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Engine rough (miss shots) when idle in flight

Camillo

Well Known Member
Hallo.

I have done a few hours of flight on my -4. It has a new Lycoming injected experimental IO-320. It has a Christen oil system.

During last months, a couple of times it runned rough after having idled preparing for landing (I have to idle way before landing in order to lose speed and height). One on ground, I pulled the mixture between half and 3/4 cut and the throttle all in to clean the spark plugs. The further flight went OK.

Yesterday during both landings engine went so rough that I had to abort the first landing fearing of an engine failure. Even increasing power gently didn't stop the faltering. Nearly the same on the homing landing, where I had to slip and lose height in order to land safely without going around.

I have to admit that during the last flights I use to power on the aux boost pump. I now suspect that the boost pump induce for flooded engine during low power settings. Actually, during the first landing I immediately shut off the pump and while for the next seconds the engine continued to miss shots, during the second approach, which became an actual landing, engine run OK (or at least I can't remember it run badly). I will for sure avoid powering on the boost pump in the future. While on ground, idle is wuperOK. Issues only when flying idle.

I was wondering is this can be a reasonable reason for the issue or if I have to investigate more. Actually, I can't trust on the engine and this makes me very upset.

Thanks.
Camillo
 
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Ciao Camillo, what kind of fuel injection do you have? Also, what kind of air filter system? Are you running regular magnetos, or electronic ignition?

How does it behave when flying? Do you have an engine monitor? If so, you can upload data to savvyaviation.com and get some interesting insights.
 
Ciao.
I have regular magnetos. Fuel pump and air filter system from Van's.
When flying is OK (apart when idling). On ground, idle ot WOT, is OK.
I do have Dynon Skyview EFIS and EMS. How can I download data from it?
Thanks.
Camillo
 
If you have the continental style injection system that ECI was selling its designed not to use the boost pump unless the primary pump fails. If you turn it on you will get a rough running engine. If it’s a more standard lycoming system then you have a issue and the pump should be on for takeoff and landing.
G
 
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Idle mixture set correctly??
On a Bendix/Silverhawk FI system (std on Lyc) the boost pump should have no effect on how the engine runs unless the pressure is over 45 psi.
 
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I have an airflowperformance aux pump. The filter is just after the pump.

Engine is a Lycoming EAIO-320 D1A part number YENPL-RT10467. As indicated in the Lyco major part sheet, engine injector is AVX-5VA1 from AVSTAR.

I bought the engine brand new (experimental) from Van's in 2014. In the engine log test sheet is hand written: "inhector aux-5VA2 S/N AV027282 assigned to this ENG". In the engine test verification there are some values below minimun requirements, if that may help.

As of idling mixture, no, I didn't. I will check how to do it. However, on the ground minimum works perfectly. Fuel pressure is around 30/32 PSI (not looked during rough engine operations).

Thanks.
Camillo
 
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Assuming you set your idle mixture correctly (I find most new engines are way rich) then I suggest you look for an air leak.

I experienced the same condition way back in my learning days. The engine ran fine when at power, but pulling back to idle it was scary rough. I ended up doing a dead stick landing as I figured that was safer than trying to nurse it in.
The cause was one intake manifold being loose (one of the two bolts missing on this rented plane). The manifold was close enough to reseat under full power, but dropped away at idle.

Your may have a harder to find air leak.

Carl
 
I would start with Idle mixture. They are often very far off (overly rich) when they come from the factory. It should be adjusted before first flight. Probably too rich. Runs ok cold, as cold engine needs/likes richer mixture, but chokes on the extra fuel when hot and wants a leaner mixture.

Larry
 
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intake leak

Check for an intake leak. Intake gaskets, rubber couplings, check valve on the bottom of the sump or the MAP sensor hose.

If you do have an intake leak you can see it on your EGTs rising quickly when you pull the throttle back. They should not do that. Would be happy to look at your engine monitoring data. Best is to upload the log files to https://www.savvyaviation.com/ and share a link to it, but you can just send me the raw log files too. Good to keep an eye on engine monitoring data for the first few hours. There are a lot of things you won't catch on the screen while flying.

Lenny
 
Thank-you for your answers.
I had a very hard work season. Shortly I will get back to the hangar and investigate, starting with downloading engine monitor data.
 
