I have a '91 D15 when started cold it takes time to idle right (dies out if I don't hold accelerator). After warm runs/idles good. I'm sure theirs something I.e. Vacuum or something that's supposed to hold idle until it's warm. I'm not sure where it is. Vacuum thing on throttle side seems to be working after its warm. I'm sure there's supposed to be something similar to hold idle when it's cold. Please help.