Will installment loan mess up credit when already approved for car?

Ok just trying to get some info. I have recently been approved for a suv i wanted and am putting money down on it. But because of this it has left me short a little. If i was to get an installment loan that says no credit checks which is my last and most dreaded option, would that mess up me finalizing my truck? Will the loan Company even check my credit again when i go in to finalize? Or is it best to wait till that's all done before doing any of that?