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looking for Opinions: Low CHT

lr172

Well Known Member
Recently, I have noticed that #2 is warming up, after cold start, about 30* cooler than the other cylinders and this is a new development. I can't find anything related to the baffling that has changed. At higher power settings, the gap closes up to at least close to where I remember it being, relative to the others.

I did a compression check and got 60, where last annual it was 74. Pulled the rocker cover and found the inner intake spring in 5 discrete pieces. Replaced the spring and a few light hits with the hammer broke loose whatever build up I had on the intake valve/seat and compression came back to 72. Presumably the lighter spring force allowed debris to accumulate on the valve or seat. I further presumed the reduced compression was causing the slower warm up.

It seemed to improve a bit on the test flight, but now back to lagging 30* behind the others during warm up. I haven't yet tested compression again to see if it is debris on the valve again.

Looking for input from the brain trust on other potential causes. Recently did a wobble test and all exh valves are in the range.

Larry
 
More info please

How many hours and years on the engine? Injected or Carb? Condition of plugs?
 
How many hours and years on the engine? Injected or Carb? Condition of plugs?

FI - 750 hours since overhaul in '15. Plugs are good. Get a solid EGT rise on both ignitions during runup and EGT is a bit on the low side and would expect a higher EGT if one plug wasn't firing. Good point though. Probably worth checking the plugs to be sure it isn't creating intermittent issues. Plugged injector doesn't make sense, though some debris in the spider could. Haven't checked that yet.
 
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Guessing here,
Tapping the valve may have just evened out the carbon deposit on the seat. You might consider lapping that valve to get proper seating again. At same time you can check if for guide wear too, if the spring fragments side loaded the stem.
 
Lower CHT and EGT would indicate that cylinder is producing less power than the rest. You mentioned you performed the exhaust valve wobble test; maybe check the intake valve lift just to validate any valve train issues are confined to the cylinder rather than something to do with the cam or lifter.
 
Larry, it would not have to be a plugged nozzle, but could be a distorted spray pattern at the low pressures. I might just clean it and see.

If not, IIRC, you have stated previously that you found some debris in the spider sleeve (or "a" sleeve . . ), and that could affect idle FF for a nozzle at idle but not at higher flows.

Edit: I see you already came to this conclusion.
 
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Larry, it would not have to be a plugged nozzle, but could be a distorted spray pattern at the low pressures. I might just clean it and see.

If not, IIRC, you have stated previously that you found some debris in the spider sleeve (or "a" sleeve . . ), and that could affect idle FF for a nozzle at idle but not at higher flows.

Edit: I see you already came to this conclusion.

Good idea Bill. I will clean out the injector for safe measure. When I posted about spider blockage, it was a friend's plane, not mine and it was brand new, so not surprising to find a bit of manufacturing debris. But I am thinking that pulling apart the spider is probably a good idea.

Larry
 
Guessing here,
Tapping the valve may have just evened out the carbon deposit on the seat. You might consider lapping that valve to get proper seating again. At same time you can check if for guide wear too, if the spring fragments side loaded the stem.

That is a good point. This weekend I will do another compression check. WHile I didn't do the wobble test on the intake valve, I did observe free movement of the valve when the spring was off. That said, you raise a good point about the binding spring abnormally wearing the guide. One of the spring pieces was no longer round, but unsure it if was one valve movement that bent it or thousands. I certainly hope that is not my issue.
 
Lower CHT and EGT would indicate that cylinder is producing less power than the rest. You mentioned you performed the exhaust valve wobble test; maybe check the intake valve lift just to validate any valve train issues are confined to the cylinder rather than something to do with the cam or lifter.

Will take a look at that. However, if I had a reduced valve lift I would expect the low CHTs to follow into the full power range. Maybe that is a flawed assumption. Once in high power regime, the 1&2 CHTs are the same and 10* cooler than 3&4 and it has been that way for a long time. It is only the lagging warm up that is new. Whenever behavior changes, I am very anxious until I can understand why it has changed.

Larry
 
Larry, what was the end result of this problem? I have cylinder #1 that has recently started running about 15-20* CHT cooler than usual, and the engine doesn’t feel “smooth” anymore. Mag checks on the run up and in the air seem fine (~100rpm drop on either mag). EGT’s seem about where they normally are.

About 10-15 hours ago, I fouled a plug in the air, and when I removed it, it had a lead ball, I cleaned and reinstalled, rotating all spark plugs also, but it doesn’t seem to have been the same since then.

Also, I think I might be showing signs of morning sickness, but I have never seen it first hand, so I am unsure what it sounds like. I don’t personally have a wobble test setup, but I might be able to russle one up.

Kind of looking for ideas of where to start or what to check before just handing it over to the mechanic.
 
Larry, what was the end result of this problem? I have cylinder #1 that has recently started running about 15-20* CHT cooler than usual, and the engine doesn’t feel “smooth” anymore. Mag checks on the run up and in the air seem fine (~100rpm drop on either mag). EGT’s seem about where they normally are.

About 10-15 hours ago, I fouled a plug in the air, and when I removed it, it had a lead ball, I cleaned and reinstalled, rotating all spark plugs also, but it doesn’t seem to have been the same since then.

