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I would check to see if there is any Vacuum hoses on the transmission.

On GM Trucks/ Vans there is a small pod on the side of the transmission that has a vacuum hose to assist in shifting esp for getting into the passing gear on the highway these hoses often break making a vacuum leak to the engine that would also make your engine race or idle too fast would be a better term. It will be obvious when you crawl under the van look on the side of the transmission around the pan depending on what type of transmission it is it could be in different locations. there are different automatic transmissions GM uses.

Look for a small round tin thing Called the Vacuum Modulator link below is a photo of a couple http://spacecoastfieros.com/tech/440-4T60/Modulator.jpg that is threaded into the transmission or bolted. if that hose is not broken at that end follow the hose up to the engine see if it has broken up there or it may have just come off for what ever reason.

That would be my guess and the first thing I would look for.

Hope that helps

If that isn't the problem then it could be something else possibly electronic if it is EFI. alternator wire disconnected could make engine idle fast on EFI.

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16y ago
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Q: Why would a 1990 GMC Vandura 3500 w 57l automatic 80k miles have problems switching gears on the freeway and even downshift in higher speeds causing a racing engine?
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