CHT differential greater than 100 degrees in cruise

[cferr59]

Hello,

I have a Lycoming O-360-A1D in a 1965 Mooney M20C with fine wire plugs, a Surefly, and a recently 500 hour'd magneto.  There has always been a 90-100 degree spread between CHT3 and CHT4.  However, it seems like after installing the Surefly, it has increased to 130 degrees.  This has me a bit concerned because one cylinder can be over 400 and the other on around 270.  Opening the cowl flaps brings the hot cylinder down to a good range, but makes the cool cylinder entirely too cold.

I went over the baffling and don't really see what the difference is between the 2 sides of the engine.  I did note that the oil cooler is immediately below cylinder 2.  I am wondering if this design reduces the pressure differential below the engine on the left side.  The engine has about 600 hours on it and good compression on all cylinders.   Leaning method is lean to rough, enrich to smooth.  The probes are all bayonet type probes.  I have an ARI cowl closure mod.

Any thoughts on how to even out the temperatures?   What CHT spreads to other C owners see? 

I don't think there is a problem with the Surefly per se, but I believe the issue is related to unevenness in fuel distribution and/or cooling airflow combined with the fact that the engine can now be leaned a bit further. 

Example from recent flight prior to opening the cowl flaps (I usually don't let it go over 400, but I was seeing where it would settle):

Cylinder - CHT - EGT

1 - 350 - 1476

2 - 385 - 1406

3 - 278 -1406

4 - 409 - 1364

After Surefly install:
https://apps.savvyaviation.com/flights/4544346/93eb250b-62ad-4d6e-91b9-76400adc5144
https://apps.savvyaviation.com/flights/4544357/df17981f-3b70-4b88-b3ea-acd357f1cd09

Before Mag 500 hour and Surefly install:
https://apps.savvyaviation.com/flights/4488319/861ac80c-28d1-4906-8142-3b1df55b6e91
https://apps.savvyaviation.com/flights/4353026/553c5162-3307-48d9-be54-563d2dd78636
 

[David Lloyd]

Any chance the #4 CHT probe is a spark plug gasket probe or is it the same bayonet style probe as the others?  A spark plug gasket probe will read higher than a bayonet style probe.

Are your CHT leads long enough to swap between a couple cylinders to see if the the cylinder really is hot, or is it the probe?

Is the Surefly set for fixed timing or advance timing?  If advanced, are these flights at an altitude that would reflect the timing advance?

I'm in the same boat with a carb engine,  EGTs and CHTs all over the place.  Change altitude, throttle, mixture and to lesser extent RPM; everything changes and not exactly in lockstep with each other.  Down low into the wind, up high with the wind will produce two charts that look like two different airplanes.

[Jblanton]

Intake/Exhaust leak is what I guess.  Also check the inner cylinder baffle on the bottom.  It can move, or be installed incorrect.  Make sure tie rods are intact pulling #4 rear baffle to cylinder head. I suggest using fixed timing on the surefly with the carburated A1D, and the pos plenum.  

 

Jeff

[cferr59]

12 minutes ago, David Lloyd said:

Any chance the #4 CHT probe is a spark plug gasket probe or is it the same bayonet style probe as the others?  A spark plug gasket probe will read higher than a bayonet style probe.

Are your CHT leads long enough to swap between a couple cylinders to see if the the cylinder really is hot, or is it the probe?

Is the Surefly set for fixed timing or advance timing?  If advanced, are these flights at an altitude that would reflect the timing advance?

I'm in the same boat with a carb engine,  EGTs and CHTs all over the place.  Change altitude, throttle, mixture and to lesser extent RPM; everything changes and not exactly in lockstep with each other.  Down low into the wind, up high with the wind will produce two charts that look like two different airplanes.

Surefly is in advanced timing mode with base timing at 25 degrees BTDC.  I typically run 21.5 inches at 2500 RPM which should have some advance.  I noticed that the engine can be leaned .5-1 gph more with the Surefly installed at that power setting.

The probes are all the same type.  I guess it is a good idea to swap the probes around and make sure I am  looking at good data.  They do show even temperatures prior to starting the engine.

