Fuel flow

[lowNslow]

It's my understanding that the max CHT for the O300 is 525 degrees, why would 465 degrees be a problem?

[ghostflyer]

You can never too much information when about to cross tiger country. The newer digital instrumentation does save you money. Years ago (too many to try and remember) we were taught how to lean a engine on cruise . Wow what a mistake I made . Leaning as per digital instrumentation has saved me heaps of money plus it's saves on wear on the engine .i haven't had no where the problems of cracks in the cylinders as before . My plugs are running cleaner ,oil temp s are better controlled and I try to keep CHT at or below 370degs. I was taught on a long descent one was supposed to warm the engine every so often . I now keep all CHT,s at a constant 320 degs ( all in the range) . No warming the engine every so often . It's Been. A saving of around 4 to 5liters a hour in cruise .

[GAHorn]

I simply can't imagine any instrument or any technique that will keep ALL cylinders at a nice 320 or any other identical temperature.

[DaveF]

Dave, I'm not challenging your personal experience, but for an educational discussion, how did a bad magneto overhaul affect just one cylinder, yet not manifest itself during an ordinary runup which could be detected using original instrumentation?
What fuel pressure problem did it detect, exactly?

The bad magneto affected all four cylinders (Lyc O-360) but only caused one CHT to run away. The engine ran smoothly on both magnetos and during preflight runup checks. The engine was smooth on takeoff, but one CHT was going over 460F, 60 degrees hotter than the next hottest. Selecting R in flight caused all four to misfire and the engine to run rough, and immediately after repairing the magneto all four CHTs were within 20 degrees. So that was the cause, but I don't know why only one cylinder ran hot.

The long version:
The high CHT was present from the very first flight after major overhaul. I kept it somewhat under control with airspeed, RPM, and lots of fuel, but it was clear number 3 had a problem. I sealed baffle leaks to no effect. In-flight leaning tests showed all EGTs peaked at about the same fuel flow, so it wasn't a lean cylinder. The monitor was installed with the overhauled engine so I had no data on CHTs before the major, but other Avcon operators said my CHTs were not normal.

The rate of EGT rise didn't indicate detonation, but I started to suspect ignition (thanks, Thad!). I retimed the mags, no change, and then did an in-flight mag check. The engine ran rough on the right mag and the monitor showed all four cylinders misfiring, so the problem was the magneto, not just number 3 plugs or wires. The misfiring came in pairs -- 1 and 3 (top plugs) acted like timing retard, with EGTs up and CHTs down, while 2 and 4 (bottom plugs) did the opposite. The engine ran normally on both and on left only. I can't explain the behavior.

My mechanic and I expected to find a bad distributor block in the mag, but all we found was excessive point gap. (I don't recommend Quality Aircraft Accessories for magneto overhaul, by the way...) We re-set the point gap and on the very next flight all CHTs were normal and within about 20 degrees.

I'd never have suspected a problem without the all-cylinder monitor. I only saw 465F, but that's because I pushed the nose over to get airspeed up. In a Vy climb I think it would have gone to redline, 500F. Not only am I not willing to run CHTs in the mid-400s, but having one cylinder 60 degrees hotter than the others was a clear indication that something was very wrong! The cylinder would not have lived very long being run that hot.

I used the monitor to debug the fuel pressure problem. About 8 hours into the new engine my fuel pressure gauge (40 years old, Bourdon tube, still works!) would start at normal 5 psi, but slowly climb to 10-15 psi. That's well over redline, and if real could be dangerous. The engine monitor told me that my EGTs were normal, so I knew I wasn't flooding the engine and the high FP was probably a bad indication. Had I not known that I'd not have flown until the problem was isolated and likely would have bought a new fuel pump. The problem turned out not to be the gauge or the AC pump. It was a blockage in the fuel pressure sense hose acting like a check valve, and engine heat caused high pressure to build in the line. The gauge was right, but so was the actual fuel pressure.

[GAHorn]

I did not realize you were discussing a Lycoming installation...my comments were made with a C145/O300 in-mind.
Thanks for the explanation....I also would have been suspicious of a dist. block or perhaps a contaminated distributor/etc allowing "tracking"... but in any case, the fact that one magneto was the problem could/should/would have been obvious from simple instrumentation.
The enjoyment of better instrumentation can doubtless be nice.
Thanks.