what

They seem just fine to me?

will check the latest beta

what causes a vehicle to make the engine gauge not work, despite a running and functional engine in it?

Seems to be a [packard] with that specific vehicle then, my bad

but i've never seen a bug like it before, so that confuses me

Does this help?

Your instrument slot def probably has an optionalPartNumber on it that's making IV look for an engine that doesn't exist. Spedometer works because that variable doesn't care what it's on, but since RPM is an engine-specific variable, the code won't ask the vehicle for that info as it knows it won't have it.

Closing as this was due to part slot being incorrectly referenced.