EGT Mystery, help please

After about 600 hrs of run time on our O-320-E2D, fixed pitch prop, I've developed a rise in EGT on just the #3 cylinder and I just can't seem to figure out the cause. This happened rather abruptly, it's not been creeping up over time.

Here's the basic scenario. At cruise power settings of 2400-2500 rpm, full rich and 1000-3000 density altitude. I'm seeing 1450-1470 F on just cylinder #3, the others are 1290 to 1320. All four CHT's are between 320-340 F.

For the past 10 years these settings would lead to about 1370 on cylinder #3, so I've basically seen a 100 F rise over normal.

This high reading seems very steady, it does not oscillate. (as might be expected with a bad exhaust valve seat).

Here's what I've checked with no improvement:
1) installed brand new spark plugs in cylinder #3
2) did a hot compression check, (all cylinders 72/80 or better)
3) swapped egt probes, high temp stays with cylinder
4) changed the intake tube seal and checked for cracks on intake pipe.
5) checked and tightened hose clamps on rubber hose that connects intake tube to sump.
6) used a shop vac in blower mode to force air into the carb intake and used foaming bubble juice to look for intake leaks on every connection going to cylinder #3.
7) confirmed that the primer system is not connected to cyl #3


So there you have it. IDEAS would be greatly appreciated!!

Thanks,
Bob Newman
40176 finish kit
Glastar N99RN flying

It might be the wire from the probe to the monitor, or the monitor itself. Swap the wires at the monitor and see if #3 is still the hottest. At least this will rule out all the monitoring equipment.
It could be a mag issue, or a plug wire issue.

As Tom said, perhaps something is screwy with one ignition on that cylinder. What happens if you run on only one ignition, and then the other ignition? All EGTs should rise when you run on one ignition. I think the increase in EGTs for each cylinder should be about the same when switching from both ignitions to single ignition.

Hate to say this, but a leaking exhaust valve could also be a cause---

When you did the comp test, did you listen for a leak at the exhaust pipe??? You are loosing a bit of pressure, but where is it going??

Yes, I know the compression is not bad, but you are not testing in exactly the same conditions of a running engine.

I'm going with an ignition problem to that cylinder... same thing happened to me.

What they said on one mag.
Plus does it peak with the others?

Ground run-up mag check shows no difference at all between left and right mag. All cylinders have egt rise running on one mag. rpm drop on each mag is about 80 rpm. I'm going to repeat this in flight, with the engine obviously under more load than the ground run-up

During compression test I didn't hear anything leaking, the pressures have been constant over many years now. I'd have to repeat this to say 100% there was nothing leaking though.

I'm going to swap the instrument to probe wires on cyl #1 and #3, I've already swapped the probes themselves and know that's not the issue.

Both mags have less than 50 hrs on them since new, brand new plugs in cyl #3

Fuel flow and IAS remain exactly as normal (ie I believe I'm making the same horsepower as ever)



keep the ideas coming!

Thanks,
Bob Newman

egt

Try to ohm check the #3 lead wire. Most mechanics have a lead tester.Marc

My issue was in the Mag...

One other issue I had back in the earliy '90's on an O-320 E3D Lycoming engine that caused this same issue was the intake tube had come loose at the sump itself and it was sucking air at the sump end of the intake tube... another place to look.

Is this a higher reading after rop or lop.

Reiley, When you had the Mag issue, did effect all the cylinders?

Also, I checked the intake fitting at the sump by using a shop vac in blower mode forcing air into the carb and then applied "gas leak bubble juice" liberally to this area. No detected issue there.

Thanks,
Bob

I recall it being noticable somewhat on 1,2 and 4 but because #3 is hotter than the rest from the get go it the #3 difference was a lot wider spread in temp change than the rest for sure. I did realize a change in performance when this issue occurred.