What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Engine Miss

Len

Well Known Member
On flight from Florida about 30 min from destination and in descent the engine seem to stumble a few times. Could see nothing on CHT/EGT. Landed and taxied in .. no issues ... run up before shut down ... no issue.

Return flight today ... couple of stumbles climbing to altitude then 3 hours of cruise and nothing. Noticed the #1 CHT was well below others in temp and EGT will above others (100 degrees). I was not able to lean as much as normal.

Landed about 30 min from home and dropped passenger. From there .... CHT was back up normal with others and EGT was back down ... and I could lean as normal.

I had something similar a few years ago that turned out to be an exhaust valve? Thoughts?
 
Carb or injected?

If injected, maybe dirt in an injector?

Either one, intermittent sticking exhaust valve?

Either one, flaky plug or plug wire?
 
Fuel Injected. Checked plugs and wires ... can't find any issue with them.

BTW ... engine has 256 hours on it since new.
 
Noticed the #1 CHT was well below others in temp and EGT will above others (100 degrees). I was not able to lean as much as normal.

Were you LOP when you observed the above condition? A significant reduction in CHT with an increase in EGT points to exhaust valve leakage. However, you need to evaluate how much CHT reduction and where you were are relative to peak EGT. If running 75* ROP and seeing a 30* CHT reduction/ 100* EGT increase, I would say debris in the injector or spider. If running 20* LOP, I say you have a leaking or sticking exhaust valve. Once your LOP, there is no way that further AFR reduction can increase EGT and rules out fuel reduction, though not excess fuel. Given the intermittency, I would be looking at a sticking valve and this is pretty common on the Lycomings, though it often presents cold and goes away when warm. However, that is not gospel. Other failure modes happen and it typically starts with intermittent appearances.

Retarded timing can reduce CHT and increase EGT. However, i struggle to see how a mag can deliver different timing on one cylinder relative to the other 3. Spark failure due to plugs or wires can be eliminated, as a missing spark will not increase EGT. Intermittency pretty much rules out cam related issues that can cause this symptom.

Larry
 
Last edited:
A reduction in CHT and a corresponding rise of 60-100dF rise in EGT is a classic sign of a failing spark plug.

Do an in-flight LOP mag check and it will tell you which one. Or replace both.

If you were LOP at the time, you could also have an intake gasket leak. The Lycoming gaskets have a propensity to leak at 250 hrs. Ask me how I know.
 
Last edited:
I changed plugs while trying to chase down the issue along with the plug wires for this cylinder. These plugs have probably 25-40 hours on them.

I was running ROP when this occured.
 
How old are the wire harness? 15 YEARS is the max service life that slick / champion say the insulation is reliable (between the core and the ground shield)

I just had an older than dirt harness fail on a certified AC. Thought I had a plug issue on one cylinder to start, but a new harness was the cure.

Also the intake tubes can be loose or intermittently leaking at the oil sump ends as well as the flange gasket/head end.
 
I changed plugs while trying to chase down the issue along with the plug wires for this cylinder. These plugs have probably 25-40 hours on them.

I was running ROP when this occured.

Recommendation above stands. Mostly likely cause is either debris in the spider or debris in the injector for #1.
 
Last edited:
You could have a cylinder that went LOP or close to peak EGT, this would give you a rise in EGT, and if close to peak a decrease in CHT.

Remember CHT starts dropping after about 40dF ROP, so you have either some very minor blockage in the injector or spider. Or, and this is quite likely, a leaking intake gasket or rubber coupling. This would do it too.
 
Perhaps you should check the "dry tappet clearance" -- an excessively tight valve train on one cylinder was the cause of an intermittent misfire on one of my O-360s.
 
Last edited:
Return flight today ... couple of stumbles climbing to altitude then 3 hours of cruise and nothing. Noticed the #1 CHT was well below others in temp and EGT will above others (100 degrees). I was not able to lean as much as normal.

Was the low CHT/ high EGT during the stumbles or through the whole 3 cruise flight? Intermittent problem points to debris, constant problem points to leak in the intake or blockage.
 
Back
Top