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Low compression on one cylinder of my Lycoming IO-540-C4D5D engine..anything to worry about?

My plane has just had its annual. Its engine is now 12 years old, but low time – I fly about 40-50 hours each year.

Cylinder number 1 of my engine showed compression of 58/80 a year ago and this year the same cylinder showed 60/80. The other five cylinders all show high 60s or low to mid-70s in terms of their compressions. The engineer thinks it’s the inlet valve on number 1 that’s the problem and nothing much to worry about but he can fix it with some work and a reasonably sized bill – it’s a day or two’s work. (I had a worse cylinder, reportedly with the same inlet problem that was repaired a few years ago and is fine now.)

My EDM 700 also shows cylinder number 1 has lower CHT than the other cylinders. The engineer thinks that the probe or the EDM unit may be at fault. I think that the obvious answer is that cylinder 1 with the low compression is also operating at a lower temperature than the others. Mind you, I’m not an expert and I don’t know if a slight leak on an inlet valve would cause a lower head temperature or a higher one.

Do I need to worry about cylinder 1? What would others here do?

Howard

Flying a TB20 out of EGTR
Elstree (EGTR), United Kingdom

Howard
No need to worry but I think that you should repair it mainly due to power lose and Elstree’s short runway. I used to fly a 2700h engine, one day when I needed the power it wasn’t there, anemic engine can get you, longer T/O run, poor climb, higher fuel consumption etc. If Riad suggests to do it I would listen to him, he is a very competent and knowledgeable engineer, he can be a pain but he is very good.

Last Edited by Ben at 01 May 11:44

I would stick a borescope in there and make sure it isn’t the exhaust valve. A leaky exhaust valve is supposedly more dangerous, not because it is leaking but because something is wrong with it.

To check the EDM, does it read the same (and correct, within about 2-3F) temperature for all 12 probes when you turn up in the hangar in the morning? If it does, and the #1 CHT probe is not physically loose, then it’s not likely to be the EDM.

Also it’s fairly normal for #1 or #2 to be cooler than the others because it gets loads of air:

It is unlikely that compression affects engine performance. If it did, a Cirrus would never get off the ground because Conti engines have terrible compressions. The burning mixture doesn’t have the time to significantly escape through such a leak. What a low compression gives you is a warning that something isn’t right.

60/80 isn’t itself unairworthy but Lycoming have guidance on the subject:

If it was me I would get it fixed, but much depends on the history. If your #1 has been 60/80 for ages, that’s one thing. But mine have always been 77/80 or better, every time since 2002, so if I got 60/80 I would be very concerned.

BTW I fixed the engine P/N in the title because if you had the C4B5 you would be a rarity

Administrator
Shoreham EGKA, United Kingdom

Thanks @Peter.

@Ben, my plane is not maintained at Elstree. I had a good relationship with an engineer elsewhere before I started hangaring my plane at Elstree, so kept on with that other engineer since. I therefore only know Riad a little and don’t use him. I do hear good reports about him and his team’s work.

Flying a TB20 out of EGTR
Elstree (EGTR), United Kingdom

There is no power loss, just because one cylinder has a slightly lower compression. Even if all cylinders were 50/80 the engine would develop the same power.

Howard wrote:

The engineer thinks it’s the inlet valve on number 1 that’s the problem and nothing much to worry about but he can fix it with some work and a reasonably sized bill

There is no reason to “think” that something is the problem, that is pretty poor advice. He has all tools available to tell you with certainty where the cylinder loses air. Once you know that, you’re in a much better position to decide whether and what to do.

Forget about leak test results and engine power, they are not linked.

One always hears about leaky exhaust valves, but I am surprised than one can have a leak in an intake valve. If so, having combustion gases leak into the intake manifold does not sound too good.

LFPT, LFPN

@achimha – I think I am misrepresenting the engineer. He is extremely experienced and well respected. If he thinks it is the inlet valve, I’m sure he must have undertaken a procedure to confirm that before telling me that that is what he thinks it is.

Flying a TB20 out of EGTR
Elstree (EGTR), United Kingdom

What kind of CHTprobe connection does the EDM700 use on your no. 1 cylinder? My CHT probe cable is running to the old factory CHT instrument. The EDM700 is connected by some backpack type to that probe. As a result the EDM 700 shows a 70°F lower value than the other cylinders. This is also described in the EDM700 pilot’s guide.

Last Edited by nobbi at 01 May 14:53
EDxx, Germany

My CHT probe cable is running to the old factory CHT instrument. The EDM700 is connected by some backpack type to that probe. As a result the EDM 700 shows a 70°F lower value. This is also described in the EDM700 pilot’s guide.

AFAIK, that is not the correct way. Contrary to what has been posted the other day on the Socata owner group, I was told by Socata that they certified the EDM700 as “primary” for CHT so there is no longer any other CHT instrument (not so on the TB21 which is why that retains the factory CHT+EGT). So the original CHT+EGT gauge should not be present (and measuring a single cylinder is rather useless anyway). That is how my aircraft came from the factory.

What I don’t know is whether the above applies to all TB20 airframe serial numbers…

The EDM700 is not STCd by JPI as “primary” but it is within the power of a Type Certificate holder to certify anything as “primary”.

If Howard still has the original CHT gauge, that could however the explanation for the lower EDM #1 reading…

Administrator
Shoreham EGKA, United Kingdom
18 Posts
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