I've done some thinking about this, and I wonder if the low CHT #1 indication could possibly be due to excessive resistance somewhere between the CHT probe and the engine monitor. Each CHT probe is connected to about 18" of metal shielded cable, then each of the two wires per probe has spade connectors that connect them to extension wires that go to the engine monitor. The CHT probes are thermocouples, and if there is a high resistance at one of those spade connections, it could lead to the engine monitor sensing a lower voltage than the CHT probe is actually creating.

Next time I have the cowlings off, I will inspect and reseat the spade connectors for the #1 cylinder, then I'll check it in boiling water again. If that doesn't do it, I'll disconnect the #1 and #3 CHT probes at their spade connectors and trade them, and check with boiling water again. It'll be useful to see if the problem follows the probe, or if it stays on the #1 cylinder.

If the problem stays with the probe, I probably need to replace it. If the problem stays on the #1 cylinder, I'll pull the connector at the EIS and check for pins that aren't fully inserted. Pulling and reseating the connector should clean off any corrosion too. Next, I'll swap #1 CHT with another cylinder by moving the pins in the connector, to take all the wiring out of the picture. if the problem stays on the #1 cylinder after all that, I'll contact Grand Rapids, as I almost certainly have a failed engine monitor.