IO360 one cylinder odd man out

I have an IO360M1B with standard compression, running the SDS electronic ignition and injection, coming up on 1300 hours mostly trouble free, running 93E10. For the last couple hundred hours my #3 cylinder has been slowly getting lower on CHT and higher on EGT, so that in cruise now (WOTLOP) it runs about 30-40 degrees cooler on CHT and about roughly 40 degrees hotter on EGT, which would normally indicate that the mixture is off or the exhaust valve is leaking. Trouble is, my GAMI spread on all 4 cylinders is 0.0 - with SDS injection I can individually trim them however I want, and they are bang on. The ignition is right, I've checked timing (29 degrees when at altitude WOTLOP, 25 degrees lower and richer), changed plugs, recently replaced the wires, the problem is still there, and only the #3 cylinder has changed - if it was the coil or the trigger, it would affect more than one cylinder. Borescope looks beautiful, no sign of a burned exhaust valve, I recently lapped the exhaust valve in-situ, no change. Compression on that cylinder is 78/80 just like it always has been.

I'm running out of things to look for. I can swap the CHT/EGT with another cylinder and check for instrumentation problems, though it's unlikely to have two sensors vary like that at the same time. It's almost like the exhaust valve is fine but opening too soon - could this be a pushrod issue?

Attached are two pics of the exhaust valve.

I’d check for an intake leak.

Intake leaks don't have much influence at WOT.

Does this engine have flat tappets? If so, I'd suspect exhaust lobe wear. Intake lobes are shared by 1+2 and 3+4 cylinders, so if it were the intake lobe, cyl 4 would be affected as well.

You could do a quick dial indicator test on the rockers to check valve lift to see if that's the issue.

hgerhardt said:

Intake leaks don't have much influence at WOT.

Does this engine have flat tappets? If so, I'd suspect exhaust lobe wear. Intake lobes are shared by 1+2 and 3+4 cylinders, so if it were the intake lobe, cyl 4 would be affected as well.

You could do a quick dial indicator test on the rockers to check valve lift to see if that's the issue.

Click to expand...

Indicator is already on back order, pending.

Can you do an inflight ignition check and run on single ignition for a short period to confirm that neither of the plugs has an intermittent miss? If one did, I think the cht would drop and the egt would rise on that cylinder.
Good Luck,
Mahlon

mahlon_r said:

Can you do an inflight ignition check and run on single ignition for a short period to confirm that neither of the plugs has an intermittent miss? If one did, I think the cht would drop and the egt would rise on that cylinder.
Good Luck,
Mahlon

Click to expand...

That was an early check, yes, both plugs are firing.

Easy test

Greg, Your system uses dual coils. That means that it's a "wasted spark" system [spark occurs at both TDC on compression stroke and at TDC on the exhaust stroke] It's rare, but I have seen those dual coils fail on 1 cylinder, but not the other. Easy way to tell. Swap the #3 ignition wire with it's mate. Watch to see if the issue is now on that other cylinder. If it does, you have a bad ignition coil. The fact that your CHT is dropping and the EGT is going up indicates that the fuel is still burning as it goes out of the cylinder. Your ignition timing can be "spot on", but if the spark is weak, the burn rate will be slower, causing the reading you see.

Charlie

One repeatable test is worth a thousand expert opinions - Bob Nuckhols

mahlon_r said:

Can you do an inflight ignition check and run on single ignition for a short period to confirm that neither of the plugs has an intermittent miss? If one did, I think the cht would drop and the egt would rise on that cylinder.
Good Luck,
Mahlon

Click to expand...

This is a tough one. I have had loss of ignition and in my experience CHT will drop but EGT will rise about 150-200. The small rise on EGT is not consistent with my experience or the normally accepted numbers. Intermittent misses should not be so rhythmic as to provide a steady 40* elevated EGT; It should be bouncing around. The long term change in steady state also points away from an intermittent problem. SOme of this may not apply to all mixture levels. You can test. Whilr in this cruise state, Drop one igniton and observe EGT rise. If the EGT rise on all the Cylinder is 40, then ignition is the culprit.

