EGT failure-Fouled plug? Engine behavior

[Pmerems]

Yesterday I took my RV-7A up for a flight after my conditional inspection. I have been flying my RV for over 10 years with only 500 hours on my bird. I know that is a bit embarrassing.

20 miles after departing the airport (KTUS) I noticed my number #3 EGT was displaying about 200 F degrees lower than normal. #3 CHT was normal. The engine was running smooth. I am flying with a Dynon Skyview and previously years with a Dynon D180 so I have always had full engine monitoring.

The EGT failure behavior I have seen in the past is rapid changes in the display, extremely temperature (2000+F) or total failure (X’d out on the display). Since #3 EGT was displaying at least 200F lower than normal and the rest of the EGT’s I was thinking maybe I had a fouled plug, magneto issue or just another EGT failure.

Out of caution I returned to the airport and upon landing the #3 EGT X’d out on the display. Now I had a better feeling the EGT failed. I did a run-up and mag check on the ground and the #3 EGT didn’t respond. Also verified it wasn't a spade lug connection or other wiring issue. So once again I am replacing another EGT.

My question for those more experienced then me is:
If I did have a one bad spark plug in a cylinder, would the EGT in that cylinder be lower than normal during cruise flight? Would CHT run cooler also? My thought considering the decrease in the burn efficiency in that cylinder the EGT might be cooler.

I have replaced at least 2 other EGT’s over past 10 years. One was a real pain because the tip of the probe swelled up and I had a very difficult time getting the tip of the probe out to the hole in the exhaust tube. Hopefully this one will be a bit easier.

Thoughts?

[flysrv10]

If you have one bad spark plug, your EGT will be higher. The combustion flame will reach the probe and cause it to read higher.

[scrollF4]

If one plug fouls (leaving only one good spark on that cylinder), the ignition happens late. That allows the fire to travel up into the exhaust, creating a HIGHER EGT reading on that cylinder. You should see that effect during your mag check when RPM drops (inefficient combustion) but EGT rises (the flame is closer to, or on, the EGT probe).

Those probes just go bad. I have replaced many, and at this point I consider them consumables and keep several on hand. Barring any other symptoms, so far IMO it’s a failed EGT probe.

[BillL]

Flame has nothing to do with it. It take time for combustion to reach the far side of the combustion chamber, so while the working plug fires on time, the effective average time of the combustion completion is longer, thus later in crank degrees. The loss of work performed on the piston leaves energy in the charge, thus higher EGT.

In the biz is is called heat release rate. Two plugs firing basically start two burn zones that meet (~) in the middle, so faster total heat release. Intake turbulence (swirl) will also increase heat release rate - think angle valve, vs parallel valve combustion chamber.

Picky off.

[kiwipete]

Paul

Save yourself the continuing pain of replacing Dynon probes and buy EI ones. Will be far more reliable and cheaper in the long run.

Regards

Peter.

[scrollF4]

Originally Posted by BillL

Flame has nothing to do with it. It take time for combustion to reach the far side of the combustion chamber, so while the working plug fires on time, the effective average time of the combustion completion is longer, thus later in crank degrees. The loss of work performed on the piston leaves energy in the charge, thus higher EGT.

In the biz is is called heat release rate. Two plugs firing basically start two burn zones that meet (~) in the middle, so faster total heat release. Intake turbulence (swirl) will also increase heat release rate - think angle valve, vs parallel valve combustion chamber.

Picky off.

I acknowledge that I have been picked. Bottom line: the hotter part of whatever is going on is going on closer to the EGT probe than normal, raising the sensed EGT.

[BobTurner]

BTW, you should see this effect (higher EGT when operating on one plug) during every runup mag check.

[krw5927]

Depending on your engine monitor wiring, you may wish to check the crimps on the terminals that the EGT probes plug onto. If you're seeing repeated failures, you may have an intermittent crimp on the engine monitor side that wouldn't be permanently resolved by just replacing the probe.