Hi John65,
I agree with the above comments. Unfortunately, if you were the one who washed the engine upon reliance of advice given to you by an unqualified mechanic or service person, you cannot sue for negligence of the unqualified mechanic. This is because from the court's view, (i) you assumed all responsibility and risk for anything going wrong by carrying out the wash yourself; and (ii) no reasonable person would rely on one opinion of an unqualified mechanic; and (iii) you did not mitigate your risk by seeking the advice of many qualified mechanic opinions or a service company (i.e. professional).
Further, consumer affairs cannot help you as you did not enter into any business transaction with a company/business, the person you asked did not advise you in their professional capacity or the capacity in which they were employed, hence, Australian Consumer Law will most likely not cover this.