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high idle in neutral and while driving.


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got an 87 4.0 5 speed manual that is idling high (up to 3000-3500) at startup. I had this happen on the last trip i took, and i found a leak in my vacuum line. since that the issue hasnt shown its face. recently it started happening again, but it is only temporary and if i restart the truck, it will sometimes go away. aside from vacuum lines, what else could be causing this? Thanks in advance for any help. 

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Its been reported that loose manifold bolts can cause high idle, i have never experienced it, but it makes sense. Its just another place where vacuum can be lost. Another, is your IAC (idle air control) valve. It could be dirty and sticking open.

 

 

This is the location for the IAC on an HO (91 models and on) On a Renix (90 and prior year models) it is simply on the front of the throttle body instead of the back.

 

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