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CHT drops after a few hours

71459

Well Known Member
Very odd phenomenon - could use some help/ideas from the group.

Engine is a carbureted O-320-E2G (150 HP) on an RV-4. SMOH is about 1400 hours.

Engine starts right off. Run-up is fine, mag check excellent. Take off and climb-out is normal - engine runs very strong. Runs great for several flights/several hours, but eventually I notice the CHT on #3 is looking cold. I don't have an engine analyzer, so the temp on #3 is all I have to go on. After another 10 minutes or so the CHT drops away almost completely and the engine starts running rough - engine seems to be flaming out (for lack of better words). Make a beeline for the airport and hope it doesn't quit. Dangerous and nerve-wracking.

I'll take the plugs out or a good cleaning. Reinstall them and the engine runs fine again, but after a few hours the problem comes back again.

Compression of very good (high 70's). Borescope doesn't show anything abnormal. Plug resistance are all at out 1000 ohms - good readings. I'll check the timing later today.

Lots of acro flown, so carb does see zero-G a few times each flight. Thinking maybe a faulty carb. Hate to exchange it if it's not the cause.

Any help/ideas appreciated.
 
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Sticking valve? Lycoming valve wiggle test? Maybe marvel mystery oil in both the feul and oil, but I don’t think changing the carb is the answer.
 
Sticking valve? Lycoming valve wiggle test? Maybe marvel mystery oil in both the feul and oil, but I don’t think changing the carb is the answer.

Could be, but when I check the plugs, all 8 are equally sooty. That's what has me thinking about fuel delivery.
 
going to be hard to diagnose with that limited data. Need a better description of running rough. If CHT goes to almost ambient and engine is still running, that kind of points to an individual cylinder vs something system wide, like a carb issue. What is the power setting when the problems start? Full rich or leaned?
 
Could be, but when I check the plugs, all 8 are equally sooty. That's what has me thinking about fuel delivery.

sooty plugs can form during the idle phase before shut down and mislead you. Likely need to set your idle mixture to stop this as it is too rich. The fact that your idle mixture is off does not point to fuel problems at high power. If the carb does well on take off and climb, it seems unlikely it will just out of the blue start forcing you over rich later in the flight. Carbs tend to have few intermittent problems like this that appear out of nowhere and then fix themselves. An exception is fuel contamination that prevents the needle/seat from closing (causing a very over rich mixture) and that can self-correct if the debris dislodges itself. Sticking valves would make a lot of sense, but the presentation is not typical and makes me think not the best candidate. I do agree that a wobble test is worth the effort to rule it out if the engine has 400 hours or more on it.
 
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Too bad you don't have a FF gauge. One educated guess is a failing fuel pump. If the engine is not really rough, but just losing power (you referenced flaming out), this is a good place to start. These often fail with intermittent symptoms and can suddenly start pumping lots of air that slowly starves the engine of fuel. If fuel drops to too low of a level, causing a heat reduction, this will cause sooty plugs, as there is not enough heat to burn it off. A FF instrument would be very helpfull here to determine if the problem is due to too much or too little fuel. Soot on the plugs (assuming it is dry) generally does not create issues with combustion and thinking that cleaning them is not likely the reason that it runs fine again the next time.
 
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leaking intake gasket?

That was one of the first things I looked at. I did that because several months ago, I had exactly the opposite problem - CHT was running high (intake leak?)! Now it's falling away cold! Anyway, I installed new intake gaskets and new intake hoses. All torqued to spec.
 
What is the power setting when the problems start? Full rich or leaned?

I'll lean pretty aggressively - not long after takeoff, and ongoing during the climb-out. Nothing uncommon.

Power is varying - lots of acro. Seems like cylinder temp reads too cool even at high power. Power is then reduced for descent, that's when the temperature really plummets and roughness kicks in.
 
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If the carb does well on take off and climb, it seems unlikely it will just out of the blue start forcing you over rich later in the flight.

Right, but wondering if acro might be upsetting the float valve. Unlikely, I know..
 
Have you ever checked the exhaust valve clearance? Ever reamed the ex valve guides? One cold cylinder may be a sticking exhaust valve in that cylinder. I would not continue to run it or fly until this is solved. If it is a sticking valve you don't want to exacerbate the issue by having it contact the piston.
 
