Thermo Coupling in a motor vehicle and unwanted noise

Who has a solution to run a thermocouple on an engine? I am trying to log RPM and Exhaust Temperature. I am getting noise which I think is caused by the ignition system.

I am using a MAX6675. If the engine is not running it will calculate the temp just fine. If I disconnect the Max6675 it will record the RMP perfectly. If I have the MAX6675 connected then it will work for a little bit until the noise (RPM) gets too high and it resets the Arduino.

How can I use a different thermocouple or optocoupler to isolate it?

I don’t think I can isolate it from the exhaust as it is screwed in and in the gas flow.

Any suggestions appreciated.