Low CHY and low EGT on one cylinder of a carbureted O320?

I experienced a few seconds of rough engine operation at 2560 RPM while flying at 9500ft. OAT was 29F.

I then watched the #2 cylinder EGT drop from approx 1400F to 1190F. I also watched the #2 CHT drop from 280F to 200F.

It stayed like this for about 30 minutes while I circled the airport at altitude. Eventually the CHT very gradually rose to 230F and EGT rose to 1200F.

I did check carb-heat and had the usual a small drop in RPM with no rise of the course of one minute.

The CHT returned to normal as I descended.

Thoughts?

Good puzzle to solve.

As you have a carburetor, the normal suspects associated with fuel injection are eliminated. Assuming you eliminated altitude impact on mags and sparkplugs, the symptoms you describe are consistent with a cylinder being way lean.

If you were anything other than WOT, you might have an air leak on that cylinder that is causing it to be lean. I'd first look at the intake pipes and gaskets. As the problem changed with time however, a simple leak might not jump out at you.

Carl

Devastator just tells you to fly more

An acting spark plug would be my guess.

Spark plugs or sticking exhaust valve are the two most likely culprits.

Wouldn't a bad spark plug result in an incomplete burn and thus an elevated EGT - similar to running rich.? Am I mixing up the effects of combustion?

The idea the cylinder went excessively lean would explain both drops. Since this is a carburetor, is the likely culprit the intake pipe? Is there an easy test for an intake pipe leak?

Less than 10 hours ago we did a borescope and the engine is "exceedingly clean" because I burn so little 100LL - probably not more than 20 gallons in the past 80 hours. This would not support a stuck valve (but would also not exclude it 100%).

I'm not an engine guy but I'd think that the plug or a sticking valve could explain it. They could clear themselves before landing, except perhaps the wire.

In both cases, they'd show up as low EGT and low CHT.

My experience with bad plugs is that they are very likely to lower the EGT for a short while. Sticking valves, in my experience, seem to show up on run-up, clearing up as the engine warms up.

The plug gives varying (that is, non-constant) EGT temps. The valve shuts down the entire cylinder until it starts working again. It's either on or off.

Now of course other indications are possible for these defects and perhaps other causes for your engine. This is merely my own experience.

Dave

Sticking Exhaust Valve Thread...

You can search the forums, there are several great threads out there about this issue.

Here's the latest, a thread I started last month and just wrapped up yesterday:

http://www.vansairforce.com/community/showthread.php?t=143125

Take a look, it could be the problem.

Thanks Rob.

I searched the forums and read the link you provided.

I will investigate the possibility of a sticky valve but suspect - given other information - the intake is a more likely cause.

I am looking for suggestions on how to test for a leak on an intake tube.

Use a compression tester

humptybump said:

I am looking for suggestions on how to test for a leak on an intake tube.

Click to expand...

You can tape over the air intake and use a compression test adapter to pressurize the cylinder on the intake stroke with shop air. The intake valve should be open with the piston at the bottom of its travel. Then use soapy water solution to inspect the entire induction system for leaks.

snopercod said:

You can tape over the air intake and use a compression test adapter to pressurize the cylinder on the intake stroke with shop air. The intake valve should be open with the piston at the bottom of its travel. Then use soapy water solution to inspect the entire induction system for leaks.

Click to expand...

Thanks for the test procedure. I'll have to take a closer look at where I will have the necessary access of the intake tube between the carburetor and the cylinder.

Update: there is a very useful video for testing the induction tube: https://www.youtube.com/watch?v=CfG4QGv-r64

We can eliminate the sticking valve and a leak on the intake tube. Both were checked today and are in great condition. The bore scope also showed the piston head looking good and the intake valve looking new.

I'm installing replacement plugs top and bottom on #2 and we'll watch for any repeat of the anomaly.

While checking the exhaust valve, the mechanics did an interesting thing. It may be common knowledge. But, for those who fear losing a valve into the engine here is the trick:


One observation was that the oil is darker / dirtier than I would expect for 18 hours since the last change. This indicates combustion particulates getting past the ring(s).

RV8JD said:

More info in the Lycoming Service Instruction No. 1425A that addresses sticking valves.

Click to expand...

I won't say 100.00% it's *not* a sticking valve. However, two A&Ps inspected the valves and performed the wobble test. The results do not indicate a sticking valve.

I need to attempt to reproduce the event. This far, attempts have not been successfull

humptybump said:

Wouldn't a bad spark plug result in an incomplete burn and thus an elevated EGT - similar to running rich.? Am I mixing up the effects of combustion?

Click to expand...

Not really. one bad plug creates symptoms of retarded ignition timing. These symptoms are reduced CHT and increased EGT, due to the late PCP. The mixture will usually still burn completely, but not as much heat converts to energy and ends up in the exhaust. You can ground a mag in cruise and watch this effect.

Larry

humptybump said:

We can eliminate the sticking valve and a leak on the intake tube. Both were checked today and are in great condition. The bore scope also showed the piston head looking good and the intake valve looking new.

I'm installing replacement plugs top and bottom on #2 and we'll watch for any repeat of the anomaly.

While checking the exhaust valve, the mechanics did an interesting thing. It may be common knowledge. But, for those who fear losing a valve into the engine here is the trick:

have a long length of natural fiber rope. With the cylinder in its lowered position, proceed to snake the rope in through the top spark plug hole until the cylinder is completely full. This gives a margin of error against accidentally pushing the valve into the engine. It's still possible to make the mistake but the odds are good the top will give enough resistance under most circumstances.​

One observation was that the oil is darker / dirtier than I would expect for 18 hours since the last change. This indicates combustion particulates getting past the ring(s).