This sounds to me as it could be a sticking exhaust valve.
If there is a sticking exhaust valve this can go bad in the air.
Don´t fly before you know more. If you don´t find the cause, I would remove the valve cover on cyl 3.
After removing the valve springs, the exhaust valve should have a small play
in the valve guide. It should be easy to move is by hand.
If you find that the play is almost non existent or the valve is stuck, all exhaust valves have to be reamed. You don´t need to remove the cylinders to do this. The first cylinder takes more time.
Follow instructions in Lycoming SB.
Lycoming Publication SSP 1776.
Lycoming Service Instruction 1425.

Good Luck
 
Too bad you don't have a FF gauge. One educated guess is a failing fuel pump.

Just 2 weeks ago, I had a different issue, though maybe somehow related - the engine would not idle. I noticed a high fuel pressure reading - indicating the mechanical fuel pump was failing high, forcing the float valve open at low power. So just last week, I installed a brand new Lycoming pump. Flew the airplane 5 or 6 times - completely normal operation - engine idles perfectly, fuel pressure back to normal. Now, cold CHT and rough operation is coming back, but I see normal fuel pressures. Confusing..
 
This sounds to me as it could be a sticking exhaust valve.
If there is a sticking exhaust valve this can go bad in the air.
Don´t fly before you know more. If you don´t find the cause, I would remove the valve cover on cyl 3.
After removing the valve springs, the exhaust valve should have a small play
in the valve guide. It should be easy to move is by hand.
If you find that the play is almost non existent or the valve is stuck, all exhaust valves have to be reamed. You don´t need to remove the cylinders to do this. The first cylinder takes more time.
Follow instructions in Lycoming SB.
Lycoming Publication SSP 1776.
Lycoming Service Instruction 1425.

Good Luck

I think this is the next thing to try. Thanks for the info. Of note - OAT this time of year is very cold, so wouldn't be surprised the cylinders are never getting that hot anyway. Thinking is a very cold OAT at altitude might exacerbate a sticking valve.
 
Right, but wondering if acro might be upsetting the float valve. Unlikely, I know..

suppose anything is possible, but lots of guys do acro with carbs and don't see CHTs go really low. I could see acro messing things up while doing it, but should immediately resolve when acro stops. Float issues are most often persistent
 
Have you ever checked the exhaust valve clearance? Ever reamed the ex valve guides? One cold cylinder may be a sticking exhaust valve in that cylinder. I would not continue to run it or fly until this is solved. If it is a sticking valve you don't want to exacerbate the issue by having it contact the piston.

Yeah, I'm grounding it for now. I'm going to proceed with reaming the exhaust guides, cleaning up the stems.
 
I'll lean pretty aggressively - not long after takeoff, and ongoing during the climb-out. Nothing uncommon.

Power is varying - lots of acro. Seems like cylinder temp reads too cool even at high power. Power is then reduced for decent, that's when the temperature really plummets and roughness kicks in.

WheN this happens you need to aggressively lean and richen via experimentation to determine if the cause of roughness is due to overly lean or rich condition; Either can drops CHTs when excessive. There are installation issues that can cause a float to hang up and not move properly. This could result in either over rich or over lean and could see how excessive G's could cause it to start the cycle.
 
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Yeah, I'm grounding it for now. I'm going to proceed with reaming the exhaust guides, cleaning up the stems.

stuck valve issues would likely be more pervasive and not just limited to lowering the power for decent.
 
Yeah, I'm grounding it for now. I'm going to proceed with reaming the exhaust guides, cleaning up the stems.

When the engine is cold, pull the exh spring off and try to spin the valve iwth your fingers. If it moves freely, valve sticking is not the cause. They are tightest when cold. Many guys have to pound the valve out with a hammer and drift to ream the guides. This is why they call it morning sickness. cold valves stick causing roughness that often abates once the guide warms up and expands,. Not saying it can't happen after acro, but very odd that it performs great when cold and sticks when hot. Opposite of physics principles. I would stick with fuel mixture as the culprit.

Also pull the strainer out of the carb and check for debris. Failing pumps can shed a lot of ****. I just had a failing pump produce what looked like sugar as well as strands of rubbr and plugged up my injectors. 2 dead stick landings.:mad:
 
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stuck valve issues would likely be more pervasive and not just limited to lowering the power for decent.

Right, though the cylinder does cool off rapidly - which is normal and expected. It's just that it keep dropping, doesn't want to come back up when power is added, and the engine runs rough. Funny though, once on the ground, it seems normal again. That has me even considering carb ice.
 
