1992 4.0L Loud hissing sound and rough idle

[sauceman]

Hello everyone.

Yesterday I started my ranger and it had a very rough idle. The RPM when sitting would shoot up causing the truck to shake like crazy. It almost sounded like it was going to stall out at some points, but it did not. When I give it gas, it is normal for a few seconds, and the problem proceeds. The truck is fine when it is getting gas, it is only when the accelerator is not applied that this issue occurs. I recently changed the fuel pump relay. My spark plugs/wires are about two years old, but I am going to replace them. Under the hood, I can hear a very loud hissing noise even when the Air conditioning is off. This is something that has just started. The truck never idled perfectly smooth, but this is the worst its ever been.

 

[Uncle Gump]

You have a vacuum leak... if you can hear it... should be pretty easy to find. There aren't many hoses/vacuum caps on a 4.0L.

 

[RobbieD]

Does sound like you bumped or broke something and now have a vacuum leak. Look and listen for a disconnected hose; you can start at the vacuum tree and follow the hoses.

And first look close at your PCV valve. I think it's on the driver rear valve cover on yours; these get brittle with age and break easily.

 

[sauceman]

Does sound like you bumped or broke something and now have a vacuum leak. Look and listen for a disconnected hose; you can start at the vacuum tree and follow the hoses.

And first look close at your PCV valve. I think it's on the driver rear valve cover on yours; these get brittle with age and break easily.

Turns out the problem was a missing cap on the Vacuum tree. Simply plugging the hole fixed the issue.

 

[RobbieD]

Thank you for posting the resolution. I'm glad it was an easy fix.