Repair guides and support for C/K pickup trucks sold by General Motors under the GMC brand, including the GMC Sierra.

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started running poorly a few days ago

My 89 GMC sierra does not have a vacuum modulator valve,it started running like crap a few days ago and shifting from first to second,it slams,shifts hard going into second. Why?

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I would take it to an AutoZone and get a free computer read out so you have even an idea of where to start looking.

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Gary Seely will be eternally grateful.
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