vans.
Mags are Slick 66GC25SFNN on left and 66GP-0SANN on right also came with engine.

Again, thanks for taking time to reply.
 
Other things to check:
- Intake leak. Make sure all your intake gaskets are in place and connections tight.
- Intake leak. Take out the sniffle valve and replace with a bolt to see if the problem goes away.
- Intake leak. Watch EGTs at 1000 rpm or so as you lean. Is one cylinder rising faster than the other?
- Spider issue. Had this recently happen. Do a fuel flow test with a shot glass to collect fuel at each injector (line from the spider disconnected at the injector and fuel directed to the shot glass). The spider has very small bypass holes for fuel flow at idle. If one is blocked this can look like a lean cylinder. This will also show up during the idle lean as you watch EGTs.

Carl
 
Other things to check:
- Intake leak. Make sure all your intake gaskets are in place and connections tight.
- Intake leak. Take out the sniffle valve and replace with a bolt to see if the problem goes away.
- Intake leak. Watch EGTs at 1000 rpm or so as you lean. Is one cylinder rising faster than the other?
- Spider issue. Had this recently happen. Do a fuel flow test with a shot glass to collect fuel at each injector (line from the spider disconnected at the injector and fuel directed to the shot glass). The spider has very small bypass holes for fuel flow at idle. If one is blocked this can look like a lean cylinder. This will also show up during the idle lean as you watch EGTs.

Carl

Thanks, Carl. I will check the spider too.
 
Carl,
I have done 3 GAMI tests while flying at 75% and 65% power. All had a GAMI range of 0.4 gpm.

Would this test show the spider and injectors are ok?

Also, all my EGT's at cruise are within 50 degr F when leaned to 100 ROP = 1300 degr to 1350 Degr. Probes are 2 inches from exhaust flange.

Max EGT spread at idle is 100 degr. Lowest is 900 degr and highest is 1000 degr.
 
NOT normal behavior. Original post says engine was still surging on intial start up (cold) and did not stop after warmed up or on taxing to run up area. Fuel will not be vaporizing in injection lines under these conditions. Sounds more like there is a leak somewhere letting air into the fuel system.

erich

Erich,

Sorry, the cold bit was buried in the text ;)

Hot it will surge, whatever you do.

If the cold surging re occurs, I would investigate. If it is only hot, I would not.

Mike
 
Carl,
I have done 3 GAMI tests while flying at 75% and 65% power. All had a GAMI range of 0.4 gpm.

Would this test show the spider and injectors are ok?

Also, all my EGT's at cruise are within 50 degr F when leaned to 100 ROP = 1300 degr to 1350 Degr. Probes are 2 inches from exhaust flange.

Max EGT spread at idle is 100 degr. Lowest is 900 degr and highest is 1000 degr.

With that spread of EGT's at idle, I wouldn't think you have a plugged bypass port. I believe (i.e. my opinion) that surging comes from systemic issues, not an individual cylinder issue, which usually presents as roughness.

It is difficult to compare absolute EGT numbers across different platforms. However, My peak EGTs are in 1400-1500 range, similar to yours. I took a look at a recent datalog and I showed a range of 900-940 at a 750 RPM idle (the range goes up and down with RPM). I was 830-880 at 650 RPM. You should compare these with yours. Given that our peaks are similar, you can use that as a guideline. By comparing mine to yours, I could possibly infer that you are running leaner at idle. My setup is on the lean side with barely a bump in RPM at ICO.

Again, difficult to compare across planes, but given this and your symptoms, I would try richening up the idle mixture a bit and see if your surging goes away. Surging is a classic symptom of lean idle. Do you ever get a hesitation when increasing the throttle quickly from idle early in the acceleration? That would be another symptom of a lean idle.

You can also do some research about lean surging on the web. 4 stroke engines all behave the same way. Our systems, like FI are different, but the fundamentals of fuel mixture are the same and much can be learned. It will also help to tighten up your definition of surging to be sure we're on the same page. Surging is not that common and can only come from a few things. It usually requires dymanic changing of the air volume, air fuel ratio or ignition timing (we can pretty much rule this out with mags) or a very lean condition that develops it's own dynamics (think of a falling leaf series of stalls).

Good luck,

Larry
 
Carl,
I have done 3 GAMI tests while flying at 75% and 65% power. All had a GAMI range of 0.4 gpm.

Would this test show the spider and injectors are ok?

Also, all my EGT's at cruise are within 50 degr F when leaned to 100 ROP = 1300 degr to 1350 Degr. Probes are 2 inches from exhaust flange.

