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the hiss will be a vacuum leak as the vacuum is at it's highest when deceleration in engine rpms is occurring ( at idle reading is around 15" of mercury but on deceleration goes to high 20's
This will cause a hiss sound in and hoses ,pipes ,brake booster, auto tranny actuator or vent controls controlled by vacuum
Your going to have to do some wiring tracing. You need an ohm meter. Since you still get a reading with the tps unplugged, you have a shorted wire or the ECM could be bad. The ECM is under the air filter. Unplug them both and check the wires for short to each other. The ohm meter shouldn't show a reading. If it does, you found the problem. Also check those wire for short to ground. Since the map sensor is right there I would also check the tps wires to the map wires for a short. Again there should be any readings. If all the wires check OK, then I would be looking at an ECM replacement.
I'm kinda guessing the "hose" you are "pinching" off is the air assist to your injectors as most IAC Valves don't have any hoses unless it's this type of system. Are you gettting any codes or is your check engine light on ? I would guess that you have a vacuum leak which would be the most common cause of this kind of thing. You can check for one by spraying some carb/thottle body cleaner on your vacuum lines and aound the intake manifold, if the idle changes as you are spraying, you have located the general area of the vacuum leaks. If you are haveing problems in the IAC valve your check engine light should come on and pulling the code(s) will help you to narrow down the problem. Comment here to let me know if you have the light on and if you are gettting any codes and what they are. This would help me to help you better. Below I gave you a description of how the system works to help you understand the system better. I would check for a vacuum leak first. I know this wasn't a whole lot of help but I hope it helped you a little bit anyhow.
Do you have a check engine light on? First thing I would do is unplug the IAC motor when the engine is warm. If the idle speed doesn't change you have a vacuum leak or a bad IAC motor. If the idle speed goes down you might have a bad TPS or IAC
Interesting problem... I myself own an 88 Ford Bronco II 4wd 2.9L V6. I have issues with my idle reving up to 1500 to 2000 rpms at stop. I have not yet resolved the issue, however, I do believe that sometimes electronic components are to blame...such as the brain or "Mother Board" of the vehicle. I replaced the "Brain" on two previously owned vehicles with instant results. I would suggest the local junkyard for a try...only as a last result. The "Brain" is located on the passenger side along the Interior wheel well. If you look along the front passenger side wheel well inside, beneath the plastic, vinyl, etc....you will find a metal enclosure that is comparable to a computers cd drive or hard drive........Approx. 4" x 6" give or take. The Board will have a wide slot plastic attachment plugged into it...very similar to an IDE slot on a computer.
This is the Brain......for your Bronco II.
Buying a used Brrain is a Gamble ...
You could get a bad brain!!!
BUT...You could get an instant repair!!
Who's to decide?
I had an 1986 BMW 325I
Suddenly One Day....
It started Reving up and down erratically....I was at a loss.
Luckily, I had a friend who was a BMW mechanic, and informed me that there was a relay in my glovebox that controlled my idle. I took a chance and tried replacing the relay with one I purchased at the junkyard...although they wanted $50 for it...I was willing.....and the new /USED idle conrol relay turned out to be an affordable fix. Thankfully