Right, though the cylinder does cool off rapidly - which is normal and expected. It's just that it keep dropping, doesn't want to come back up when power is added, and the engine runs rough. Funny though, once on the ground, it seems normal again. That has me even considering carb ice.

The problem is that you do not know if it is one cyl or the whole engine. Very different diagnostic paths. 8 sooty plugs kind of points to a systemic issue, which brings you back to fuel delivery and likely too rich, though sooty plugs can be misleading.
 
When the engine is cold, pull the exh spring off and try to spin the valve iwth your fingers. If it moves freely, valve sticking is not the cause.

I've done thhs, many years ago. Yeah if I recall, if the value moves freely, there's not need to ream. But I'll check the procedure again.

I just had a failing pump produce what looked like sugar as well as strands of rubbr and plugged up my injectors. 2 dead stick landings.:mad:

Good grief!
 
I've done thhs, many years ago. Yeah if I recall, if the value moves freely, there's not need to ream. But I'll check the procedure again.



Good grief!

NEVER discount the problems that can occur downstream of a failing pump. Definately pull the screen out of the carb and check.
 
The problem is that you do not know if it is one cyl or the whole engine. Very different diagnostic paths.

Agree - I only have one CHT gauge, but all cylinders get sooty.

One additional thing.. last flight, once on the ground, I taxied to the runup. Ran it up and the CHT stayed cool. Then I leaned it out aggressively and the temp shot up. I was surprised how fast it changed. This is why fuel delivery (carb problems/too rich) has been first on my mind.

On the other hand, this happens when the cylinder temp gets cold (power reduction in very cold outside air). Rough running, power loss, and cold temps together are the classic signs if sticking valves. I'll take out all 4 exhaust valves, inspect, clean, and ream the guides this weekend.

Thanks to everyone for their input - appreciated!
 
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Well I removed the #3 exhaust value - looking for signs of sticking. The valve slid out easily, however it had quite a bit of deposit material on the stem. I wonder if that's normal after 1000 hours, or if it's normally bright polished steel and any deposits at all are a sign of trouble? I also reamed the guide as well - some resistance encountered and a small amount of deposit material removed, but really only a tiny amount.

But, now I'm struggling terribly trying to get the valve back in. Has anyone done this procedure successfully? Is there a tried and true method of getting that valve to line up and slide back into the guide? Special tools required or fashioned? - Thanks in advance for any help with this.
 
I've had good luck with one of those 3 jaw pickup tools. Grab the valve close to the head and use the top of the piston to square up the valve, then have an assistant rotate the prop gently in the direction the makes the piston go up.
As the stem gets close look through the guide and try to match it up, use gentile piston pressure to get it started.

If after a few tries this will not work you can remove the exhaust and reach through the port and put the valve in the guide that way.
 
I've had good luck with one of those 3 jaw pickup tools. Grab the valve close to the head and use the top of the piston to square up the valve, then have an assistant rotate the prop gently in the direction the makes the piston go up.
As the stem gets close look through the guide and try to match it up, use gentile piston pressure to get it started.

If after a few tries this will not work you can remove the exhaust and reach through the port and put the valve in the guide that way.

Great! Thank you for the ideas - I'll give it a go tomorrow.

Would you have a link to the pickup tool that you have? I have one, but it seems too flimsy to do the job.
 
I don't think single cylinder issue points to a 'common problem' like a carb issue but stranger things have happened.

The sticky valve was my first thought but when the guide is clogged it tends to show up first on the first climb out of the day when the valve is heating up quicker than the head/guide. When I had mine that's what it would do, it never gave another problem until the next start. If you aren't seeing that it adds to the mystery.

To eliminate the valve the rope trick and dropping the springs off doesn't take long and mine was clearly stiff. I cleaned the stem and reamed the guide with the cylinder in situ as demonstrated on many YouTube vids. One armed wallpaper hanging skills are handy to get the valve out the plug hole and back in the guide afterwards.
The clean up kept me flying till I was ready for the cylinder removal though mine was being caused by a glazed cylinder oiling things up. It could be seen on the plug, something else you're not getting though I suppose it could be pure lead. Do you fly aero's full rich?

Later after blasting the pistons on an intervention at 1000 hrs (to hone all 4) the imprint of the valve on the piston crown was there. Thankfully a parallel valve engine.

You can get an idea of the wobble without all the gear though the acceptable wobble is actually very big so worth measuring before any deeper intervention. Though his may not be the issue at all, don't you love aero engines?

Good luck, Regards, Clive
 
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