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Van accelerating on it's own and is now overheating

My 2001 Chevy Venture van is accelerating on it’s own and is now overheating. The heater works, the AC works but the defrost doesn’t. I can be at a dead stop, put it in drive and it starts going on it’s own, even up a hill. It’s also developed a ‘knock’ or ‘jump’ with it automatically shits gears. I simply cannot afford a new engine, we did that last year when it overheated and blew the gaskets.


You’re just going to have to take it someplace. You described a whole big mess of things without really providing the info that people would need to try to help you sort it out. My first guess is that you have at least 3 completely different issues (idle speed, hard shifts, overheating), but there is a huge list of different combinations of problems that can explain all of that.

If you do want some ideas then provide some more info. E.g. when the engine was replaced was this a new engine or old one? How many miles are on the van? The engine? What has the maintenance history been like (“regular oil changes” is meaningless - describe the more intensive service stuff like plugs & wires, filters, etc).

Is the check engine light on? Any other warning lights? If so, then the van needs to be scanned for error codes. Many auto parts stores will do that for free. You need the exact codes, such as “P1234”

What do you mean that the heat/AC work but not the defrost? Are you saying that you put the selector in the defrost position but the air continues to blow out someplace else?

Have you checked the coolant level? The transmission fluid? Check out the throttle body for sticking.

Here are a couple of possibilities off the top of my head:

  • you have a leaking intake manifold gasket (very common on these engines) that is giving you a vacuum leak (high idle) and coolant leak (thus overheating). The overheating is also overheating your transmission fluid.
  • you have a bad throttle position sensor interfering with the idle and giving hard shifting, and then a completely different cooling system issue.
  • you have a vacuum leak associated the HVAC controls (thus no defrost & a high idle) and a completely different cooling system issue that then may or may not have anything to do with the transmission.
  • I suppose I could go on…

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