Hallo to everyone.
Yesterday I made some fix/checks on the plane.
I checked the exhaust and intake tubes and they are all tight.
I checked the fuel line and it seems that they are OK. The line is covered with firesleeve and shielded in a couple of positions. See photos:





Then I checked the idle mixture. The RPMs rise was 30/40 instead of 50. When leaning, the RPMs went to 1.030/1.040 before dropping.

I enriched slighlty turning less than a turn (three "notches"). The RPMs rose to 1.050/1.060. I came back one notch (so 2 notches enriched) and obtained the 1.050 rise.

I didn't fly, yet. However, minimum on the ground is perfectly round (600 RPMs).

Could that slight difference be the culprit for the engine coughing in flight while idle in a hot day?



I downloaded the Dynon EMS logs and uploaded to savyaviation website.

This is the first flight when the issue occurred (2021_09_25_14.36Z):
https://apps.savvyaviation.com/my-flights/36732/9934c7c2-67a9-44c9-b175-4cc3d944dd25
Problem started a few seconds before minute 26.30.
At 24.03 engine was idled.
From 25.27 RPM dramatically went down (approx. slightly low 1.000 RPMs), even if MAP was left unchanged.
At 26.30, after the engine "hiccup", I gave power and made a 360° close to the runway.
At 28.30 I landed.

I noticed that just before the RPM drop pitch raised by 5° (from 5° to 10°). Maybe the airflow disturbed the idle...

This is the relevant part of this flight. Note that the first circled area is when the problem started and the second one is the final part of flight, where RPMs normalized. It is strange that for same MAP RPMs changes very much.


And this is the second one, in the same day (2021_09_25_10.29Z):
https://apps.savvyaviation.com/my-flights/36732/9934c7c2-67a9-44c9-b175-4cc3d944dd25
Here I can say that at minute 18.22 I switched on the fuel pump, while reducing power and entering downwind. On 19.22 I think I had the engine coughing and I thaught it could go off. After a few second I switched off the pump but engine continued to go high and low on RPMs. Then I landed.

RPM seems to be well over 1.000. However, I had the clear sensation of a close to quit engine while in flight.

Thanks.
Camillo
 
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Did you use the Precision Airmotive SIL RS-67

The service letter goes through setting idle and mixture.

I’m just wondering as I am not sure about the numbers,…you seem to indicate it goes to over 1000 RPM when leaning mixture from idle,… yet idle is 600 or 700.

Also looks like you haven’t flown all that much and mixture has been a question,…. How are the plugs looking?

The folks at Saavy should be able to give you some ideas on what to go look for.
 
Thank-you.
Idle RPM is 700.
Engine manual says to perform the test at 1.000 RPM.
RPM increase was 30-40 RPM. So, within limits. While I was there I adjusted it to 1.050. However, I suspect the issue was not dued to the idle mixture setting.
No, I didn't check for spark plugs. I have to.

Precision document at the end says:
If the system is set in accordance with this letter (prior to extended engine runs or flights), and the mixture does not meet the criteria after flight (especially in warm weather), vapor formation may be indicated. See the publications referenced in paragraph D for further information. If no other discrepancies are found, the servo should be removed and sent to an approved repair facility for evaluation".

The issue rose in a very hot day, with a hot engine. So, vapor formation may be a more possibile cause...?
 
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When the problem is occurring, does manually leaning the engine fix it? If so, it is going too rich. If it doesn't, it may be too lean or the problem is not fuel related. If it shows too rich, check the raw fuel pressure and if the raw fuel pressure is staying within normal parameters, then the center body seal in the servo may be sticking. To test start at idle with engine running well. Increase rpm from idle to say 1500rpm without going past 1500rpm and note the fuel flow. The continue up to full power and then retard the throttle to 1500 RPM, again without going past 1500rpm. and note the fuel flow. This reading should be very similar to the reading you had on the way to power. If it is substantially different the servo needs to be removed and sent in for repair.
If the raw fuel pressure is out of limits high, it will cause the engine to run way too rich and want to quit.
Does the engine monitor show one cylinder with very low egt as compared to the others when it is happening? If so that would indicate the cylinder with low or no egt isn't functioning and causing the roughness. the Trouble shoot that cylinder for not firing, too rich/lean, ignition, compression.
Happy New Year,
Mahlon
 
I haven't tried to manually lean while making the issue. I will try during the next flight at a safe altitude.

What do you mean for "raw" fuel pressure? Without aux fuel pump? I will try this method.

I only have EGT on cylinder #3...

Thanks.
 
