CHT probe location/type anomalies "ouch thats hot"

[garytex]

I am new and learning my M20 F, and tweaking things.  It has a JPI 700 that showed #3 running warmer than the rest but not exceeding 385 in climb, 340 in high cruise.  The original CHT gauge is also still in place, and wobbles along at about 375 in climb. Baffling is in good shape, SWTA cowl, with leprechaun cylinder bottom wraps.  All good, nice cool running motor, no?  

My mechanic says " Oh, by the way I put the spark plug ring temp sender for # 3 on the bottom, where it is supposed to be".  I concur.  # 3 does not use a bayonet sender, the original gauge sender is positioned there so it uses an under spark plug ring.

So I take off, and am making a nice climb, on pretty day with lots of vis.  Texas hill country, pretty lakes, WOT, 2700, 200 ROP, going up like stink, and look over and #  3 is showing 445 on the JPI.  Oops something isn't right.  So I level it off, reduce MP to 21" run the mixture out to about 100 LOP, and it slowly gets down to about 385.  All the rest of the cyls are around 300, EGTs even around 1350.  I'm making about 120 mph in level flight.   A mag check shows an intermittent miss on # 3, right mag (bottom plug).  Back on the ground, alls well with the baffling, inside the bottom of the top of the offending massive electrode plug there is some brown staining in the very bottom of the area around where the spring contacts the electrode, maybe the spark is jumping across there, the combustion area is clean (recently cleaned, probably not clean from detonation).  I slap in a fine wire, no more miss, but still hotter than a firecracker.  I flew over to Fredericksburg Sunday morning and had to run cowl flaps open, way LOP and 21" to stay below 420.  Still blew past a couple of 182's heading in for breakfast there.  That was nice.  All the while the original gauge with the bayonet probe in #3 wavers well below 400.  

I am going to pop that original sender in some boiling water and see what it says, but I bet its going to read around 210.  I think, and am looking for some of you with more experience in type to comment, that both the original gauge, and the JPI are both accurately reporting.  And if that is the case, then just how hot is the bottom spark plug base on # 3 getting in a hard climb?  500 maybe?  No wonder the stinkers crack between the spark plug boss and the exhaust valve.  The cool cylinders are all reporting from bayonet locations.

I still have some places that I can close up between the front baffle plate and the cooling air inlets.  There is some baffle material there, but it is obviously folding back under the bottom of the cooling air inlets, and I do not see an easy fix for that.  There isn't anything to lay the forward facing baffling material up against except on top of the bottom of the cooling inlet.  Maybe a little second flat can be fabricated from aluminum that will be below the cooling air inlet that the forward facing baffling material can tuck into.  Comments from the other SWTA cowl guys welcome, please.  Maybe I should see how the 201 cowls address this.

 

Gary  

 

[danb35]

I understand that the spark plug washer probes typically read 50-75 deg. hotter than a bayonet probe--that could be part of your problem.

[jetdriven]

Gary, ours was set u similarly when we bought it.  Yes the bottom plug washer reads hotter than the bayonet location.  At 21" of MP there is NFW you are actually getting over 400 CHT.

Our fix was to install a 3/8" washer style thermocouple under the factory bayonet probe on #3.  it reads 40 degrees cooler than a true bayonet would but the factory guage helps out there.  They cost 10$.

[Cruiser]

this is what I have. It still reads 25-40°F low but works better than the spark plug rings.

CHT Adapter Probe http://www.jpitech.com/probes&sensors.php

[Shadrach]

 

The sparkplug ring thermocouples do read higher, but in my experience it's between 30-50df higher, not 50-75df. Are you sure that you don't have a slight exhaust leak in the area?  A slip joint or a slight leak around the down tube gasket can heat up the area. That being said, I know of a plane or two that get's a visit from the mx fairy right before and after annual causing a miraculous rearrangement of the thermocouple wires. All year long the JPI reads from the 4 bayonet probes and right before annual JPI CHT #3 somehow gets switched back to the sparkplug ring gauge...and then back again post annual.

As for #3 in general in some applications it tends to be the hot cylinder, this thread illustrates why it's so in one particular application and shows a method that worked to improve it... YMMV.

http://mooneyspace.com/index.cfm?mainaction=posts&forumid=2&threadid=2691

[garytex]

Thank you all for your responses.  Good information and ideas in every one.  

Shadrack, I did notice a slight exhaust leak on the bottom of the spring loaded fastener muffler fitting, with grey on the fasteners, and bottom of the cowl.  It does not look like enough leak to heat the cyl ot spark plug..pretty far away, but needs fixed anyway.

I think I may try a JPI probe in the bayonet location, just to try to see what is really going on there, and then use one of the above mentioned piggyback fittings to legal things up.

Byron, that's what the guage reads, so my question is, is the guage wrong, or is something else wrong.  That is what I want to find out next.

Cruiser, I noticed that the JPI ring probe instructions say to put the ring probe on top, and on the cyl that gets the most cooling airflow.  I am starting to wonder if the # 3 bottom plug doesn't run up there in the hot range all the time, and we fly along, blissfully unaware.

 

[jetdriven]

Gary, I think the bottom spark plug gasket reads 50 dF to high, and if its showing 445, your real bayonet CHT is kinda warm but alright.  Rumor is you put the spark plug gaskets on the bottom on Lycoming, and top on the Chinatenintals.  I should have taken a photo, but send me your address I will send you a 3/8 thermocouple CHT probe, you put it under the factory #3 probe as a washer.  Then it reads 50 cool but the factory guage is OK.

[garytex]

Byron, what a generous offer, thank you.  Do I understand correctly that the 3/8 thermocouple washer probe sends to the JPI?  I haven't decided that I do not want to see the lower spark plug temp.  I think that I may put the factory gauge under the spark plug just to see that temp., and put a JPI probe in the bayonet location so as to have a equal sensing comparison to the other cyls.  I also think that when I get the front baffles tightened up (they are terrible on the SWTA cowl), that I will see lower spark plug base temperatures, as more air will be forced through the fins, rather than just blowing around the top of the cylinders and exiting to the bottom of the cowl around the fromt baffles.  My mechanic wont give me trouble about the factory guage sensor location, he is more interested in function than form.  Good guy.  

If I decide to piggyback, I'll come begging for the thermocouple, but as of now, since I'm not sure what I am going to ultimately do, I do not want to impose on you for the shipping hassle, etc.

Sincerely, Gary