Engine idle speed increases when coasting in neutral. When truck comes to a stop ( traffic light ) Engine Idle Speed drops to a normal level. This problem happens in the spring and summer when hits 65 or 70 degrees. Engine idles ok in winter. I have replaced o2 censor / I.A.C. VALVE / CANISTER PURKE VALVE. Please advise.

Thanks for your help.

glens99ranger

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1999 2.5 L Engine I4 5 Speed Manual Cubic Inches 153 Horse Power 119 @ 5,000 rpm Torque 146 lbs. ft. @ 3,000 rpm fuel 87 Octane Firing Order 1-3-4-2 Compression Ratio 9.4:1 Extra Long Bed Mud Flaps.

Could be normal. When you 'coast' at speed, the computer does not know... And is designed, in some emissions configurations, to keep the throttle plate (IAC in this case) open a bit on 'coast down' to cut back emissions. Deceleration can cause high vacuum in the intake manifold, and create more emissions than not developing that level of vacuum. Open the IAC a bit on coastdown, IN GEAR, and the high vacuum is not developed.
Try not coasting?
tom