I'm tempted to try different combinations of RPM and MP to see if any change the behavior.  

[David Lloyd]

I did have an EGT probe in another plane that read same as others before start up, seemed to lag behind others under power and about 200 degrees less than others during cruise.  Got tired of wondering, replaced the probe and enjoyed normal indications thereafter.

[jaylw314]

40 minutes ago, cferr59 said:

Surefly is in advanced timing mode with base timing at 25 degrees BTDC.  I typically run 21.5 inches at 2500 RPM which should have some advance.  I noticed that the engine can be leaned .5-1 gph more with the Surefly installed at that power setting.

The probes are all the same type.  I guess it is a good idea to swap the probes around and make sure I am  looking at good data.  They do show even temperatures prior to starting the engine.

I'm tempted to try different combinations of RPM and MP to see if any change the behavior.  

Do you have a primary certified engine monitor like the EDM 900, i.e. you have no more factory CHT gauge?  That's the only way all four CHT probes would be identical.

If not, you probably have bayonet probes in all 4 cylinders, but the #3 cylinder is the factory bayonet probe that goes to the factory CHT gauge.  your engine monitor would use a ring probe that either fits under the factory bayonet probe or under the spark plug.  It is not uncommon for a spark plug probe to run 50-75 degF hotter OR cooler depending on the installation.  The bayonet probe is significantly better, my guess is +/- 25 degF.

[cferr59]

1 minute ago, jaylw314 said:

Do you have a primary certified engine monitor like the EDM 900, i.e. you have no more factory CHT gauge?  That's the only way all four CHT probes would be identical.

If not, you probably have bayonet probes in all 4 cylinders, but the #3 cylinder is the factory bayonet probe that goes to the factory CHT gauge.  your engine monitor would use a ring probe that either fits under the factory bayonet probe or under the spark plug.  It is not uncommon for a spark plug probe to run 50-75 degF hotter OR cooler depending on the installation.  The bayonet probe is significantly better, my guess is +/- 25 degF.

Yes, I have an EDM-930 and the factor guage is no longer there.

[cferr59]

47 minutes ago, Jblanton said:

Intake/Exhaust leak is what I guess.  Also check the inner cylinder baffle on the bottom.  It can move, or be installed incorrect.  Make sure tie rods are intact pulling #4 rear baffle to cylinder head. I suggest using fixed timing on the surefly with the carburated A1D, and the pos plenum.  

 

Jeff

That is a good thought.  I'm going to try to head over to the airport later to smoke check the intake and pressure test the exhaust.  If I can't keep the CHTs in check to, I might switch the Surefly to fixed.

[cferr59]

3 hours ago, jaylw314 said:

Do you have a primary certified engine monitor like the EDM 900, i.e. you have no more factory CHT gauge?  That's the only way all four CHT probes would be identical.

If not, you probably have bayonet probes in all 4 cylinders, but the #3 cylinder is the factory bayonet probe that goes to the factory CHT gauge.  your engine monitor would use a ring probe that either fits under the factory bayonet probe or under the spark plug.  It is not uncommon for a spark plug probe to run 50-75 degF hotter OR cooler depending on the installation.  The bayonet probe is significantly better, my guess is +/- 25 degF.

So I was wrong.  The cool cylinder has a different type of probe.  Anyone know what kind this is?  To me, it looks like a gasket type probe that would be used at the spark plug hole, but it is installed at the location where you usually would have a bayonet type probe.

[jaylw314]

5 hours ago, cferr59 said:

So I was wrong.  The cool cylinder has a different type of probe.  Anyone know what kind this is?  To me, it looks like a gasket type probe that would be used at the spark plug hole, but it is installed at the location where you usually would have a bayonet type probe.

That's the bayonet ring probe.  That's weird, I would have figured a 930 install would have included all probes of the same type, and it looks like they took the factory probe out.  I think that could mess with your temperature readings, since there's now an empty space where the factory probe was?

[carusoam]

Aka piggy back probe...

For fun, swap a probe from a neighboring cylinder... and see if the problem clears up... another CHT sensor costs about 0.11 amu...

