xrs135 Posted July 19, 2015 Report Posted July 19, 2015 My #4 cylinder has always run hotter than my other three cylinders in my 68' M20C. Probably a solid 40-50 degrees hotter. Seems strange to me since it is usually #3 that runs the hottest - yet in my situation, this is my coolest running cylinder. Really though, #1, #2, and #3 are all pretty close in line with one another. Part of me wonders if the probe/wire might just be bad? ...but I don't really know how to swap the probes to test it myself. Is there anything else I should be looking at that could possibly be causing this?
Marauder Posted July 19, 2015 Report Posted July 19, 2015 My #4 cylinder has always run hotter than my other three cylinders in my 68' M20C. Probably a solid 40-50 degrees hotter. Seems strange to me since it is usually #3 that runs the hottest - yet in my situation, this is my coolest running cylinder. Really though, #1, #2, and #3 are all pretty close in line with one another. Part of me wonders if the probe/wire might just be bad? ...but I don't really know how to swap the probes to test it myself. Is there anything else I should be looking at that could possibly be causing this? What is the EGT doing on #4? Sent from my iPad using Tapatalk
xrs135 Posted July 19, 2015 Author Report Posted July 19, 2015 What is the EGT doing on #4? Sent from my iPad using Tapatalk It's normal and in line with the others from what I can remember.
Shadrach Posted July 19, 2015 Report Posted July 19, 2015 It's normal and in line with the others from what I can remember. It's normal as in the raw number or it's normal in how it acts while leaning? Which cyl is richest? Which is leanest?
takair Posted July 19, 2015 Report Posted July 19, 2015 When you say hottest, are you talking CHT or EGT. What temperatures are you getting? Do you have the same type of thermocouple on the cylinders? Is it possible they are swapped? Have you checked the baffles...big opening near number 4?
xrs135 Posted July 19, 2015 Author Report Posted July 19, 2015 I'm talking CHT. The raw number is always higher than the others in all phases except for maybe immediately after start. It's always higher in climb, and cruise. It's a C, so it runs hot. If I work to keep temps down, #1/2/3 will all be just under 400 in climb whereas #4 will be ~430. In cruise everything else is about 365, and #4 is just under or at 400. As mentioned earlier, egts are all in line... I had all the intake gaskets replaced at last annual. I'm not super savvy when it comes to engine stuff, so I'm not sure if my thermocouples are the same. The previous owner said the doghouse is all sealed tight, but I don't even know where exactly to look to verify this.
carusoam Posted July 19, 2015 Report Posted July 19, 2015 Not being savvy adds to the challenges. But, you came to the right place for information... 1) you have a carburetor. 2) your carb has two fuel jets. A main jet and the one that comes on at or near full throttle. 3) fuel distribution is always going to be a challenge. The one carb randomly feeds liquid that is sprayed and evaporating into four intakes. 4) having the engine monitor is key to being able to identify any other issues normal and un normal. 5) collect EGT data during cruise as you lean the engine towards peak. How close are the numbers to each other? Can you go LOP? 6) do this at full throttle and at less than FT. See if you can tell when the secondary fuel jet is operating. 7) partial throttle is known to improve fuel distribution. See if this works for you. 8) using carb heat is also known to improve fuel distribution. See if carb heat effects your EGTs. Do you have a carb temp gauge? 9) before running random leaning experiments, be sure to know about the Red Box theory of safe engine operations. The red box theory explains high power settings combined with lean mixtures can be harmful. 11) check your own dog house for each and every crack and hole. Whatever the previous owner said, is old news by now... 12) the dog house is known for it's crummy cooling capacity. Air prefers to flow out of it than to flow through it evenly. Some hope is around the corner on this issue, as one MS member is building a new cowl front complete with known airflow improvement parts.... 13) check the engine logs for any notes regarding engine timing. The timing should match the specs for the plane/engine combination. This would effect all cylinders evenly These are things that come to mind from discussions over the years. If you don't have a modern engine monitor, get one... If you don't have a carb temp monitor, get one... I am only a PP, not a mechanic, but I did enjoy the ownership of a less than fine M20C Best regards, -a- 3
xrs135 Posted July 19, 2015 Author Report Posted July 19, 2015 Thanks for all the info. Sounds like I need to do a little bit more investigation and tests in the air. One bit of information, talking to the previous owner, it sounds like #4 always ran hot, even before the engine overhaul. So I'm guessing that rules out an internal cylinder issue. It must be a probe/wiring issue, OR a leak in the doghouse somewhere...