42CE61CD-9F22-4416-B967-F265FF3F44CC.jpg

I had kind of the same issue and I chased it for four months before we went looking inside the 0- 320 , very concerning this was occasionally jamming under the valve cover and causing a miss once every couple hours 🤔
 
I haven't tried to manually lean while making the issue. I will try during the next flight at a safe altitude.

What do you mean for "raw" fuel pressure? Without aux fuel pump? I will try this method.

I only have EGT on cylinder #3...

Thanks.

Raw fuel pressure is the pressure at the fuel servo inlet. either boost pump on or off. If it is significantly higher and out of limits with the boost pump on that may be your issue. Normally less than 25-35psi in either case but should be no more than 45psi.
Good Luck,
Mahlon
 
Disclaimer: I haven't read all of the posts here.

Looking at the posted pix. Even with the radiant barriers and fire sleeve, you're not doing yourself any favors with the related proximities. There's not enough velocity in the lines to provide much active cooling. If it's not a vapor pressure problem now, don't rule it out in the future.
 
Garry, my engine is brand new... The problem rises only during hot weather (as far as I experienced till now).
 
Something leaking on the intake manifold makes sense, Intake/ induction leaks are greatest at idle when the difference between manifold pressure and ambient pressure are the greatest, and least at full power.
 
View attachment 20467

I had kind of the same issue and I chased it for four months before we went looking inside the 0- 320 , very concerning this was occasionally jamming under the valve cover and causing a miss once every couple hours ��

Garry - any idea what caused that? Must have been quite a surprise when you saw it!
 
As Freemasm wrote, the first thing I thaught was a vapor issue.

I may add that also during winter, when idling during flight (i.e. before stalls), engine is not very smooth. And that is why before every flight I run the engine WOT and cutting mixture to clean spark plugs. Or at least I thaught it was a sparking plug issue. Until september, when I flew in a very hot day.

The problem arose during both trips, after having idled for approx. a minute and after having pulled down flaps, turning in base, maybe in a nose up attitude. Both times with electric pump on (and immediately turned off). And after a few seconds of "up and down" RPMs (say 900-1200; however Dynon only shows a steady 900 RPMs) RPMs normalized, during the final approach and up to the flare.

So, it may be something connected with heat AND attitude/different air flow inside the cowl? When I slow down from 150 knots to 80 to lower flaps no problem. Maybe during this phase a lot of air passes. Then, when passing from 10° to 30° of flaps and speed reduces to 70 knots, with nose higher than before, maybe heat builds up inside the cowl and heats the fuel line.

There should be something which is a mix with heat and airflow.

Strange thing is that in both trips during the final approach (65/60 knots.), nose slightly up for landing the RPMs are OK (no swing). And on ground, idle speed is very smooth. Problem shows only in flight and only after more than 30'' idling.

In order to be on the safe side, I just ordered a couple of exhaust heat shields and a SCAT-4 1'' tube and flange to bring fresh air to the fuel line, as for magnetos.

Thanks.
Camillo
 
Garry - any idea what caused that? Must have been quite a surprise when you saw it!

No real way of telling what caused the casting to break , they were first run cylinders and I had flown behind the engine 1000 hrs before it was overhauled, so 3000 hobes hours total time on the cylinders before this incident ,guessing it could have been metal fatigue , I understand Most places today that are overhauling engines are re-working cylinders because of the long backlog in new ones ,
 
Update:
I installed 2 more shields on the exhaust, close to the fuel line.
I checked for the minimum idle setting and screwed clockwise two notches (it was only slightly leaned).
I got used not to idle the engine during the downwind, base and the first part of the final.
I only made a few flights from the issue, and not with that hot day when the issue occured. However, no more RPM fluctuation occured...
Thanks.
Camillo
 
Rough engine

Camillo,

I’m no engine expert but your fuel line that attaches to the throttle body just above the throttle arm is VERY close to 2 exhaust pipes. Even though it’s fire sleeved I suspect that the fuel may be getting too hot and introducing some vapor. I might suggest rerouting that line up and over the intake tube to completely avoid the exhausts (or making a new fire sleeved fuel line to fit).

Just my two cents but you’ve ruled out the obvious intake leak and mixture.

I may be full of it and get flamed out in this forum but it wouldn't hurt and it’s certainly worth a try.

