wishboneash Posted August 5, 2012 Report Posted August 5, 2012 Hi all I have low CHT on one of the cylinders of my M20J after the cylinder was overhauled. Heres' the background. After annual (late May this year), the compressions on all cylinders were reported normal by Top Gun (mid 70s and above). However after getting the plane back and on subsequent flights I noticed very low EGT on cyl 1 (temp diff around 500F or so) at low power settings (around 1000-1500 rpm/idle). I called Top Gun and asked them and their response was low EGT at low RPM is not an issue. The JPI 700 manual also says something similar. OK. Fine. As time went on, the temp difference began to go up (700-800F) and I could sense the engine not running as smoothly at low power settings. Then 2 weeks ago, the engine began to run rough during run up before a long X-country flight. I couldn't clear it up by leaning etc. I shut down and took the plane to a local shop for diagnosis. They found fouled plugs on cylinder 2 (not 1). They cleaned those out and the rough running engine symptom was gone, but the low EGT on cyl 1 continued. Engine still not quite smooth at low power. I asked for a compression check and they found ZERO compression. I had this corroborated with another A&P and I had the cylinder pulled. Couldn't see anything obvious in the cylinder (no cracks, valves not sticking or bent or burnt). Could be some carbon build up behind the seats causing a leak. The cylinder shop (San Carlos) told me there was pitting around the valves. Nothing stood out as obvious. They re-ground the valve area and lapped the valves and tested for leaks. We put the cylinder back this weekend and checked for suction in the cylinder by rotating the prop. Looked OK. Started the engine and the EGT issue at low power was gone. I was seeing reasonable EGT on idle (just like before the annual). Next, took the plane up and ran it on high power for about 45 minutes. EGT on cyl 1 didn't rise much above other cylinders. This was surprising. The engine definitely ran smoother throughout the power range. That was good. But here's the strange thing... CHT on Cyl 1 wouldn't go above 200F! Other CHTs were in the 360 to 380 deg. So what could it be? Still low compression and less power output on cyl 1? A&P suggested to fly it a few more hours and then he would do a proper compression test. If any other ideas, I would love to hear them. Thanks for reading my long-winded story. Sriram '78 M20J Quote
Bill_Pyles Posted August 5, 2012 Report Posted August 5, 2012 Perhaps you have a faulty temp sensor on cyl# 1. Quote
NotarPilot Posted September 30, 2013 Report Posted September 30, 2013 Turn on the engine monitor when the engine is cold and see if you get the same temps on all for sensors at ambient temp. They should within a couple degrees of each other. Quote
pinerunner Posted September 30, 2013 Report Posted September 30, 2013 You could check the probe by swapping with another or dipping in boiling water and looking for 100 degrees C (212 fahrenheit). I've spoken to the folks at IE and that's OK on theirs if it helps your peace of mind concerning your probes. However they also confirmed what I already knew having worked (as in made them; its easy) with thermocouples on chemical instruments; when they fail they tend to go completely and not just drift off. the "goodness" of calibration is basicly fixed and is a function of how tightly the composition of the alloys used for the dissimilar metals is controlled. I had had one fail and would give good readings intermitently and then drop out. So your gauge is likely telling you the truth, assuming its still in place and connected right (you've already looked at it, right). I should think low compression would drop CHT's since it lets the hot gas escape the cylinder prematurely. Maybe you should send data to folks at Savy Aviator for a read. I forget what they charge but so low a CHT shouldn't be left to stand. Their response would make a great post. Quote
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.