carusoam Posted July 19, 2015 Report Posted July 19, 2015 Check the balance between... Heat generation: aka EGT in cylinder 4. Balanced EGTs is a sign of balanced fuel flow. Tough to get with a carburetor. Heat dissipation: aka CHT of cylinder 4. Good dissipation equals low CHTs... If EGT of that cylinder is higher than all others, the CHT is likely to be higher as well. If the EGTs are close to each other, they are generating the same heat. A hot cylinder on an engine that has well balanced EGTs, probably has an airflow problem around the hot cylinder... Carbureted engines are not known for well balanced EGTs and dog houses are not well known for balanced cooling. Definitely make sure the systems are working as they are supposed to. Best regards, -a- 1
N201MKTurbo Posted July 19, 2015 Report Posted July 19, 2015 One thing to check for is that the probes are in the correct cylinders. I have seen quite a few instances where after many years of maintenance they inadvertently get swapped. 1
xrs135 Posted July 19, 2015 Author Report Posted July 19, 2015 Check the balance between... Heat generation: aka EGT in cylinder 4. Balanced EGTs is a sign of balanced fuel flow. Tough to get with a carburetor. Heat dissipation: aka CHT of cylinder 4. Good dissipation equals low CHTs... If EGT of that cylinder is higher than all others, the CHT is likely to be higher as well. If the EGTs are close to each other, they are generating the same heat. A hot cylinder on an engine that has well balanced EGTs, probably has an airflow problem around the hot cylinder... Carbureted engines are not known for well balanced EGTs and dog houses are not well known for balanced cooling. Definitely make sure the systems are working as they are supposed to. Best regards, -a- This probably isn't much help, since you can't see the individual numbers of each cylinder... But I randomly found this photo I had on my phone from typical cruise flight and cropped it in as best I could to the Jpi. You can at least visually see how the egts look. Not perfectly even, but I'd say normal enough for a carbureted bird. As you can see, #4 actually isn't the highest EGT, but it's CHT is probably 395ish.
Shadrach Posted July 19, 2015 Report Posted July 19, 2015 Highest EGT does not correlate to highest CHT. You need to see where #4 is running in cruise. To check for a mixture problem. Climb to altitude and set up the plane as you normally would for cruise including leaning on whatever cylinder you normally lean on (should always be your leanest cylinder, not necessarily the one with the hottest EGT). Now, set the JPI for #4 make a note of EGT number, then start to lean and see what happens. Find out where the #4 cylinder is in relation to peak, If it's lean of peak then I would make a not of the fuel flow at peak and do the same check the other cylinders You may have an induction leak. It may be a baffle seal problem. Have you inspected all of the material? Have you ensured that the seal is properly dressed facing forward (I have to do this almost every time someone else changes the oil). Improperly faced baffle seal will open up like a hole under the effect of ram air and most of your cooling air will spill back into the accessory case.
MyNameIsNobody Posted July 19, 2015 Report Posted July 19, 2015 If the previous owner said it always read this way...I would only be concerned if it suddenly went way higher than it's "normal". My #2 is always 30-40 degrees hotter than 1, 3 & 4. Is what it is. Has read that way for 550 hours.... So...It IS normal to read there. If otherwise THEN it has my attention...
xrs135 Posted July 20, 2015 Author Report Posted July 20, 2015 It may be a baffle seal problem. Have you inspected all of the material? Have you ensured that the seal is properly dressed facing forward (I have to do this almost every time someone else changes the oil). Improperly faced baffle seal will open up like a hole under the effect of ram air and most of your cooling air will spill back into the accessory case. Looking up into the copilot's air inlet on a C model, is this how the orange seal is supposed to look? Or is it a different baffle seal you are talking about?
takair Posted July 20, 2015 Report Posted July 20, 2015 Do you have a cowl mod? I see fiberglass toward the fron of the cowl...don't recall that on early C models...
Shadrach Posted July 20, 2015 Report Posted July 20, 2015 Looking up into the copilot's air inlet on a C model, is this how the orange seal is supposed to look? Or is it a different baffle seal you are talking about? ImageUploadedByTapatalk1437352662.839358.jpg You have the plenum (dog house) set up so no baffle seal material to bother with at the rear. Is the plenum sealed tight? Gaps or openings?
xrs135 Posted July 20, 2015 Author Report Posted July 20, 2015 ^That's what I was thinking with the doghouse setup... Like I said before, I don't know much about what a tightly sealed doghouse actually looks like... But to my eye, it looks "pretty good." Is sealing tiny cracks/openings that I find something that I could do myself? What kind of high heat silicone is recommended?
xrs135 Posted July 20, 2015 Author Report Posted July 20, 2015 Permatex RTV high-temp red silicone? Would that work?
Shadrach Posted July 20, 2015 Report Posted July 20, 2015 Talk to your IA first. The plenum should be sealed for maximum cooling.
Recommended Posts