96 ranger hisses..cant find vac leak

[MAKG]

It's not the plenum (aka upper intake) because that's not going to hiss for two seconds after shutoff. Unless that was a red herring (A/C). About the only vacuum systems that can do that are leaky check valves, vacuum reservoirs, or vacuum motors (or the lines between them). Your power brake booster is one of these. Your climate control is another. Some EGR control circuits do this, but I think yours is electrically actuated. It can't be the intake manifold itself because it leaks down one volume of vacuum at idle in at most 2 crank rotations (if it didn't, you wouldn't be able to idle the engine). That's about 10 times faster than you're reporting (I calculate 150 ms for 800 RPM idle -- and assumed intake volume is about equal to displacement, as is common).

 

[96xltV6]

i did not touch anything other than the things i had to move to get the plenum off when i was finding the source of my misfire, so i dont know how any valves could all of the sudden leak. i've had a leakdown test and i passed recently.

 

[MAKG]

What makes you so sure it isn't a coincidence?

A "leakdown test" is performed on the engine for diagnostics. It is not related to a vacuum leak (though it might explain low vacuum if it failed). It is only applied to the cylinders; your intake manifold can be gone and you can still pass a leakdown test.

 

[96xltV6]

since i can't find it myself, what should i tell the garage to test for, since a leakdown test doesnt find a leak? I don't want to keep paying for things that won't fix my truck.

What has to be done? And why does my AC have anything to do with the sound i hear? I don't even have my heat on, and its 10 degrees out.

 

[MAKG]

There is no reason A/C can't be warm. That dries the air very efficiently, and your model will turn it on for you in several selector positions for that reason.

I run my A/C year round. Winters are very wet, and it makes the difference between being able to see the road and being stuck.

There really isn't much in the vehicle that can hiss after the engine is off. The A/C orifice tube is one of them.

 

[96xltV6]

If its foggy, i just turn defrost on. cars and trucks without ac have a defrost.

anyways, how can i disable it so that when i turn the truck off, i wont hear the noise that it makes? it might be throwing me off from discovering the culprit.

 

[96xltV6]

Hmm. I should have replaced the EGR port's gasket that seals the inlet..because I RTV'd the outside of it for the second time last night, completely coating the lip, and this morning, i unplugged my battery to clear the code, and it runs like it did before I had the vacuum leak It still has RPM's that stick around 1500 for the first ~5 seconds with the clutch in or the stick in neutral at a stop before it drops to 1000, but at least it's not running lean anymore. by the way, what i just mentioned is how it used to run as well.

 

[96xltV6]

 

[Tsestak45]

I know this is old and you probably don't even have that truck anymore but I have solved the problem for rangers that idle at like 1500 until you come to a complete stop. It is 1,000% your TPS. I know this because I spent about 4 months working on it replacing things one by one until I got mine to stop doing that. Your TPS is telling the computer that you're giving it more throttle than you are until you come to complete stop and then the computer realizes that on its own and drops it down to the correct idling RPM