High EGT after cylinder replacement

Interesting.

Clearly you don’t have GAMI injectors because the flow rates would most likely have been slightly different

So, what appears to be left is, as you say, the harness, and that includes the magneto cover plate which contains the distributor contacts. That portion is not overhauled. The part which is overhauled is this:

At what sort of power setting (total fuel flow?) is the 300F EGT difference seen? It cannot be at a relatively high power, say 50%, because the EGT would then be about 1800-1900F which is physically impossible for avgas.

The other thing worth checking is whether they actually moved the EGT probes. If they did, say they swapped #5 and #6, then the 300F increment would be showing on #6 on the instrument. If it is showing on #5 then you have a problem on the instrument, on that input.

placido wrote:

They have performed the mag checks and it clearly shows that the EGT drops exactly on the cylinder 5 when the engine runs rough on one cylinder.

In summary,

• you replaced cylinder 5,
• cyl 5 EGT is 300°F higher than the other cylinders (and than before the cyl replacement)
• you have roughness associated with an EGT drop on cyl 5 when on one mag

One half ignition harness is about 300 €, I would think. 6-700 € for both sides.

You are making progress, but they could have started with the easy (cheap) things first. Like doing a run-up and a mag check.

And you definitely need an engine monitor capable of downloading data. It would have been clear from the engine monitor data that one cylinder shut down during mag check.

So the probe that was in cyl 3 went into cyl 5 without changing any connections, and at that point cyl 3 was 300 ℉ hotter than the others. That rules out a failed probe, but also I strument error.

Also, prior to the cylinder change, cyl 5 showed 300℉ lower than what it does now? Dirkdj is right that the absolute number is not important. But what is important is the baseline iow what it used to show before the cylinder change. I assume they reused the same exhaust pipe so nothing has changed wrt the position if the probe.

So maybe the 300℉ is just due to the ignition issue, although it is a big difference. On my engine I see max 140 ℉ difference between one and two plugs firing.

One way of detecting a faulty wiring harness is to run the engine in the dark.

. . . . or find a buzzer coil – or someone with same. You´d hook it to the mag side of the HT wire and see what arrives at the corresponding spark plug. Then do same test without spark and watch, what gap the spark does from HT wire to earth at maximum. That way you can do tests with all wires, including the mag cap if you like. You should see same spark gap at the end like the one you feed into the HT wire, else it finds an easier way in between and gets lost . You would not like to look into all places with the prop spinning ! Vic

@placido, Vic is referring to using a high voltage cable tester. I have used it myself, but I do not know how good it is. Worth a try.

Actually I did not know that test equipment and my idea was a real ignition coil that is used on the Yak or Lanz Bulldog (tractor) for starting before you switch over to the magneto. The coil has an automatic buzzing contact breaker in it to produce continuous 15 mm long sparks. That is quite useful to do a real high tension test on all critical parts in the ignition system. Vic