Q: I fly a Cessna 182S registered HP1401. It has a Lycoming IO-540-AB1A5, 3 blade prop. Sea level OAT is 90°F or so year round.
It had a single probe analog CHT display connected to cylinder #1, which always remained around 320°F.
I then installed a digital engine monitor, which showed the rest of the cylinders easily getting into the 400°F range. So I started flying lean of peak with GAMI injectors and cowl flaps fully open all the time and got the CHTs into the 360°F range.
The original engine reached its 19th birthday and some 1,900 hours, so I decided to replace it with a factory rebuilt.
The test data sheet from Lycoming shows all temps completely normal at the factory.
I just flew the first two hours with the new engine, and it’s running much hotter than the old engine, particularly cylinder #2. In order to keep the temp at 400°F or below, I have to keep cowl flaps full open and keep the mixture around 19 gallons per hour.
- At 19 gph CHT1 reads 303, CHT2 reads 402, other CHTs around 375;
- At 18 gph CHT1 reads 311, CHT2 reads 408, other CHTs around 384;
- At 17 gph CHT1 reads 316, CHT2 reads 412, other CHTs around 390.
I’ve read all the Lycoming literature I could find, which boils down to: Baffles, mag timing, induction leaks, exhaust leaks, and obstructed injectors. All of these look good, so I sent Savvy Aviation the EDM download, but they didn’t find anything really wrong.
I then called Lycoming and they told me to check baffling again and to play with mixture and power and continue break in, avoiding continuous flight above 435°F CHT.
A friend who is an A&P as well as a mechanical engineer recommended I reduce the spark advance from 23° which is specified, closer to 20°.
Would you have any recommendations for me? Should I continue breaking in? Should I lean further as long as CHTs stay below 435°? Should I try reducing the spark advance?
Henry Stec
A: Henry, welcome to 2018. Your question is a good way to start the new year.
After reading and digesting your situation, I first want to point out the fact that any engine recently rebuilt or overhauled will typically show higher cylinder head temperatures as a result of the tighter tolerances compared to your previous engine with 1,900 hours.
The most important thing I can tell you is all of the temperatures you provided are well within the specific limits for your engine. I would not alter the specific engine timing under any circumstances!
One thing to remember is that the airframe manufacturer decides which is typically the hottest cylinder following their cooling climb tests during the certification process. Cessna has chosen the #1 cylinder for a single probe system location because their tests confirmed that was the hottest running cylinder in this specific installation.
If this engine were installed in a different aircraft with a different cowling design, the hottest cylinder may turn out to be #5.
My suggestion for you would be to continue to break the engine in as recommended in the Cessna Pilots Operators Manual or the Lycoming Service Instruction 1427C.
I agree with the Lycoming guys recommendations and wouldn’t recommend additional leaning. Remember: The two least expensive things you can put in your engine are gas and oil.
Also, running with the cowl flaps open causes drag, which in turn means consuming more fuel, so continue to keep the cylinder head temperatures below that 435° point and keep the cowl flaps closed if possible.
If I were to do anything else in your situation, it would be to really take a close look at the engine baffling again. I feel you may be able to make some improvements in that area.
We know the engine baffling lives in a very hostile environment, so in many cases where an engine is replaced, it makes sense to replace a good part, if not all, of the baffling so that it fits much better than some of the high time baffles that have taken a set over time and may not seal as they did when new.
How do I post a new question? I can reply to a thread but don’t know how to start my own?
Help.
Please ignore. Two seconds after posting I found the way.
Darnit.
Never change the magneto timing to anything different than that of the TCDS specs. Doing so can cause other damage to the engine. New rings in new cylinders will leak and need to lap in during the high power settings for a overhauled engine. Don’t lean excessively either. The extra fuel will cool the engine as long as it does not carbon up the interior of the cylinder (fat chance). Change power settings and altitude during the break-in.
Well you expect a new lover to be hotter! Why not a new engine?
I replaced most of the baffles after doing a top overhaul.
Since it is impossible to see the upper and side baffles, I used a digital camera, held inside, and pointed it to the sides and rear. I pics showed that the baffles were lifted by the stiffeners in the upper cowl. So I had to cut a notch in the baffle to get a tight fit. It took 3-4 tries to get all the baffles to fit tight.
The new cylinders initially ran up to 400 degF, but after 10 hrs are now 350-360 degF..
Be sure the CHT probes are clean and that there is no foreign material in the cylinder CHT probe holes. Also, the probes may not be transmitting an accurate temperature readout. If prior temps have been recorded, and depending on the type of CHT system installed, it may be possible to swap probes between the hottest and coolest cylinders, and not changing anything else, refly the aircraft in the same manner as before, then record your CHT temps and compare. Any significant temp change may indicate a CHT system problem.
Yes, I would expect any engine to run hotter after an overhaul, but I would also look at baffling first. After spending years chasing down high CHT issues in my plane I started to learn a lot about baffling and you’d be surprised at how many little details are overlooked when installing (or re-installing) baffles by even the most seasoned A&P’s.
Baffles may appear to be “in good shape”, but you should pay close attention to a few things when installing the cowling over the baffles. I discovered on my PA28-180 that if I was not careful when putting the cowling back on (after an oil change or other routine maintenance) that the rear baffles would fold back instead of folding forward. This would cause the airflow to push the baffles backward and own allowing precious cooling air to escape behind the engine.
Another issue I discovered is that many people to not even consider how baffling sections should overlap. For example, the most forward part of the baffle seals should overlap the next section behind, not “underlap”. Imagine the air coming into the engine compartment being able to lift a seal allowing air to escape. Finally, on some aircraft there are seals at the very front of the engine at the bottom of the cowl opening. These should be folded down to provide a tight seal against the cowl.
As I said before, even the most seasoned A&P’s tend to miss these details. Yet they can make a big difference in engine cooling. I always look inside the cowl with a flashlight after installation to ensure that all seals are folded and overlapped in the right direction. You always want to see the seals situated in a manner such that when air pushes against them they provide a tighter seal rather than create an opening for air to escape.
Oftentime, the rubber “flaps” that are riveted or stapled to the rear vertical baffle have taken a set, or have become weak and floppy, so even if it is possible to fold them forward prior to installing the upper cowling, they will blow backwards and unseal on the next flight. It would be best to replace these rubber strip seals in the factory manner. This may also apply to the seals on either side of the engine which run longitudinally. I ran into this issue with older C-310 aircraft.