Some strange happenings with CHT and EGT

In addition to Roger's advice you may like to consider removing all the earth/ground connections you can easily access. Give both terminals/sides a "wire brush"/sand to remove any potential corrosion (make bright). Dab on a little electrolytic grease before reinstalling. This will ensure a good, durable, contact.

Sean, do you mean dielectric grease?  If so, keep in mind that dielectric grease is non-conductive and it should not be applied to the mating surfaces of a connection.  For open connections like a ring terminal on a stud, clean the surfaces, tighten the nut down to achieve a gas-tight mating of the components, then you can apply dielectric grease to prevent moisture from reaching the parts.  For a connector (like a Deutsch, for example), the grease is applied to the seals to help prevent moisture ingress.  It should not be pushed into the sockets where the pins go when the connector is mated.

The proper use of dielectric grease is often debated, and it is logical to think that a non-conductive grease could interfere with the continuity of a connection. However, many automotive manufacturers now fully flood all connectors before assembly.  And in fact, the Rotax maintenance manual for the 912iS says to coat all connector pins of the wiring harness (except the ECU) with dielectric grease after cleaning them.

 The reason the connection is not compromised is that silicone dielectric grease has a very low shear strength and is easily wiped aside by the connector pin as it’s inserted, or the pressure of tightening the nut on a ring or fork connector.  The remaining grease then tightly surrounds the portion of the connector not in direct contact with its mating surface, and insulates it from corrosion and arcing.  You can easily test this with an ohm meter, you will find that resistance is not increased across a (good) connection when fully coated with dielectric grease.  

Thanks Roger.

the ground in the case of the CHT is right next to it on the Cyl head and is tight.  I marked it too, so I know it has not moved.  Checked it, still tight.

So very annoying.

The one thing I am considering is the wires are cable tied to the motor support, and it runs right near the exhaust, and I am wondering of they are just getting too hot?

I concur with Roger. I was bedeviled for almost two years with what turned out to be false readings on oilP & oilT. The instrument is an AvMap EngiBOX “glass” device. After much testing and thought and brainstorming with guys like Roger and Brett at LEAF and my A&P Buddy Don Forse, I fixed the problem.

My thinking is that these ‘glass’ instruments are basically computers and are much more sensitive to ground issues than steam gages or other analog devices. Also, the sensors I am using are grounded to the engine block where the coils and ignition are grounded. I reasoned that this was a bad ground “environment” for these sensors.

Solution: I removed the EngiBOX ground from the ground block I use for other devices and gave it and my EFIS and my fuel computer their own “direct to battery” ground. I also put them on their own circuit breaker. Then I wired a ground direct from the battery to one of the bolts on the oil pump because the oilP and oilT sensors ground to it.

Problem solved.

This is a RANS S-12 with a 1991 Rotax 912UL - I’m the forth owner…