96 ranger hisses..cant find vac leak

[96xltV6]

hey guys. heres a little background on this ranger. I bought it from a used stealership, big mistake. the dealership kept it for 2 months trying to get it to pass emissions so they could sell it to me. i got it for cheap cuz of that.

it developed a misfire. i had so many things replaced..cat converter, iac, maf, tps, coil pack, wires, plugs, and none of it fixed the misfire.

a couple weeks ago i took my plastic intake manifold plenum off, and the misfiring cylinders fuel injector plug was on upside down. i flipped it around and put it on correctly, threw everything back together, and it doesn't misfire anymore.

Now, there is a hissing noise that i can hear from inside the cab. My idle sits at 2500RPM idling.
this hissing noise did not exist before i took the plenum off. I've checked all the vacuums i can find, and i even have a haynes manual, but i can't find it. It can best be heard when i turn the engine off, cuz it hisses for about 2 seconds after the engine is off. i've tried the unlit propane torch, water, wd40, brake cleaner trick, spraying it near the engine, and the idle doesn't change. every vacuum hose is on tight.

a couple days ago i took my plenum off and the gaskets look fine, and i orange rtv'd everything. it didn't make a difference. i still hear the hissing noise.
How do i diagnose this hissing noise? I can't feel it because of the wind that my fan creates.

I've been trying to fix this truck since march, 2007.

 

[97ranger4x4]

Thats weird that your idle is so high. I am guessing you connected all of the sensors. No check engine light? I wonder if you need to replace the gaskets.

 

[MAKG]

A commonly overlooked leak site is the power brake booster, and it's particularly likely if you can hear it in the cab. Some models also have vacuum controlled climate control; the motor(s) live on the passenger side under the dash.

Try squeezing its vacuum line with a pliers and see if that does anything to the idle. Alternatively, pull off the line at the intake manifold and cap the nipple. Then idle the vehicle to see if the hiss is gone.

 

[baddis]

+1 on makg's response. i have seen many brake boosters go bad

 

[Silverado]

One way I've seen to find a vac leak is to take a propane torch, open the valve a little, and place the nozzle near suspect areas.

If the vacuum takes in some propane, the engine should bog down.

YMMV... just one thing I've seen done. I'm not sure how safe it might be? I sure wouldn't be smoking while I did it LOL

 

[Evan]

It sounds like a brake booster or HVAC line. I would take disconnect the lines at the manifold one by one. If you remove a line and the idle increases, reconnect it. When you remove a line and there is no change in idle speed, you've probably found the culprit. The next step is to find out where the line goes, then trace it till you find the leak.

EDIT: I missed MAKG's plier suggestion above....that's an even easier way.

 

[96xltV6]

it didn't have this problem before i took it apart and found the 5th cylinders injector plug on upside down. the sound of the hiss when i have someone turn the truck off does not come from the general area of the brake booster, though.

oh. I used to have p0174 and p0305 when the injector wasnt plugged in correctly.

Now i have p0171.

 

[Evan]

Maybe somehow the intake manifold didn't get bolted down squarely. Or, one or more of the studs holding the fuel rail onto the lower manifold got loosened. This could cause a hissing sound and vacuum leak.

 

[smokey]

lean code, a vaccum leak will cause that code to be thrown.

 

[96xltV6]

 

[pillen140]

its easier to use carb clean to find a leak, brake clean smokes like crazy if it gets into the intake, not good for you either. the heater control is also known to go bad, (hoses melt or dry rot)

 

[Wes]

I just paid to have my check engine light turned off... Had codes telling me that both banks were running lean. The shop used a smoke machine to see where the leak was. It was under the intake plenum. Had them replace every gasket under there. Now idle's good and that damn orange light is gone. Let me know if you want a list of the gaskets they replaced and I'll find my reciept.

 

[MAKG]

FYI, it's NORMAL for the air conditioner to hiss for a bit on shut-off if it's been running. It comes from the bottom of the evaporator, near the passenger side frame rail.

 

[rangertotty]

i had the same problem on my other truck... it ended up being the plenum gasket

 

[96xltV6]

which plenum gasket? the one thats inbetween the EGR input port and the plenum near the throttle body? I Rtv'd that, and the gasket is there, too. Also, i taped around it with electrical tape, and i still hear a hissing sound.