Also, I think I might be showing signs of morning sickness, but I have never seen it first hand, so I am unsure what it sounds like. I don’t personally have a wobble test setup, but I might be able to russle one up.

Kind of looking for ideas of where to start or what to check before just handing it over to the mechanic.

In my case, it was isolated as a slow warm up CHT and once warm and full power, CHT was normal ans behaved as it did for 800 hours. FYI, it is still doing it and never figured out why. If yours is a CHT that is now 10-20 lower than previously at full power or cruise power, that is different and points to a drop off in heat produced during combustion and therefore power reduction. First thing to check is for a partial restriction in the injector (will raise CHTs to a point when ROP and drop them when the leaning is substantial or when LOP - think about how mixture moves CHT). Usually this drops CHT a variable amount and will also drop EGT as well. Unfortunately this could be mixture, ignition or compression. A sticky exhaust valve can give variable results. It can stick a little or a lot, It can mostly move but not fully, etc. Could also be a piece of debris that is preventing a valve from fully closing, but a compression test should find that pretty easy. One plug not firing can cause this, but should be obvious during a run up. Sometimes ignition only misbehave at higher MAPs (the higher the MAP, the more energy required to jump a gap) so an inflight mag check would be helpfull.

The onset of a lack of perceived smoothness would imply that one cylinder is producing less power than the others.

It's been a long time since I had two mags, but 100 RPM drop seems like a lot. How old are the mags? Maybe others with more mag experience can chime in, but this is suspicious to me and would spend more time on ignition issues. During the mag checks, you should be observing the EGT rise on each cylinder, looking for differences in the rise across cylinders. Any stand outs should be further explored.

To start

remove injector and inspect and clean
do an in flight mag check at full power, while observing each individual EGT rise.
do a compression test on offending cylinder
check for FOD in baffling or cyl fins on offending cyl
If able, wobble test on exh valve in offending cyl. For these purposes, you can get away without the test tool. If the valve is sticking due to a tight guide, you will easilly feel the tightness once the spring is off. Many guys have to whack it with a hammer to get it to move. If you can twirl it with your fingers and perceive a very loose movement, it is quite unlikely that as sticking valve is causing your issues. If it doesn't appear loose and easy to move, you have possibly found a cause or that reaming is in your future.

If you want a wobble test tool, I can send you mine to borrow or can make you one for half the price at ACS. I made a couple for other members here.

Larry
 
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Thanks Larry, this gives me a few ideas to start with.

I forgot to mention that it's an O-360-B, so no injectors to clean. I initially suspected an induction leak, but I couldn't find anything loose or out of place on visual inspection. I suspected the induction leak because when I lean to peak, #1 would start rapidly falling in temp compared to the others, but if I enriched, it would come back up, albeit to it's 15-20* cooler than before.

Lycoming says 175rpm max drop for a mag, 50rpm difference. Left and right on runup give right around 100rpm drop, so no real difference, and less than the tolerance. My mags are older, but not a ton of hours since last 500 overhaul. Inflight mag checks seem consistent also.

I should note that this all seemed to have started around the time about a month/half ago that I was out flying when it was pretty cold. I did some touch and goes, and on the last one, I got engine roughness, and did an inflight mag check and realized I had fouled a plug. I landed, pulled the plugs, one was lead-ball fouled. I cleaned and rotated top/bottoms and reinstalled. I _think_ this is about the time it hasn't _seemed_ to run smooth since. Perhaps valve fouling also?

So, I will start with:
Documenting better CHT/EGTs in flight, during rich and lean (I need to find my JPI data box so I can actually download the data)
Do a compression test, might find something obvious
FOD and baffles have been checked, I see no change there
Do a poor mans wobble test/valve freeness/at least remove the valve cover and have a look test

I will start with these and see where that gets me. Thanks for the help!
 
I should note that this all seemed to have started around the time about a month/half ago that I was out flying when it was pretty cold. I did some touch and goes, and on the last one, I got engine roughness, and did an inflight mag check and realized I had fouled a plug. I landed, pulled the plugs, one was lead-ball fouled. I cleaned and rotated top/bottoms and reinstalled. I _think_ this is about the time it hasn't _seemed_ to run smooth since. Perhaps valve fouling also?

Always best to start with the most logical steps. I would examine the ignition closely on that cylinder if the problem started immediately after messing with it. Could have damaged a plug, accidently closed a gap, messed up the spring int the terminal cap, left the plug loose (low compression), compromised the wire, etc. If one cylinder is running on one plug, I would expect CHT to drop about the amount you are seeing, but it would also raise EGT.

Lead balls on the plugs are common and not an indication that there is lead fouling elsewhere. Looking at the valves does nothing. Need to pull the spring off and feel the looseness or tightness. This will require a method to hold the valve closed (rope or compressed air) and a tool to compress the spring (home made or purchased - I made one from rebar, but requires welding, but creative grinding on a wrecking bar will do.), so you can get the keepers off.
 
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Do a compression test, might find something obvious

Definately. If a piece of debris got lodged into the valve face or seat, it will drop CHT a bit (due to reduced compression ratio) without much change in EGT. A compression test will dentify it and tell you which valve.
 
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