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NEED some CHT advice

blaplante

blaplante

Well Known Member
OK, I've been helping on this 7A for a while now. Been over the baffling multiple times, still find the little gap here and there. It does have the 'washer trick' installed on #3, which helped a bit. Here's a takeoff on an 80F day.

OK #3 runs hot (433F peak here). Engine (new Lycoming) is using very little oil, Phase 1 is now just completed so really broken in by now. O-360-A1A (carb).

Odd EGT signatures on the take off. Note how #2 droops, and then rises. Thoughts? No louvers installed ... yet. If we can't figure this out, that's probably the next step.

The intake pipes hardware were checked for security a while back.

Things are OK in cruise.

Savvy analysis image:
https://photos.app.goo.gl/s1P5zPgJjydvx1zbA
 
I’d say that #3 CHT probe or its wiring is faulty.
No way the CHT ΔT is that rapid up and down. I don’t even think preignition would do that. But maybe worth checking for a damaged plug in that cylinder.
Looks erratic in other parts of the plot too.
 
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I doubt it is wiring, as it behaves pretty normal. #3 is higher at idle, so the higher climb temps are to be expected. However, near the peak you observe some type of disturbance. Temps rapidly move up and down a few times, then it jumps. After a bit of high temps, you get another disturbance and it falls and eventually matches the others. EGT does not follow this trend. Struggling to guess what that might be, though moderate intermittent detonation seems possible, but unsure if the EGT would behave in the way that it did. The higher EGT implies a somewhat leaner mixture in #3 and not uncommon for a carb (3 & 4 are often the leaner ones).

There are more up down disturbances as the power comes back and that would not be detonation, so maybe it is an an issue with the probe or a bad crimp. The up down swings are withing 5* so it seems more likely that the temp is really moving vs wiring. I would expect a loose crimp to show bigger swings, but don't have a lot of experience with failing probe wiring.
 
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Probes

They wear out, spare one on the shelf will never go bad. Replace probe with new or move existing to different cylinders and see if issue follows. In my experience, weirdness in the likes of yours have been probes. Easy cheap troubleshoot, no change, then move on.
 
Easy probe check

Before you change probes with new ones (at >$130 each, "cheap" has some perspective ;)), you can prove whether it's the probe or something else by simply swapping the probe(s) you suspect with each other, or any others you think are working fine. If the symptoms continue as before on the original cylinders, then you know the probes are fine and then can move on to other troubleshooting steps. If the symptoms stop, then you know if it was connection related and you've resolved the issue. If the symptoms continue on following the probe to the new location, then you know it is the affected probe - replace it.

The added benefit of this process, aside from peace of mind, is that you often resolve probe issues by simply cleaning up the connection. Ensure you have good star washers between the probe terminals; this provides bite between the two terminals and a solid electrical connection.

Erratic probe operation generally falls into three different categories: faulty indicator (highly unlikely using an EFIS), internal wiring fault or external voltage influence. Note that thermocouples generate millivolts, so any resistance added via the connector will cause issues with the unit's operation.

You can eliminate most probe faults by ensuring a good connection and that the probe wires are insulated and protected from external influences, such as spark plug wires.
 
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Yep, the very first thing we did at the airlines was the swap gauges to see if the problem followed the gauge, in your case with one engine it would be probes.
 
lr172 said:
I doubt it is wiring, as it behaves pretty normal. #3 is higher at idle, so the higher climb temps are to be expected. However, near the peak you observe some type of disturbance. Temps rapidly move up and down a few times, then it jumps. After a bit of high temps, you get another disturbance and it falls and eventually matches the others. EGT does not follow this trend. Struggling to guess what that might be, though moderate intermittent detonation seems possible, but unsure if the EGT would behave in the way that it did. The higher EGT implies a somewhat leaner mixture in #3 and not uncommon for a carb (3 & 4 are often the leaner ones).

There are more up down disturbances as the power comes back and that would not be detonation, so maybe it is an an issue with the probe or a bad crimp. The up down swings are withing 5* so it seems more likely that the temp is really moving vs wiring. I would expect a loose crimp to show bigger swings, but don't have a lot of experience with failing probe wiring.
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I don’t think it’s detonation. That’s practically impossible in a NA lycoming unless you’re running silly lean at wot and already have CHTs approaching redline and perhaps some gross error timing advance as well.
It’s clearly flowing enough fuel at 16+ gph and the EGTs would suggest that it’s plenty rich.

