Just recently, I noticed something after a drive. While sitting there idling, fully warmed, the engine will slow rotation down to about 600RPM and idle somewhat rough. It doesn't feel like it's going to stall, and is very steady.

However, if the idle is raised just slightly, the roughness instantly goes away. There are no other symptoms. This makes me consider it's a vacuum leak, but there is no check engine light and I don't hear the iconic hissing sound of a vacuum leak.

Is this normal? 1999 Flex Fuel 3.0 automatic, 114,5xx on the clock.

 

Something to note is, it doesn't idle like that all the time. It's intermittent. I don't think it's anything to worry about, but I like to be sure and not just guess.

 

Elm 327
Torque pro for android
Obd fusion for Apple

Check fuel trims at idle

A vac leak would show the long term trim over +5
Learn fuel trims if you don't know.....you tube video.