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Advice needed on in-flight engine hesitation

N

neonbjb

Member
Hi guys,
I had a bit of a "pucker" moment yesterday when I was climbing out of my home airport. At about 1500' AGL the engine lost an significant and audible amount of power for about 1-2 seconds, then went back to full power as if nothing had happened. It sounded as if it had encountered a heavy load and then overcome it.

After looking around and figuring out my exit plan, I started analyzing the engine instruments looking for a problem. I did not see anything. Fuel & oil pressure were fine. Oil temp was great, CHT and EGT on all cylinders was fine. Alternator was producing power. I did a mag check and both mags were working fine.

Being that I could not see anything immediately catastrophic occurring from reading the instruments, I decided that rather than landing out I would continue flying to see if anything else would happen. I climbed to 3500' AGL then went to cruise power for a little over half an hour. Nothing further happened.

My question for you guys:
Is this something that would cause you any concern? Is it cause to ground the plane for inspection? If so, what would you look for?

A few more salient points:
- OAT temp was ~75F at 4000' MSL. I regularly fly in much hotter temps with no problems.
- Fuel tanks were 10gal/5gal, so relatively low.
- Engine is an IO-320 B1A and has about 400 hours SMOH. I have never had an issue like this before. Oil consumption is very low.
- Was running full rich at the time of the incident. I generally do not lean on climb out until 6000' MSL.
- Aircraft is regularly hangared. It hasn't been exposed to rain in well over a month and a few fuel tank refills. I sump it before every flight.
 
Last edited:
Thank you very much, this is almost certainly the issue. The engine runs at very low power levels and relatively cool very often. I'll take a look.
 
I had a similar experience in an O-320 powered flying club Cessna 172, several times. The theory was it was plugs getting fouled from running too rich on the ground. We started leaning aggressively on the ground during taxi, and that seemed to work.

It certainly will get your attention when the engine gives a little cough on the climb out, though. Definitely ups the "pucker factor".
 
DaleB said:
I had a similar experience in an O-320 powered flying club Cessna 172, several times. The theory was it was plugs getting fouled from running too rich on the ground. We started leaning aggressively on the ground during taxi, and that seemed to work.

It certainly will get your attention when the engine gives a little cough on the climb out, though. Definitely ups the "pucker factor".
Click to expand...

That's what I was going to suggest. But, you cannot rule out the sticking valve.
 
Since you state that, "CHT and EGT on all cylinders was fine", it appears that you have an engine monitor. If it has a recording function, that could tell you which cylinder was at fault. EGT will fall to around the CHT temperature in a cylinder that quits firing for whatever reason. Thus, your search can be considerably narrowed.

To minimize carbon buildup, ground operations can be conducted "aggressively leaned".

Cheers, David
RV-6A
 
Unfortunately my monitor doesn't have a logging function, though I wish it did at this point. I do lean very aggressively when taxiing and I did a mag check with no found problems before taking off, so I'm pretty sure it wasn't an ignition issue.
 
Four hundred hours puts your engine exactly at the point where the "wobble" test for valve sticking is recommended.
Trembling after a cold startup or a momentary hesitation in flight are the classic symptoms.
Fortunately, the fix is relatively easy and well documented elsewhere here.
For prevention, I would search for the definitive article on Lycoming valve lubrication by Bill Marvel and Bill Scott, over at another type-specific website.
 
find a mechanic who can do the exhaust valve SB. report back. it is kind of like having a blip in your heart beat. :)
7EF45B86-8242-4F72-81CB-10122666787F.jpg
 
I would not go down the road of adding unapproved items to fuel..

Avgas has lead scavengers that are activated by fuel burn. Leaning appropriately is a great practice. With a carbureted engine though that process is limited. Keeping CHT within normal limits is very important. Stuck valves occur from High CHT. Keep in mind actual EGT values are immaterial. A complete clean combustion event is important, hence the leaning thing.

Mike Busch has excellent articles and podcasts on this subject. I would reach out to him via Savvy aviator. He has helped me in the past..

Let us know what you find..


Regards

Mike
 
I would not go down the road of adding unapproved items to fuel..

Avgas has lead scavengers that are activated by fuel burn. Leaning appropriately is a great practice. With a carbureted engine though that process is limited. Keeping CHT within normal limits is very important. Stuck valves occur from High CHT. Keep in mind actual EGT values are immaterial. A complete clean combustion event is important, hence the leaning thing.

Mike Busch has excellent articles and podcasts on this subject. I would reach out to him via Savvy aviator. He has helped me in the past..

Let us know what you find..


Regards

Mike
 
You probably have your answer and my guess would be a sticky valve as others have pointed out.

Apart from that, I am not sure why you would not lean your engine below 6000??
Especially since your engine is running very "cool" as you reported.
Note your EGTs on take off and start leaning as soon as you get out of the pattern. Maintain take off EGTs until you reach cruise altitude.
You'll do your engine a favor when you run it "lean and clean".

Was running full rich at the time of the incident. I generally do not lean on climb out until 6000' MSL.
Click to expand...
 
I had to look at the thread title twice; on the first scan I thought it said in-flight engine installation...
 
Pilot Dog Ship said:
Four hundred hours puts your engine exactly at the point where the "wobble" test for valve sticking is recommended.
Trembling after a cold startup or a momentary hesitation in flight are the classic symptoms.
Fortunately, the fix is relatively easy and well documented elsewhere here.
For prevention, I would search for the definitive article on Lycoming valve lubrication by Bill Marvel and Bill Scott, over at another type-specific website.
Click to expand...

