High EGT after cylinder replacement

Just wondering, is it possible for any ignition issue to produce a 300F increase, at low power?

Instrument issue has not yet been eliminated.

One plug not firing isn’t going to give a 300F increase, that sounds like at least half an order of magnitude higher than any change you’ll see from one spark plug not firing.

We had exactly the same problem with roughness on one mag with our engine not 3 weeks ago (ours isn’t as well instrumented though, and we hadn’t changed a cylinder, nor were EGT values anything special).

Problem: Engine ran rough on one magneto (on 3 cylinders).

This is how we troubleshot it.

1. Find the bad cylinder (you did that already). I did this by running the engine at idle for about 45 seconds on the mag that it was running rough on. Find the exhaust that’s cold. Confirmed by the EGT reading on that cylinder showing a rapid drop off when on the bad mag.

Now start from the easiest thing to do and work up in complexity:

2. Swap the spark plugs over on that cylinder to eliminate a problem with a spark plug.

Problem did not move with the spark plug, showing the spark plug is fine.

3. Next check the harness.
a. Low voltage test: Multimeter showed a known good lead had a resistance of < 2 ohms, the suspect lead was open circuit.
b. high voltage test: High voltage tester also showed open circuit on the suspect lead.

So there was the problem. Start with the easiest stuff first (swap the plugs over, I think you said you did that already). Then test the harness – if you don’t have a multimeter, they aren’t expensive (and your mechanic ought to have one anyway), and your mechanic ought to also have a high voltage tester.

If you find the harness is bad and replace a side, also consider the other side of your ignition harness – is it a similar age/condition? I replaced both sides on mine since the harness was 20 years old and I didn’t want to replace one side only to have in 6 months time the other side let me down on some unattended airfield in the middle of nowhere.

If the EGT is still 300 degrees higher and the engine runs smoothly and correctly on both mags, and the fuel flow is correct on all cylinders, then you have a problem with the instrument. You can also confirm that by watching how the EGTs on all cylinders change as you lean the mixture (do they all peak roughly around the same time?)

alioth wrote:

One plug not firing isn’t going to give a 300F increase, that sounds like at least half an order of magnitude higher than any change you’ll see from one spark plug not firing.

Yes. That had me surprised from the start. I see max 140°F difference during a mag check. The other piece of information that was missing from the start was engine roughness on one mag, and we do not have a baseline from before the cylinder was replaced.

Rough engine when running on a single mag – Probable causes

• Bad plug Plugs were swapped with no change
• Bad HT lead
• Problem in the mag distributor cap

Higher than normal EGT on the new cylinder – Probable causes

• Bad probe Probes were swapped between two cylinders but high EGT remains on the same
• Poor probe connection
• Instrument problem I do not think it can be instrument problem because when the probes of cyl ##3 and 5 were swapped (without touching the connectors), the higher EGT moved to cyl #3 on the instrument IIUC
• Leaky exhaust valve New compression test was made of the new cylinder and dry tappet clearance was checked
• Cylinder running leaner than the others The amount of fuel from the injectors was measured
• Ignition timing Would not have affected just one cylinder
• Valve timing Would not have affected just one cylinder

In addition to what alioth suggests, you should also disconnect the cyl #5 probe, clean the connector, put some Deox or Stabilant on it, put it back together and see if that cures the higher EGT reading.

There is an easy way to test the particular channel on the instrument. You need two resistors, a battery (say 1.5V, or a 9V PP3 will do) and google for thermocouple tables and look up the voltage required to simulate a Type J or Type K thermocouple.

However assuming that somebody checked the ambient (cold engine) readings are all the same on all 12/13 probes, this is an unlikely fault, but not impossible. It remains possible because a thermocouple instrument does not read the probe temperature, it reads the temperature difference between the probe and the connector on the back of the instrument. There is compensation for the temperature of the connector, of course… Whereas all other temperature indicating instruments (PT100, thermistor, AD590, etc) do read the probe temperature.

I normally have one but not currently, due to this. It is in the USA…

Also I don’t believe a mag could produce a +300F result, unless you are feeding nitromethane into that cylinder at the same time

Maybe I missed it above but what is the CHT like, on a ground run at say 2000rpm for a few mins? If it is in line with the others, it is probably an instrument issue. I outlined a test method above; that really needs to be done to eliminate that.

Stickandrudderman wrote:

If you don’t trust your mechanic to provide the answer and the solution, WTF are you paying him for?

Mechanics are trained to fix things (i.e. known problems), not trouble-shoot. If you find a mechanic that is a good trouble-shooter, then you’re lucky and should stay with him. But that is not what he’s trained to do. Trouble-shooting starts with the pilot/operator if the aircraft is privately owned. And that’s what all the input on this thread is about – helping placido trouble-shoot.

Mechanics are trained to fix things (i.e. known problems), not trouble-shoot

That is just another way of saying that mechanics have no idea what they are doing, beyond changing parts True in many cases…

But, yes, aircraft ownership does require owner involvement at a technical level, in most cases.

Well this mechanic spends most of his working life trouble-shooting so perhaps I should have a different title?

Yes. You are an engineer