Probably a Borg Warner unit. Not as bad as Duracrap at least.(I'd still bring it back and warranty it) And any time you are working with any of the sensors, the negative battery cable should be disconnected. Anything electrical for that matter.

 

I know this is an old thread, but my question is relevant to the topic. Plus, it doesn't seem that the issue at hand was solved.

What I've done:

Hughes Plenum Plate/Gasket
Intake Gasket
VC Gaskets
PCV valve/hose
Plugs/Wires
Cleaned everything on the top end of the engine.

This happens randomly (both before and after replacing the belly pan). Only at idle. The engine just starts hissing very loud and running like crap. It's seems like it would be a vacuum leak, but I just don't see where from. Everything was put back together correctly and torqued in sequence.

Like the OP, the hissing is most prevalent around the TB area. No CEL is set.

Is it possible that a faulty O2 sensor could be causing the engine to run lean? I've unplugged it and have been driving like that for a day now. But, there is no way to force the engine to do it...it's only random. So I can't really tell whether it helped or not.

I'm tempted to replace the IAC (When I had everything apart, I did clean the IAC. It was caked with carbon.). Just don't wanna start throwing parts at it.

If more info is needed, let me know. It would be great if we could solve this issue.


Thanks.

 

If it idles correctly, it is not likely to be a faulty IAC. I would double check the small hard plastic vacuum lines. They are prone to breaking.

 

Unfortunately cleaning the IAC does not always work. And you did remove the negative battery cable before doing so correct? Maybe the cat is broken up inside and things are moving around and once in a while they block it off. Try unscrewing the o2 instead of just unplugging it. Also, the o2(s) should be replaced anyway when doing the plenum.

 

if it helps my truck started doing that when I tested smaller than stock injectors and installed a 50mm TB but reused all my old sensors... so I think it may be related to what the pcm is trying to deliver fuel to airflow wise when cold or at idle....
maybe to have partially plugged fuel line /filter or injectors?
I removed the air horns on my TB when boring to 50mm and I read some others have had this issue when doing that .... funny because all the other aftermarket and modded tb's have that done too...

 

Yeah, I'm aware that cleaning sensors is a 50/50 deal. Just threw that in there because it was SO carbon'd up.

Negative cable is always unhooked when doing pretty much any work under the hood (minus testing of course).

I did take the pre-cat O2 out for a while. But, because the issue is so intermittent, it's hard to tell whether or not it actually helps. Also, they will be changed in a few days. I beat around on the cat to see if I heard anything bouncing around, but nothing.

I'll check/double check all my vacuum lines just to make sure. Just wish I still had access my Snap-On Modis...those things help so much when diagnosing hard problems.


Thanks for the help.