JAX57 - a mechanic wouldn't know if you had a relief valve in your airbox simply by working on your car - unless they were changing the air filter - and that doesn't happen often - 30,000 kms I think. The relief valve is a requisite addition to any SC. Without one it's only a matter of time before your airbox blows up from a backfire. When I bought my SC it was one of the first things I check - it the pop off valve had been installed. Hard to blame the mechanic for this one. But I reckon it's a question they would be worth asking any incoming sc owner - have you got a relief/pop off valve installed? Hope the car running better now.