WD40 will also work, it's basically to identify an air leak, if you spray it sparingly around the manifold with the engine running and the revs rise it shows there to be an air leak.

Have you checked the clutch switch I think it would have one, as you say it's when you released the clutch very much doubt it but it's worth a look as it has been known for the connector to be kicked off by a stray boot!

Martin