I think the CHT gauge in my new 180 is gunnysack. After re-rigging the cowl flaps, I did some local flying and never even hit 300 degrees -- either in a short climb with them open or in cruise with them closed. Granted, it was only about 60 degrees out and I only had the cruise power set at about 20" / 2200, but I woulda thought I'd get a bit more than that.
I hate to change out the gauge without knowing that it's the problem-- anyone know how to troubleshoot one of these?

Typically an open circuit (assuming it's a thermocouple) will get you some completely weird number that makes absolutely no sense (like -9999 the exact number will depend on your particular system and how it reads open circuit). If you are seeing a temperature and it's not fluctuating, it is probably a real measurement. Might check to see that the probe is properly seated in the hole.

My one cent, since I'm not an electrical engineer.

Greg

Last summer bought a PA20 in Arkansas...didn't have a CHT meter. Installed before the trip home to Alaska. CHT temps were 330 to 350. Oil temp 180-185.. Checked with an infrared thermometer...discovered the CHT meter was accurate.. Had to run full rich to keep 330 on number three. The right gear leg came uncorked first landing at my home airstrip. Insurance company totaled it. Now in the midst of a complete rebuild... Building an O320B2B. Anyone flying with ECi's CGR30? Would like an all in one primary instrument....
Mark M.

m_moyle wrote:Last summer bought a PA20 in Arkansas...didn't have a CHT meter. Installed before the trip home to Alaska. CHT temps were 330 to 350. Oil temp 180-185.. Checked with an infrared thermometer...discovered the CHT meter was accurate.. Had to run full rich to keep 330 on number three. The right gear leg came uncorked first landing at my home airstrip. Insurance company totaled it. Now in the midst of a complete rebuild... Building an O320B2B. Anyone flying with ECi's CGR30? Would like an all in one primary instrument....
Mark M.

what is wrong with 330-350?

Is this an original Cessna gauge or similar?? On the flight profile you just mentioned, you should be in the green arc most of the time...maybe go just below the green on an extended glide at idle.

Take a look at the thermocouple, it should be a "gasket type" that installs under a spark plug...cessna changed the cylinder placement a couple times in the early years, mine is on the #2 cyl (back left). Right where it trAnsitions from the ring to the double wire is where I found corrosion on mine. I replaced it with an alcor lead, and all is fine now.

Another thing I would check is the actual lead itself. The lengths of wire coming from the thermocouple are critical for the calibration. If your seeing needle movements on your gauge that go in the right direction, but the numbers just don't seem right, then somebody might have installed the wrong lead at some point. My lead coming from the thermocouple was about 10-12 inches. The alcor website has some info about the specific leads.

scottf wrote:

m_moyle wrote:Last summer bought a PA20 in Arkansas...didn't have a CHT meter. Installed before the trip home to Alaska. CHT temps were 330 to 350. Oil temp 180-185.. Checked with an infrared thermometer...discovered the CHT meter was accurate.. Had to run full rich to keep 330 on number three. The right gear leg came uncorked first landing at my home airstrip. Insurance company totaled it. Now in the midst of a complete rebuild... Building an O320B2B. Anyone flying with ECi's CGR30? Would like an all in one primary instrument....
Mark M.

what is wrong with 330-350?

Perhaps I was being overly cautious? The former owner replaced number three...three times due to heat issues......19 years. Wanted to get home...to my shop and parts....install an Alcor CHT/EGT meter/four cylinders. Then find my comfort zone with more information...

The thermocouple is usually a type K unit. You can use a good voltmeter to test it...with a table like this:

http://www.ni.com/white-paper/4231/en/

Just make sure you have the terminals you are probing at the same temperature (best not to let the probes get warm fr holding them).

Hook it all up, make sure it is grounded. Put a small pot of boiling water on a ladder under the engine, lay the probe in the water, and the CHT should read 212 last I checked. If it does, then you have HALF of a good data point, in that it works at the lower end of the range. If it doesn't even read 212, then you can at least not waste time with a lot of other tests or spare parts. If it reads 212 then it may be worth continuing with more expensive or time-consuming options.