The big moves up and down on the CHT are more like 10-15deg practically instantaneously. With no other changes. That’s v suspicious.
 
Regarding the strange CHT indications, are your CHT probe wires run together with an ignition lead, by chance?
 
00Dan said:
Regarding the strange CHT indications, are your CHT probe wires run together with an ignition lead, by chance?
Click to expand...

Lots of good suggestions. I didn't wire this plane, so I don't know if the star washers were properly installed or not. And I don't recall the wire routing. It wouldn't be the first thing that wasn't tightened properly on this bird. We just went through round 2 of 'autopilot servos not online' due to loose wires. Not to mention the 4 or 5 pitot static leaks I've chased down.

In the past the CHT readings were relatively well behaved and so I completely missed the glitchy readings in the one I posted. Another set of eyes!!!
 
Richard Connell said:
I don’t think it’s detonation. That’s practically impossible in a NA lycoming unless you’re running silly lean at wot and already have CHTs approaching redline and perhaps some gross error timing advance as well.
It’s clearly flowing enough fuel at 16+ gph and the EGTs would suggest that it’s plenty rich.

The big moves up and down on the CHT are more like 10-15deg practically instantaneously. With no other changes. That’s v suspicious.
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I don't think it is detonation either, but I promise you that it can happen to a lycoming at 16 GPH. Many factors that go into that beyond fuel flow and brand. However it is a possibility that should not be ignored in troubleshooting. The fact that it also occurs in lower power regimes almost gaurantees that it is not detonation and I referenced that. quiick movement is highly suspicious, but it is taking several seconds to make the move so it cannot be assumed. I left the cowl plugs in once and am here to tell you that the CHT needle is capable of moving moving QUITE rapidly when the conditions are ripe. I went from 260 -> 410 from the time I went WOT to about 500 AGL. That is 150* in about 10 seconds. Stated another way, 15 degrees per second temp rise. From my view, the OP's CHT is only going up and down 5-10* in the high power area of the graph and it is taking 3-4 seconds for each full up down swing. From my view, there is 30 seconds between each vertical bar.

To the OP - I had something similar on my 10, though the swings were MUCH larger (100+ degrees), and it turned out to be the CHT wire being too close to the spark plug wire.
 
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Richard Connell said:
I don’t think it’s detonation. That’s practically impossible in a NA lycoming unless you’re running silly lean at wot and already have CHTs approaching redline and perhaps some gross error timing advance as well.
It’s clearly flowing enough fuel at 16+ gph and the EGTs would suggest that it’s plenty rich.

The big moves up and down on the CHT are more like 10-15deg practically instantaneously. With no other changes. That’s v suspicious.
Click to expand...

If the OP confirms fuel type and CR (if he did, I didn't see it), this could easily be put to bed. If he's burning Mogas especially with tall pistons, I'd be finding a borescope.
 
I did find the EGT and CHT wires co-bundled with the spark plug leads.
That has been fixed today.

The EGT/CHT connections are all crimp quick (spade type) connectors. Not the best for sure.
 
blaplante said:
I did find the EGT and CHT wires co-bundled with the spark plug leads.
That has been fixed today.

The EGT/CHT connections are all crimp quick (spade type) connectors. Not the best for sure.
Click to expand...

Indeed - the wire is so small it's hard to crimp it properly. I had a few of mine come loose, and then I bought some of the screw connectors for the next time it happens - and thankfully it has not happened again.

https://www.aircraftspruce.com/catalog/inpages/eioverlapolc-1-10-05470.php
 
Yep, those set screw connectors work very well, I make sure the two connectors are touching inside. ECI supplies them with their probes.
 
rv8ch said:
Indeed - the wire is so small it's hard to crimp it properly. I had a few of mine come loose, and then I bought some of the screw connectors for the next time it happens - and thankfully it has not happened again.
Click to expand...

Semi-pro tip on crimping small wires in an 18-22 awg connector. It is possible to crimp two 22 awg wires into an 18-22 awg connector. So.....in the case of the thermocouple wires where you only have one wire coming into the connector here's what I do. Strip the wire twice the length needed, then fold the bare wire stub end in half on itself. Now, crimp the folded wire into the connector. These will not pull off (assuming using a PIDG crimp tool....NOT the cr*ppy one from the auto parts store.)

Not really a fan of screw on (clamps down on the wire) electrical terminals in a high vibration environment.
 
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