Great pic... can clearly see the piggyback... and the empty hole left by the other CHT gauge...

Oddly, the JPI 930 should have come with a set of TCs specific for primary instruments... the piggy back probably doesn’t qualify as primary... as it’s data is off the scale required to display the redline.... based on the odd location...

You may still have six CHT TCs that didn’t get used..?

Best regards,

-a-

[cferr59]

I'm going to purchase another bayonet probe so that all probes are the same type.  I suspect this will reduce the delta between the cylinders.  I also found exhaust leaks on cylinders 1 and 4 by pressurizing the exhaust with a reversed shop vac.  They were so small (especially on 4) that I'm not sure they were causing any issues and I was able to resolve them by torqueing the nuts to spec.

[cferr59]

7 hours ago, jaylw314 said:

That's the bayonet ring probe.  That's weird, I would have figured a 930 install would have included all probes of the same type, and it looks like they took the factory probe out.  I think that could mess with your temperature readings, since there's now an empty space where the factory probe was?

I did not install the JPI so I don't know exactly what happened here.  However, this is not the first non-standard configuration I found on the JPI.  The oil pressure sensor was installed in a non-primary location and the old oil pressure sensor was disconnected and left in the primary location.

[jaylw314]

8 hours ago, cferr59 said:

I did not install the JPI so I don't know exactly what happened here.  However, this is not the first non-standard configuration I found on the JPI.  The oil pressure sensor was installed in a non-primary location and the old oil pressure sensor was disconnected and left in the primary location.

Ugh, that sort of stuff is a little worrying.  The A&P's here can give more specifics, but I thought I recalled the 930 needed to be installed with its own sensors and in the correct way to qualify as primary, and you wouldn't be able to remove the factory gauges if it did not.

[cferr59]

1 hour ago, jaylw314 said:

Ugh, that sort of stuff is a little worrying.  The A&P's here can give more specifics, but I thought I recalled the 930 needed to be installed with its own sensors and in the correct way to qualify as primary, and you wouldn't be able to remove the factory gauges if it did not.

I believe that is true.  I have since corrected that issue.

[cctsurf]

18 hours ago, jaylw314 said:

That's weird, I would have figured a 930 install would have included all probes of the same type, and it looks like they took the factory probe out. 

The JPI does come with bayonet type probes, from the install manual: 

14.Cylinder Head Temperature (CHT) Probe Installation Use the J2 connector harness 700700 or 700702 labeled C1 through C4 or C6. The JPI probe is a bayonet probe P/N 5050 that has a captive 3/8-24 boss that is screwed into the head of each cylinder

There is a ring style probe, but it is supposed to be used on radial engines and around the spark plugs, not the CHT probe hole:

15.Radial Engine CHT (spark plug gasket) Cylinder head temperatures are measured with a spark plug gasket type probe placed under the rear spark plugs. The spark plug gasket probe, P/N M-113, replaces the standard copper spark plug gasket on one spark plug.

as to the oil pressure, that probably falls the closest to normal:

18.Oil Pressure Sensor Installation Usethe J3 connector harness 790422 labeled OIL-P for a pressure sensor three wire capsadense sensor (0-5vdc

Oil pressure sensor mount using an aluminum clamp MS21919. Mount to firewall. Mount the pressure sensor to the pressure line using a flexible hose and fittings (not supplied) as depicted in the drawing below. Use aluminum clamp to mount the pressure sensor to firewall. Do not mount the sensor directly to the engine. Connect the other end of the hose to the engine manufacturer’s recommended location for engine oil pressure.

18.1 Alternate method of installation keeping the original sensors in the aircraft operational The oil pressure sensor should tee off the oil pressure line feeding the original aircraft gauge or the oil pressure switch is removed and the sender is installed in that location.

[cferr59]

JPI bayonet style probe is now installed and this has resolved the issue.  Thanks for the help everyone!

[Browncbr1]

For other viewers of this thread in the future; the piggy back probes must be installed with about 1” of fire sleeve zip tied over it.    The outside air rushing over the cylinders to cool the heads also cools the probe otherwise.  