My guess is valve train. For some reason one of the valves is not fully opening OR fully closing. Have you done a wobble test to check for tight guides? Tight guides can create all sorts of issues, both opening and closing. A wearing cam lobe best explains the long slow drift from the previous state to what you have now. Pretty easy to put a dial indicator on the rockers and measure valve lift for a known good cyl and the problematic one. Also if guides were tight, one sticking event can bend the pushrod, thereby reducing lift, even if it only happened shortly and disappeared. However, this would have shown as a sudden change and not a slow drift. It would also result in a distinct lifter tick noise (think solid tappets)

If the cam lobe is worn, there would be metal in the oil filter.
With no metal in the oil filter I would check "dry tappet clerance".
Lycoming Service Instruction 1011L. Service Instruction 1424A, and Service Letter 206A.
Dry tappet clearance is the gap between the rocker arm and the valve stem cap when the lifter (tappet) is dry and fully collapsed.
For most Lycoming engines the dry tappet clearance is between .028 and .080 inches.

Good luck

chaskuss said:

Greg, Your system uses dual coils. That means that it's a "wasted spark" system [spark occurs at both TDC on compression stroke and at TDC on the exhaust stroke] It's rare, but I have seen those dual coils fail on 1 cylinder, but not the other. Easy way to tell. Swap the #3 ignition wire with it's mate. Watch to see if the issue is now on that other cylinder. If it does, you have a bad ignition coil. The fact that your CHT is dropping and the EGT is going up indicates that the fuel is still burning as it goes out of the cylinder. Your ignition timing can be "spot on", but if the spark is weak, the burn rate will be slower, causing the reading you see.

Charlie

One repeatable test is worth a thousand expert opinions - Bob Nuckhols

Click to expand...

Valid check, and easy to do - that goes on the short list.

Avanza said:

If the cam lobe is worn, there would be metal in the oil filter.
With no metal in the oil filter I would check "dry tappet clerance".
Lycoming Service Instruction 1011L. Service Instruction 1424A, and Service Letter 206A.
Dry tappet clearance is the gap between the rocker arm and the valve stem cap when the lifter (tappet) is dry and fully collapsed.
For most Lycoming engines the dry tappet clearance is between .028 and .080 inches.

Good luck

Click to expand...

Equally valid and only slightly more difficult to do - I'll get on this next week.

Avanza said:

If the cam lobe is worn, there would be metal in the oil filter.
With no metal in the oil filter I would check "dry tappet clerance".
Lycoming Service Instruction 1011L. Service Instruction 1424A, and Service Letter 206A.
Dry tappet clearance is the gap between the rocker arm and the valve stem cap when the lifter (tappet) is dry and fully collapsed.
For most Lycoming engines the dry tappet clearance is between .028 and .080 inches.

Good luck

Click to expand...

When checking for this kind of problem, you are much better off doing a traditional test at the rocker shortly after an engine run (lifters full). This form of test will also show a collapsed or leaking lifter that is causing reduced lift. A dry tappet clearance test is done on the low side of the cam circle (i.e. not the lobe). It will provide NO meaningful data on whether or not your lobe is wearing away. The lobe can be completely gone and still get a good dry tappet clearance. This test also cannot tell you if the lifter is capable of holding in oil to take up that clearance (primary job of the plunger assy).

wiring

I would still swap the elec wires and see if the problem stays. I've had a low EGT before that was caused from a pinched wire. Chances are slim but a quick test.

I had a very similar problem on my number two cylinder on O360/carb using lightspeed electronic ignition on right side and it turned out to be the coil. I think the bad coil was intermittent (worked fine sometimes) which made it much more difficult to trouble shoot. Seems as the engine warmed up the problem got worse. Ignition checks while airborne were not conclusive. Finally, one day I did a run-up ignition check during taxi back after a long flight and it was obvious I had a misfire on #2 cylinder. I had already checked/replaced plugs and ignition wires. Replaced the coil and has been running smooth with normal EGTs and CHTs since then. Good Luck!
Rascal

I would agree with the above post. Coils have a failure mode that only happens at altitude. This makes it tough to troubleshoot on the ground. If the EGT is normal on a ground check I would take a look.