Only if you have the old style valve guides.

If you have a "C" in a circle stamped on the valve cover flat surface of the head you have the later guides and then the SB 338 recommendation is to start at 1000 hrs.
 
The local shop finally had a chance to do the wobble test per the SB and found no issues. Oddly enough I was hoping they would. I guess at this point I keep flying and try to keep an emergency landing site in mind for the next few hours.
 
N427EF said:
You probably have your answer and my guess would be a sticky valve as others have pointed out.

Apart from that, I am not sure why you would not lean your engine below 6000??
Especially since your engine is running very "cool" as you reported.
Note your EGTs on take off and start leaning as soon as you get out of the pattern. Maintain take off EGTs until you reach cruise altitude.
You'll do your engine a favor when you run it "lean and clean".
Click to expand...

You're probably right. I choose 6000ft because that's when my MP and tach indicate I'm under 75% power (I have a FP prop). Also during the really hot months (when OAT is > 100F) the engine will wander above 400F if I lean in a slow climb. I suppose there is some room for improvement here in my process, I'll look into it again.
 
neonbjb said:
Hi guys,
I had a bit of a "pucker" moment yesterday when I was climbing out of my home airport. At about 1500' AGL the engine lost an significant and audible amount of power for about 1-2 seconds, then went back to full power as if nothing had happened. It sounded as if it had encountered a heavy load and then overcome it.

After looking around and figuring out my exit plan, I started analyzing the engine instruments looking for a problem. I did not see anything. Fuel & oil pressure were fine. Oil temp was great, CHT and EGT on all cylinders was fine. Alternator was producing power. I did a mag check and both mags were working fine.

Being that I could not see anything immediately catastrophic occurring from reading the instruments, I decided that rather than landing out I would continue flying to see if anything else would happen. I climbed to 3500' AGL then went to cruise power for a little over half an hour. Nothing further happened.

My question for you guys:
Is this something that would cause you any concern? Is it cause to ground the plane for inspection? If so, what would you look for?

A few more salient points:
- OAT temp was ~75F at 4000' MSL. I regularly fly in much hotter temps with no problems.
- Fuel tanks were 10gal/5gal, so relatively low.
- Engine is an IO-320 B1A and has about 400 hours SMOH. I have never had an issue like this before. Oil consumption is very low.
- Was running full rich at the time of the incident. I generally do not lean on climb out until 6000' MSL.
- Aircraft is regularly hangared. It hasn't been exposed to rain in well over a month and a few fuel tank refills. I sump it before every flight.
Click to expand...

There is a lot of focus on a sticking valve, but that would be an audible miss and roughness....
Were you over 40 hours since the previous oil change when this occurred.
Changing oil at 25 or 30 hours vs 50 hours might prevent a recurrence.

If it was not a valve, then perhaps vapor lock, or your fuel pressure dropped for a moment. Had you just filled with fuel? Did you have a longer than normal run up or an extended wait in the take off line up? Was a hot engine shut down for fueling previous to take off?
 
F1R said:
There is a lot of focus on a sticking valve, but that would be an audible miss and roughness....
Were you over 40 hours since the previous oil change when this occurred.
Changing oil at 25 or 30 hours vs 50 hours might prevent a recurrence.

If it was not a valve, then perhaps vapor lock, or your fuel pressure dropped for a moment. Had you just filled with fuel? Did you have a longer than normal run up or an extended wait in the take off line up? Was a hot engine shut down for fueling previous to take off?
Click to expand...

Some sort of fuel system problem is my guess as well at this point. The thing is nothing about this flight was out of the ordinary in any way whatsoever. I fly this plane a lot - 2-3 times a week for the last 2 years. It was an average temperature day, fuel was not freshly filled. The plane had been in a hangar. Run-up time was shorter than normal. It hadn't rained in several weeks & I always sump the plane regardless of whether or not it rained.

I am considering putting some sort of dash cam set-up looking at the engine instruments for the next few hours of flight time. If it happens again I would be really interested to see if it corresponds to a quick drop in fuel pressure. If the issue is vaporization that would register on the gauge, right?
 
A little late but, I have read of departure accidents that were caused by water in fuel, in the right conditions, ( my first thought tanks 1/4 full on one ) scenario, pre flight and sump tanks, ( this does not remove water droplets on top of tanks ) then you taxi and take off with the fuel splashing and now those water droplets are now on their way through the fuel system. I always rock the wings before preflight to get those droplets off the top of tank and then sump at end of walk around preflight inspection. is there a big temperature and humidity delta, night-day in your area?
 
I had something similar today, and think it may be the fuel boiling off in the F/I fuel divider. I am still in the run in period and my CHTs have run high. It was fine once I lowered the nose and got the CHTs down a bit. A second flight today I left the electric fuel pump on during climb and also climbed at a higher airspeed and did not experience anything unusual.

I am using a plenum and the fuel divider is pretty close to the case so it may be more prone to this. Hopefully temps will come down a bit more as the engine breaks in.
 
I have seen this happen because of a loose rotor tab on a Slick mag. it was intermittant and would briefly stumble the engine.
 
brooksrv6 said:
I have seen this happen because of a loose rotor tab on a Slick mag. it was intermittant and would briefly stumble the engine.
Click to expand...

That should cause a slight loss in rpm though, right? Not a full power loss? I've had an in flight mag failure before and this was definitely not that.
 
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