Max EGT spread at idle is 100 degr. Lowest is 900 degr and highest is 1000 degr.

This thread has nice photos and description of how the spider works:http://www.vansairforce.com/community/showthread.php?t=131769&highlight=spider

In short, the spider functions differently at idle compared to at power. Recommend you:
- With the engine at 1000 RPM or so, lean while watching EGTs. Is one cylinder EGT rising faster than the others? If one cylinder goes down while leaning it is already LOP - and is a problem.
- If the above test shows a problem, do the shot glass flow test (throttle set at fast idle, move the mixture to full rich, engine off, boost pump on). You are looking for close to the same fuel out of each spider tube.

On EGTs, absolute value is nice to know but is only a rough indication. Fuel flow when each cylinder peaks is the data you want.

0.4 gph GAMI spread is what one would expect with stock injector nozzles (0.028"). Find out which cylinder is peaking last (more fuel than desired) and install a 0.027" or 0.0275 injector. Don at Airflow Performance sells these in various sizes for ~$21 each.

I assume you have already eliminate intake leaks or ignition/spark plug issues, and have verified mag timing.

Carl
 
Maybe these pictures will help.

Here's one of my GAMI tests that showed 0.1 spread. This was back in January on a cold day. About 38F OAT.

GAMI_zpsrzgoewrp.jpg





Here is the EGT right after landing on my last flight that had the surging. Note that the engine is NOT surging here. Taxi was normal. Only started surging after fueling up at the pumps.
Only showing this to see about the EGT's at taxi leaning. I did not download the file for the taxi from the pumps to the hangar.
In the middle of the graph at 1:55 was when I increased RPM to 1100 to turn off the runway and then I leaned the mixture for taxi.
You can see that EGT 2 was low by 110 degr before leaning and 80 degr after leaning. Does not look too significant.

Taxi%20Lean_zps7rkphsqn.jpg





Here's the cruise part of the flight running 100 degr ROP just for reference.

Cruise_zps64prcl6r.jpg


The engine runs smooth so I have not checked the resistance on the plugs, but they are all gapped correctly.
Even when it is surging, it is not like a stumble. It is more like a rolling surge. I will get a file downloaded if it does it on my next startup.
Thanks for all the help on this puzzle guys.
I will plug the sniffle valve on the next flight.
 
Yes the video is exactly like mine. A rolling rpm change of about 100 to 150 rpm.

Did you pull the injector and find any trash?

The fact that mine went away and returned many hours later may support the trash theory.
 
I have done everything basically except to pull the motor. No change. very frustrating.
 
Many Lycoming engines with mechanical fuel injection (i.e. Bendix, Airflow Performance) do this when the engine is hot, as the fuel boils in the fuel injection lines above the engine when the fuel flow is low. This causes uneven fuel flow and rpm variations. The surging stops when you increase power, because the higher fuel flow requires higher pressure in the lines, and the boiling point of the fuel is now higher than the actual temperature.

Switching to slightly smaller injection nozzles (sold by Airflow Performance) increases the pressure in the fuel injection lines, which increases the boiling point of the fuel. But, the smaller nozzles pass less fuel at full rich, so there is a risk of being too lean at high power. I've chosen to live with the issue, as I don't want to risk being too lean at full power.

Yep, this is normal, and Kevin nailed it?..as he often does ;)
 
Yep, this is normal, and Kevin nailed it?..as he often does ;)

Why would it continue to surge on startup after the engine is cold?

Would there be residual bubbles in the lines left from the prior shutdown? Even with the boost pump on?

I haven't been to the hangar due to the holiday last weekend but once I check for leaks and remove the sniffle valve I'll report back with any new data.

Thanks to all.
 
