Weird Idling Issue

[AmericanGunner]

Hi all,
I am having a odd idling issue with my 91 Explorer Sport 4.0L, about two days ago I went out to leave for work when I started my truck it gave a small back fire and started idling a little rough until this occurrence I have had zero issues with my truck.

Today it started idling high then low then high ( you get the picture) BUT here is the odd thing it only idles high when I am driving my truck,it can be in neutral (5 speed) and it will idle around 1500rpm but here is the weird part when I come to a complete stop it will idle down to 800rpm or so please keep in mind this is with the truck free wheeling and my foot off the gas I can't understand what would cause such a issue.

Almost forgot to mention I think there maybe a vacuum leak it sounds like it is coming from the top firewall area of the engine.

Thanks for the advice

 

[rookieshooter]

How about the obvious....the leak? Move all the hoses while idling and listen for any rpm change or leak. I have heard of people using a sray bottle of water and spraying all the hoses and supposely this will slow the leak enough to hear a change. How about doing a Mass air sensor cleaning.

By the way welcome and also I don't know it this should be in the 911 section

 

[lifeguard379]

mine did the same thing on the back of the menifold there is a fiting with all the vacuum lines are ther is one that has a cap on it that prob blew off cover it and it shuld be ok

 

[Infragate]

The vacuum tree is located on the drivers side of the manifold, when you inspect it you will see that one of the input lines is probably just sucking air as you already know it is leaking by the sound its making, but just make sure that it was a cap that blew off and not one of the lines. If it was a line, then you are gonna have problems if you just cap it. The one i see blow off the most is the one that is going to the fuel regulator, so have a look around and make sure it goes back the way it was.

 

[AmericanGunner]

mine did the same thing on the back of the menifold there is a fiting with all the vacuum lines are ther is one that has a cap on it that prob blew off cover it and it shuld be ok

That was precisely the problem thanks for the help everyone

 

[ManixT]

Good call on the hidden vacuum line lifeguard!
and you too infragate, what you described was part of the issue with my car smelling like gas.
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