I had that same prob, but a new injector pump cured it. P.S. I made my own pmd relocation above the intake (by about 1-1/2") two years ago and no problem since. The mounting bracket is made out of finned aluminum I cut off of an old sterio amp and I put an electronic grade heatsink between the pmd and bracket. The factory mounting locatiion is a great one for keeping it cool, but a terrible location if it needs replaced.

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Woody35 had a great point. Check the cheap and easy things first. A ground problem isn't that hard to diagnose or fix. Also, if the pmd has been replaced there is a little bracket that fits over your contacts. I don't know for sure what it does, but in the process of relocating mine it fell out and wouldn't idle correctly. I asked the guy at gem state injector service and they seemed to know all about it. They actually gave me a new one told me how to install it (real easy) and no more worries.