There is a product called Temp-a-Dot. These are little stickers that you stick on the cylinders, and they turn color at a certain temperatures. Find the right ones for 300F or whatever the CYHT is actually telling you it is, and put them on the cylinders very near the CHT probe location. If you go through several stages of the the temp-a-dot, at different temps, and the dots tell you that they have been exposed to 350 or 400F, then your CHT is not working well at the higher temperatures and cannot be trusted with your engine's health.

lesuther wrote: The thermocouple is usually a type K unit. You can use a good voltmeter to test it...with a table like this: http://www.ni.com/white-paper/4231/en/ .........

I couldn't quite make sense of the j-probe chart-- temps across the top OK, readings in the chart, but could't figure out what the numbers down the left side were. (in case you haven't guessed, I'm no rocket scientist.....)
I had already thought of the boiling water thing, that makes sense for an oil temp gauge but for a CHT gauge you're more interested in the 300-500 degree range. But at least with the boiling water test you'd know where it was at the low end.
My gauge is aftermarket, but no brand name on the face. The spark-plug-gasket type probe is on the #2 cylinder, like Bart's and like the Cessna book calls out for the 1953 model. I did find something in the Cessna series 100 service manual about testing the lead/probe of the standard Cessna CHT gauge, resistance through them should be in the 2 ohm range. I'll try that. When I had the lower cowl off to re-rig the cowl flaps, I did pull that plug and clean up the ring probe with scotchbrite. It was already looking good so that didn't do much. The leads & connections all looked good and correct.

The boiling water thing is the first thing to do since it is so easy. We used a solder pot for higher temps, with a IR thermometer to check against. But the numbers in the tables refer to the voltage you will measure. You need a pretty good voltmeter to check accuracy, but you probably have enough significant digits to tell the difference between 200F and 300F on a regular voltmeter. You just do that to see if the problem is the meter or the probe. On mine, the failure mode is either an open circuit or very high temp readings.

Don't know if it's applicable to an aftermarket gauge, but the Cessna book said to test resistance through the leads & probe, should be 2 ohms. Took the leads loose from the gauge and tested them, and got a reading of 2.2 ohms. Pretty close- but don't know if pretty close is good enough in this case. Might be off enough to make the gauge read 275 when the cyl's really at 350.
I'm thinking I'm just gonna replace the gauge, probe, & leads, and start off fresh. A couple hundred bucks for that is pretty cheap compared to the cost of replacing a cooked cylinder. Most of the aftermarket gauges in the Spruce catalog look kinda cheesy though. My panel's not all that original, but I'd really like to have something that looks like the original Cessna gauge. Any ideas?

So far, looks wise, Uma looks the best. Maybe Alcor. I want a quality, accurate CHT gauge, but I don't want something that looks like it belongs on an ultralight. Don't want a digital/electronic gauge either.

hotrod150 wrote:Most of the aftermarket gauges in the Spruce catalog look kinda cheesy though. My panel's not all that original, but I'd really like to have something that looks like the original Cessna gauge. Any ideas?

Sure, find someone with a factory gauge and buy them a CGR-30P.

Just kidding- I've been known to go way overboard myself to get minor details the way I want them.

-DP

Put my CHT gauge to the boiling water test yesterday. It failed miserably-- the gauge read about 120. I'm just gonna buy a whole new set-up. Any suggestions on analog-type gauges? I'm also thinking of getting a screw-in probe, instead of the spark-plug-gasket type-- I never liked those, thought they're kinda cheesy, but most people seem to run them on these 470's. Anybody know what thread the hole in the cylinder is? Forgot to check it when I had the cowl off yesterday. From probe descriptions in the Spruce catalog, it looks like a choice between 3/8-24 or 1/8 NPT.

Pulled the cowl off (again) and measured the probe hole in the cylinder. 3/8-24 FWIW. Ordered a UMA 0-600 CHT gauge on-line this evening from Spruce, along with a bayonet type j probe. Also a new throttle cable, which was way overdue. Added a few other things, and then was surprised to get a pop-up message on the shopping cart page saying if I ordered six more bucks worth of stuff, I'd qualify for the free shipping (over $500 order). Damn, that didn't take too long.

Hotrod,

Hate to play the role of Captain Obvious, but are you sure that you have a Fahrenheit gauge? 120 C on your gauge wouldn't be far off.

Not when the green arc is from 300-425