Occasionally, my 2004 Ford Ranger 4x4 Automatic will rev up to about 2500 RPMs on startup then immediately idle down to 1200 and then 750.

It has 585XX miles on it and I recently replaced the IAC (From Ford) and spark plugs and wires were changed at 50k. I cleaned the throttle body but still having this issue.

It's almost like a vacuum leak but that would stay constant and I can't hear any whistling.

What else could it be? It's not throwing any codes.

Thanks in advance.

 

I would suspect the Intake Air Temperature sensor and the water temp sensor. The PCM maybe getting fooled into thinking the air temp is colder than it really is or that the engine is cold and richen the air/fuel mixture at startup .