01164-C71-F60-A-4949-83-D2-C6-F7-F6-A8-DF7-E.jpg
 
Camillo,

I’m no engine expert but your fuel line that attaches to the throttle body just above the throttle arm is VERY close to 2 exhaust pipes. Even though it’s fire sleeved I suspect that the fuel may be getting too hot and introducing some vapor. I might suggest rerouting that line up and over the intake tube to completely avoid the exhausts (or making a new fire sleeved fuel line to fit).

Just my two cents but you’ve ruled out the obvious intake leak and mixture.

I may be full of it and get flamed out in this forum but it wouldn't hurt and it’s certainly worth a try.

01164-C71-F60-A-4949-83-D2-C6-F7-F6-A8-DF7-E.jpg
I agree with Oly - this looks like a vapor issue probably caused by the long runs of your fuel line near the hot exhaust. In fact, you've pretty much confirmed this since adding heat shielding to the exhaust pipes reduces the problem.

You can get hose of any size, shape, and clocking from suppliers like these guys: http://www.aircraftspecialty.com
 
Engine Miss

Camilo,

When I overhauled my engine I replaced all the fuel and oil hoses with new ones from TS Flightlines. They can make any custom length, size and whatever type of fitting you want. Check out the link below:

https://www.tsflightlines.com/products.html

I think it’s also recommended to have a fresh air blast tube to the mechanical fuel pump.
 
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Thank-you, all.
I will try to reroute that fuel line.
What should I inspect for the intake leak? I already checked the intake tubes. Somewhere else?
Thanks.
Camillo
 
Thank-you, all.
I will try to reroute that fuel line.
What should I inspect for the intake leak? I already checked the intake tubes. Somewhere else?
Thanks.
Camillo

Here is a procedure to test if you have an induction leak:

https://resources.savvyaviation.com/in-flight-diagnostics/

It's really hard to visually see if something is leaking. There are some other methods that don't involve flight, like using a vacuum or some pressure, but I have not tried them.
 
Fuel Line Routing

It looks like you have the fuel line that runs between the servo and the spider manifold routed from the injector servo all the way to the rear of the engine on the cylinders 2/4 side where I’m assuming it runs up to and through the rear baffle then back forward to the spider. This is a very long and convoluted path that is likely doing you no favors in terms of heating the fuel, in addition to the issues noted by others.

Lycoming normally routes this line up between cylinders 1 and 3 through a grommeted hole in the inter-cylinder baffle, then nearly directly to the spider manifold. This is a much shorter and more direct route for this line that minimizes the areas where it runs near the exhaust system.

Skylor
 
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Engine miss

Camillo,

As Sklor pointed out, it looks like your fuel line is pretty long. Not the same setup as yours but I have a purge valve on the right side and the fuel line goes down between the #1 and 3 cylinders in the shortest possible length direct to the fuel servo (see picture). He’s right that your fuel line may be too long by going around the back of the engine on the left side.

99-D4-B43-B-9483-41-F6-808-D-8-D8604-B4-BAF1.jpg
 
Thank-you both.
Yesterday I went flying. Medium temperature spring day (actually, a medium summer hot day). 26° C OAT.
In the first flight all went OK. I idled on final and engine run smooth.
I left the aircraft parked for 10 minutes, then I started the engine again and at 1.300 RPMs, engine started to miss shots. It was an evident vapor bubble issue. After a while RPMs stabilized, thus indicating that fresh fuel was reaching the engine.
So, I believe you are right on the necessity to change the fuel line path.
I will update this post for further news.
Thanks.
Camillo
 
As Skylor argued, yes, my line goes from the spider back through the baffle and the fore, under cylinders #2 e #4. It is quite long. It comes close to the exhaust in one location, just before (or after) the outlet from the injector servo.

I will look if the different route through a hole between cylinders #1 and #3 will be suitable.

In the meantime, I rerouted the other hose. Now it comes close to exhaust in two locations instead of three and is way more far from the exhausts than before.

 
I must add that I also changed my flight behaviour, because I used to idle well before approaching the field, to let speed go down. Now, I reduce power at 1.500/1.700 and idle only when in final. I never experienced the issue again in the same way (even if I flyed just a few hours last year).

However, the engine still continues to miss some hit. I believe I should add a fuel bypass system to let the fuel return to the tank.
 
Camillo,

In post # 12, the line from the servo to the spider passes quite close to an exhaust pipe which does not appear to have a heat shield on it. This line could probably benefit from being rerouted also.