[Buckeyechuck]

On 12/31/2020 at 11:33 AM, Browncbr1 said:

For other viewers of this thread in the future; the piggy back probes must be installed with about 1” of fire sleeve zip tied over it.    The outside air rushing over the cylinders to cool the heads also cools the probe otherwise.  

I’ve got a similar issue.  About 80 degree difference between back cylinder CHT’s. Have removed and reinstalled the piggyback sensor to try to improve but no change. I have the fire sleeve. The factory CHT on no. 3 is close to the JPI CHT on no. 4. I just use the JPI on no. 3 for trending. Any suggestions other than upgrading to 900 series?

[Browncbr1]

13 hours ago, Buckeyechuck said:

I’ve got a similar issue.  About 80 degree difference between back cylinder CHT’s. Have removed and reinstalled the piggyback sensor to try to improve but no change. I have the fire sleeve. The factory CHT on no. 3 is close to the JPI CHT on no. 4. I just use the JPI on no. 3 for trending. Any suggestions other than upgrading to 900 series?

Did you install the piggyback probes using the insulated copper washer on top?  The fire sleeve should go right up against the cylinder..   it may take a few iterations if zip tying it to get a good seal from outside cooling air.    Still, piggy back probes are known to read 20-30 degrees cooler than actual.  So if you have bayonet probes on some and piggyback probes on others, that is where some of the spread would come from.   I knew that going into it, so I elected to use piggyback probes on all cylinders rather than mix and match.   

I would look at the firesleeve and probes on the coldest cylinders.     Also, cooling spread is different for different phases of flight and ROP vs LOP..   I’ve used some aluminum tape on front cylinders and swapped injectors so that things are best balanced peak-LOP in cruise.  My #3 is hottest and #4 coolest..  

Edited January 2, 2021 by Browncbr1

[Buckeyechuck]

The piggyback probe is installed against the cylinder head and insulating washer is between the piggy back probe and bayonet mount. The fire sleeve is butted up to the cylinder head. I’m due for annual in March and plan to do more work on the doghouse to tighten it up and will also work to tighten up the fire sleeve.  Front to back cylinder CHTs are running about 40-50 degree difference. I’m still doing the engine break in.  I see some folks have used high temp RTV to close up small holes in the doghouse.  Is there a benefit or issue with trying some to seal the fire sleeve?  

 

[jaylw314]

On 1/1/2021 at 7:21 PM, Buckeyechuck said:

I’ve got a similar issue.  About 80 degree difference between back cylinder CHT’s. Have removed and reinstalled the piggyback sensor to try to improve but no change. I have the fire sleeve. The factory CHT on no. 3 is close to the JPI CHT on no. 4. I just use the JPI on no. 3 for trending. Any suggestions other than upgrading to 900 series?

Stop worrying about it, maybe?  It's just a number, and as long as you know what it usually reads compared to your other cylinders, you'll notice if it changes.  Think of it another way--your CHT's are about 400 deg F and if you're low by 27 deg F, that's the difference between 462 and 477 deg K or about 3 % error.

[Buckeyechuck]

1 hour ago, jaylw314 said:

Stop worrying about it, maybe?  It's just a number, and as long as you know what it usually reads compared to your other cylinders, you'll notice if it changes.  Think of it another way--your CHT's are about 400 deg F and if you're low by 27 deg F, that's the difference between 462 and 477 deg K or about 3 % error.

You are correct.  It’s the curse of being an engineer.  Everything needs to be right.  

 

[PT20J]

On 12/31/2020 at 8:07 AM, cferr59 said:

JPI bayonet style probe is now installed and this has resolved the issue.  Thanks for the help everyone!

Curious what the difference in temps was between the bayonet and ring on cyl #3. 

[cferr59]

1 hour ago, PT20J said:

Curious what the difference in temps was between the bayonet and ring on cyl #3. 

Looked at the JPI data and it looks like the temperature has risen about 105 degrees under similar conditions.  Note that the ring terminal was installed with an adapter in the bayonet position, not under the spark plug.