One cold EGT probe and RPM restrictions

[jasong]

Hi all, just learning my new 1979 M20J and have a couple of questions for the group.

First, one EGT reading (cylinder #2) is always 6-800 degrees colder than the other cylinders.  I'm *assuming* this is probably due to a bad probe but just wanted to see if anyone had any other experiences that might lead to a different conclusion.

Second, I want to understand a little bit more about about an RPM restriction that is placarded on the panel.  It states that you should not maintain 1500-1950 RPM with MP below 15.  Why is that?  When should I be concerned that I've stayed in that range for too long?  On base/final I've found myself running between 13-14 inches MP and at that seems to get me right into that yellow arc for a majority of the rest of the approach, which might last another couple of minutes depending on the conditions.  Is that bad?  Are there different approach techniques I should be considering?

[jetdriven]

I thknk it could be the probe. Check the wiring connections for resistance first (often bolts and nuts). 3 of our EGT are within 20 degrees of each other and one is 80 degrees off, for eample.  The probe generates a small electrical currnet the more it is heated. EGT probes are resistance wiring so a bad connection will cause it to read low. So a bad connection = low volts = low EGT reading.

 

The RPM restriction is for continuous ops.  I believe it is due to an engine/prop resonance or harmonic vibration that can detune the crankshaft counterweights. They dont define continuous but our plane is in that band sometimes during the landing descent and I dont worry. Don't have it there from 10,000' the whole time.   MP is useless after the gear is down and descending in the pattern, try using the prop as it is on the fixed pitch stop at that point.  You could  try flying a closer, steeper patern near idle power, or a slightly wider shallower pattern at 1900-2000 RPM.  Or ignore it.

[carusoam]

Jason,

 

Welcome back to Mooney ownership.

 

Your yellow arc sounds like you need to avoid glider ops with your mooney, extremely low power settings and low rpm may also result in some really cool cylinder temps.

 

Best regards,

 

-a-

[jasong]

Quote: jetdriven

I thknk it could be the probe. Check the wiring connections for resistance first (often bolts and nuts). 3 of our EGT are within 20 degrees of each other and one is 80 degrees off, for eample.  The probe generates a small electrical currnet the more it is heated. EGT probes are resistance wiring so a bad connection will cause it to read low. So a bad connection = low volts = low EGT reading.

 

The RPM restriction is for continuous ops.  I believe it is due to an engine/prop resonance or harmonic vibration that can detune the crankshaft counterweights. They dont define continuous but our plane is in that band sometimes during the landing descent and I dont worry. Don't have it there from 10,000' the whole time.   MP is useless after the gear is down and descending in the pattern, try using the prop as it is on the fixed pitch stop at that point.  You could  try flying a closer, steeper patern near idle power, or a slightly wider shallower pattern at 1900-2000 RPM.  Or ignore it.

[jasong]

Quote: JimR

Other than on the ground, when was the last time you continuously operated your engine at less than 15" MP, or at less than 1950 RPM for that matter?  In my flight ops, at least, never.  So I just ignore it.  Byron is right about why it is there but I'm not sure that it has much practical effect. 

Jim 

[jasong]

Quote: carusoam

 

Jason,

 

Welcome back to Mooney ownership.

 

Your yellow arc sounds like you need to avoid glider ops with your mooney, extremely low power settings and low rpm may also result in some really cool cylinder temps.

 

Best regards,

 

-a-

 

[Cris]

Check to see if you might have a gasket probe on the errant EGT cyl. which fits in place of the spark plug gasket. That is used to enable both the factory probe & the aftermarket probe to both operate on the same cyl but is the cheap way to do it. That gasket probe will give you a differant reading than the other cyls. If that is the case you can get a spring adapter probe that will work with both probes from the EGT mfg. I may have a spare dual adapter that I'd be happy to give you if you find that you need it. However it is from a Continental IO550 so you'll need to verify the fit. Also if you have a GEM 610 you can "adjust" for the differant cyl temp with the software. I assume that can also be done with late JPI or ECI EGT's but I really do not know.

[danb35]

Cris, I think you're confusing CHT and EGT probes--AFAIK, nobody has gasket EGT probes.

[Cris]

You are absolutely right. To long a day I guess. I was describing The CHT probes 

[wishboneash]

Quote: jasong

Hi all, just learning my new 1979 M20J and have a couple of questions for the group.

First, one EGT reading (cylinder #2) is always 6-800 degrees colder than the other cylinders.  I'm *assuming* this is probably due to a bad probe but just wanted to see if anyone had any other experiences that might lead to a different conclusion.

Second, I want to understand a little bit more about about an RPM restriction that is placarded on the panel.  It states that you should not maintain 1500-1950 RPM with MP below 15.  Why is that?  When should I be concerned that I've stayed in that range for too long?  On base/final I've found myself running between 13-14 inches MP and at that seems to get me right into that yellow arc for a majority of the rest of the approach, which might last another couple of minutes depending on the conditions.  Is that bad?  Are there different approach techniques I should be considering?

[jetdriven]

Quote: jasong

Thanks jetdriven.

Out of curiosity how do you fly an approach?  Kill the power or fix it to some degree at a specific MP setting?  Or just whatever conditions require?

[Cruiser]

If you do a quick GAMI lean test you will probably find that #3 peaks first and #1 peaks last. The #1 cylinder is almost always the richest on the IO360. So the richer you run (idle/low power) the cooler #1 will be compared to the rest. As you increase power (RPM) the temp will rise but always be cooler than the rest. IF you go LOP, #1 will be the hottest.

[jwilkins]

1. I'm having a probe replaced on the JPI today for the same reason. It was working fine and then became erratic when the shop repaired the broken clamp. If there had been a probe in stock at the shop (usually is) we would have just replaced it.

2. It has been my understanding that the RPM / Manifold combination restrictions were due to possible harmonic vibrations. Which makes sense that you can replace your prop and get rid of many of the restrictions. Is there another reason for the restrictions?