The fuel moves very slowly through these lines at idle, so each cc that passes has a lot more time to pick up heat. Also, at high power settings, the passing fuel actually cools the inside of the lines.

Hope this helps.
 
Camillo,

In post # 12, the line from the servo to the spider passes quite close to an exhaust pipe which does not appear to have a heat shield on it. This line could probably benefit from being rerouted also.

The fuel moves very slowly through these lines at idle, so each cc that passes has a lot more time to pick up heat. Also, at high power settings, the passing fuel actually cools the inside of the lines.

Hope this helps.

Also at idle that fuel is at very low pressure. At low pressure the fuel can boil in the line easier.
 
Thank-you for your input. I am getting crazy for this issue.
I have a new plane which I don't trust. I operate in a medium lenght strip (900 meters long) and during each landing I have to hope the engines doesn't betray me.

The day before yesterday, during landing at idle (short final) the engine missed some hit, from time to time. After taxing, beside the hangar, idled @ 650/700 RPM, after a while it quitted. Ok, I know: it was idled and medium temperature OAT (hot inside the cowl); however when engine is cold it maintains regularly the idle RPMs.

I will try to reroute and shield protect the fuel lines. However, for rerouting the fuel line from FF to the throttle body, I fear I can't pass behind, a la R14 on Van's site, unless I use some 90° elbow fitting. Otherwise the fuel line will make a sharp bend.

Ciao.
Camillo
 
I have to add that spark plugs are black (and not grey): So the culprit may also be a too rich mixture...
 
I have to add that spark plugs are black (and not grey): So the culprit may also be a too rich mixture...

After seeing your post the other day I was going to suggest a rich idle mixture. Black plugs just confirms that. Strongly suggest you set the idle mixture. They tend to come filthy rich from the factory and if no one has done this, no surprise. DO a search for methods of setting it.
 
...
I will try to reroute and shield protect the fuel lines. However, for rerouting the fuel line from FF to the throttle body, I fear I can't pass behind, a la R14 on Van's site, unless I use some 90° elbow fitting. Otherwise the fuel line will make a sharp bend.
...
Ciao Camillo, a 90 degree elbow fitting is no problem on the high pressure side of the fuel pump. Avoid sharp bends on the low pressure side - between the pump and the fuel tanks.
 
lr172, I had already adjusted the idle mixture. It was only slightly rich before the adjustment.

So, why are the plugs black? Where is the black residue? Is it on the outter ring at the end of the threads? On the insulator and center electrode? On the ground electrode? Is the residue shinny or dull in appearance? Is it a light dust that comes off with a finger wipe? If so, does it feel oilly or like dry dust?

Black plugs can be oil burning or excess carbon from over rich operations. The answers to the questions will help determine which yours are. Typically fuel deposits created under moderate or higher power operations are burned off the plug. Excess fuel deposits found on plugs are usually from idle operations where the heat is not high enough to burn it off the plugs.

While I do not discount that possibility that vapor is causing this issue, I struggle to see vapor as the sole problem while travelling at 100 MPH approaching the airport at idle. Not saying it can't happen, but you need to have a set up that is FAR worse than the average. On the ground is a whole different issue, but 100 MPH provide A LOT of cooling air. Further, you mentioned that you used to slow down by going to idle and assume that was more like 120 MPH; even more cooling air. A simple test to rule that in or out in the lower pressure area is to replace the 2 lb spring with the 4 lb spring in the spider. That raises fuel pressure between the servo and spider and this higher pressure increases the boiling point by a small amount. The servo adjusts fuel flow by modulating pressure, so it is lowest at idle and ramps up gradually as power increases. Pressure between servo and spider is only a couple PSI at idle. The pressure up to the servo is 25 PSI, so if you are getting boiling, it is far more likely to be happening in the low pressure area.

Are you sure you got the idle mixture right? sooty black plugs are a very good indicator of an excessively rich idle mixture. There are 1000's of these planes built and most have less than optimal fuel line routing and only a very small number have fuel vapor problems, excluding the hot soak start and taxi to take off, where it is very common. It is somewhat rare to hear about fuel vapor issues while idling at 100 MPH. Far more typical in ground ops or WOT climbs at low airspeed. It is somewhat common in pusher configurations and the fix for them was the 4 lb spring. There problem was the line between the servo and spider. HOWEVER, the pusher designs often struggle with cooling airflow at lower speeds, unlike our tractor configs with better access to ram pressure.
 
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