Click to expand...

How many hours on your engine? Given the new data point about oil darkening prematurely, along with your other symptoms, I would also consider a stuck ring. A stuck ring can reduce your compression, lowering both CHT & EGT. The rings can also unstick themselves, resulting in intermittent symptoms.

Larry

lr172 said:

Given the new data point about oil darkening prematurely, along with your other symptoms, I would also consider a stuck ring. A stuck ring can reduce your compression, lowering both CHT & EGT. The rings can also unstick themselves, resulting in intermittent symptoms.

Click to expand...

"Stuck ring?" I assume you meant valve. Bother were checked. No deposit build up and proper amount of "wobble" - e.g. Wiggle the end of the valve and there should be a tiny amount of play.

The borescope did show initial signs the cylinder crosshatch is getting faint. The oil getting dirtier is likely past the ring.

BTW: the oil condition isn't "bad" but it's not as good as I remember 100 or 200 hours ago. The engine is at 500 hours from its major overhaul now so it's not "high time" but it's not new either. Also oil consumption remains low so it's not burning oil.

I'll continue to keep an eye on the oil and continue to see if I can reproduce the #2 CHT+EGT drop.

Good to know they "teach" the rope trick

Cht's and egt's

Glen, altitude will really test the insulation of the ignition system in other words the spark will bridge a greater air gap at altitude it's possible that the cylinder in question is jumping spark somewhere from the magneto to the spark plug itself I would check very carefully to see if there are any abrasions or discontinuities in that particular wire, good luck

humptybump said:

"Stuck ring?" I assume you meant valve. Bother were checked. No deposit build up and proper amount of "wobble" - e.g. Wiggle the end of the valve and there should be a tiny amount of play.

The borescope did show initial signs the cylinder crosshatch is getting faint. The oil getting dirtier is likely past the ring.

BTW: the oil condition isn't "bad" but it's not as good as I remember 100 or 200 hours ago. The engine is at 500 hours from its major overhaul now so it's not "high time" but it's not new either. Also oil consumption remains low so it's not burning oil.

I'll continue to keep an eye on the oil and continue to see if I can reproduce the #2 CHT+EGT drop.

Good to know they "teach" the rope trick

Click to expand...

I meant a stuck ring. I read your posts and see that you have eliminated the valve guide. A stuck ring is a good probability fit with the following symptoms:

sudden Loss of compression (evidenced by tandem reduction in EGT / CHT)
Sudden increase in rate of oil saturating w/deposits (i.e. new, excess blow-by)

Deposits on the valve or seat (causing incomplete seating and therefore compression loss) could also cause the first symptom and would show as intermittent behavior (assuming they clear themselves, which they often do) and explain why your mechanic found no problem with the valve guide. It doesn't account for the second though. That said, it sounds like you may have other issues causing this based upon the borescope.

Remember, oil burning is usually due to issues with the oil control rings and/or cylinder wall surface. Excess blow-by is an issue with your compression rings.

Larry

Interesting. I'd not heard anyone describe a "stuck ring".

We're taking about piston rings - correct?


(noob alert)

How can they be "stuck"? I'm not visualizing the situation?

humptybump said:

Interesting. I'd not heard anyone describe a "stuck ring".

We're taking about piston rings - correct?


(noob alert)

How can they be "stuck"? I'm not visualizing the situation?

Click to expand...

Yes, referring to piston rings. The compression rings float around in piston ring lands, which are cavities cut into the piston to contain the ring. This cavity is sized to provide about .004" clearance between the piston and the ring in the vertical plane. The piston develops carbon deposits on it's side and occassionally the deposits can form between the ring and the land. When enough build up occurs, the rings becomes stuck and is unable to float. The ring provides a seal to the cyl wall via tension (ring is shaped larger than cyl bore and the compression to make it fit (i.e. tension) causes the seal, in addition to the expanding gasses from above.

If the ring sticks, this tension is lost and the gasses from above cannot further force the ring against the cyl wall, resulting in reduced compression, reduced power and increased bloby.

This is a bit of a long shot diagnosis, but it does match the symptoms so wanted you to be thinking about it. You EGT/CHT behavior points to a compression loss and further points to a ring/wall issue due to the increased blowby (you need to better confirm this as it can help include / exclude various failure modes). I suggest you continue to explore all scenarios that could cause intermittent and continued compression loss. It is possible that compression is down a bit and occasionally gets worse. You can lose a lot of compression before you will see that in the CHT/EGT readings. In the early stages, the rings can stick and unstick, causing intermittent behavior.

Larry

lr172 said:

If the ring sticks, this tension is lost and the gasses from above cannot further force the ring against the cyl wall, resulting in reduced compression, reduced power and increased bloby.

Click to expand...

Thanks for the explanation. I understood the behavior of the rings but had not thought they could get "stuck". From your explanation, I can envision a number of resulting issues including the dirty oil I've observed.

Thanks for taking the time to outline the scenario to a "noob"

Temp issues on recent flight

On a recent flight back to Seattle from SoCal, I had #3 CHT significantly lower than the others. Probably 80 degrees or so. I am a RV7 with IO 360 and around a thousand hours. Slick mag on the bottom and Lightspeed Plasma 2 on the top. Good compressions at recent annual with cleaned plugs. 100LL all the time but am considering mogas.

Any thoughts if I should worry much? What should I look for first? Swap out the plugs before anything else? Champion plugs are ten years old. Time for Tempest? Automotive plugs on the EI side are about four years old. All plugs get cleaned and gapped once or twice a year.

Have a number of trips planned soon and want to get ready....