Hot idle performance

From the aircraft that customers bring to our facility for rough idle performance, 90% of the time we find the installation is the culprit. That?s not to say that there is an issue with part of the fuel injection system, ignition, or engine itself. We typically find excess hose routing, flow meters mounted on the firewall, gascolators on the firewall, loops in the hoses that can trap air bubbles etc. Sometimes we find issues in the airframe, poor flairs on tubing, flow meters on the suction side of the pumps, cut O-rings and bad pipe thread connections. All these things contribute to poor hot idle operation and hot restart problems.
Even if the installation is correct, poor idle operation can be an issue in tightly cowled or pressure cowled installations. There?s no magic here, we all know avgas boils at around 130 degrees F at sea level. So if the nozzle line temperature gets to 130, the fuel boils in the nozzle lines at idle where the pressure at that point at idle is around 0.04 PSI for a 4-cylinder engine. Raising the idle nozzle line fuel pressure to 5 PSI would be great but then you would need 900 PSI to make the system work at 16 GPH. Certainly you can install smaller nozzles up to a point and that would help. And NO, it will not reduce the take-off fuel flow if the size is calculated to work with the existing inlet fuel pressure and system pressure drops.
One thing to check is to run the engine on the ground with the cowl off. If proper running is achieved, then you know you have a heat issue due to the cowling, so then concentrate in trying to reduce under cowl temperature, reduce the heat transfer to the fuel or increase the nozzle line pressure. If poor operation at idle remains with the cowl off, then look for air entering the fuel supply (typically this is on the suction side of the pumps).
Running a re-circulation system may improve hot idle operation, but then there are fuel management and plumbing issues to deal with.
 
I believe I have found the RPM drift problem.

I removed the sniffle valve and found it to be stuck open.

I put a plug in the sniffle valve port and made two test flights.
The open sniffle valve was acting like an air leak.

The engine still surges at idle when hot which is likely vapor in the spider, but when cold it idles smooth again. And in flight RPM is back to steady.

After soaking the valve in acetone for a week, it is still quite sticky so I will just leave the plug in the intake sump.
 
I removed the sniffle valve and found it to be stuck open.

Same thing happen to me after my third flight. I was able to get another one for free by calling Superior directly. The new one has a few flights and it is doing ok but I carry a "plug" just in case it fails again away from home.
 
...but I carry a "plug" just in case it fails again away from home.

So do I. It's installed in the sump, about 4 years now ;)

Actually checked the inside of the intake plenum at 500+ hours. There was no significant evidence of oil or blue dye. I'm not saying everyone should just ditch the sniffle, but it seems possible to get along without one.
 
From the aircraft that customers bring to our facility for rough idle performance, 90% of the time we find the installation is the culprit. That?s not to say that there is an issue with part of the fuel injection system, ignition, or engine itself. We typically find excess hose routing, flow meters mounted on the firewall, gascolators on the firewall, loops in the hoses that can trap air bubbles etc. Sometimes we find issues in the airframe, poor flairs on tubing, flow meters on the suction side of the pumps, cut O-rings and bad pipe thread connections. All these things contribute to poor hot idle operation and hot restart problems.
Even if the installation is correct, poor idle operation can be an issue in tightly cowled or pressure cowled installations. There?s no magic here, we all know avgas boils at around 130 degrees F at sea level. So if the nozzle line temperature gets to 130, the fuel boils in the nozzle lines at idle where the pressure at that point at idle is around 0.04 PSI for a 4-cylinder engine. Raising the idle nozzle line fuel pressure to 5 PSI would be great but then you would need 900 PSI to make the system work at 16 GPH. Certainly you can install smaller nozzles up to a point and that would help. And NO, it will not reduce the take-off fuel flow if the size is calculated to work with the existing inlet fuel pressure and system pressure drops.
One thing to check is to run the engine on the ground with the cowl off. If proper running is achieved, then you know you have a heat issue due to the cowling, so then concentrate in trying to reduce under cowl temperature, reduce the heat transfer to the fuel or increase the nozzle line pressure. If poor operation at idle remains with the cowl off, then look for air entering the fuel supply (typically this is on the suction side of the pumps).
Running a re-circulation system may improve hot idle operation, but then there are fuel management and plumbing issues to deal with.

Hi Don,

Not attempting to be a Smart A here, but where else but the firewall would you mount a gascolator; the only other place I've seen one mounted was in beneath the floor of a footwell, and it was only that way due it's design?

Regards,

Quentin
 
Hi Don,

Not attempting to be a Smart A here, but where else but the firewall would you mount a gascolator; the only other place I've seen one mounted was in beneath the floor of a footwell, and it was only that way due it's design?

Regards, Quentin

Don is currently banned from the forums because he decided not to renew his advertising. You'll have to contact him directly at Airflow Performance to learn why he finds fault with a gascolator on the firewall.
 
Don is currently banned from the forums because he decided not to renew his advertising. You'll have to contact him directly at Airflow Performance to learn why he finds fault with a gascolator on the firewall.

Banned? Kicking someone off the board because they didn't continue to advertise doesn't sound like Doug's style. You don't even have to donate and he still allows you to play.
Don's contributions to the Forum have been huge. He has helped many of us offline as well and has always been approachable and willing to help, even when it wasn't his stuff.
I hope this isn't true.
 
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