So after R&R'ing the wheel bearings and the truck sitting for a month, I fired it up and found the truck to be running really really rough. It was smoking heavily, so I figured it was just fouled with oil and needed the cobwebs blown out. I did noticed a louder than usual sucking sound (I've had a "suck" sound since I bought the truck that I could never track down). The noise was louder than the engine itself (which has a rotted exhaust and holes in the cat). So I took it out and blew out the cobwebs and it seemed fine after that.

Yesterday I was putting a can of R134A in the AC and had it running in park with the air on MAX and the truck started running really rough again, no smoke this time, but it kept trying to stall and the sucking sound of a very loud vacuum leak started up again, getting louder and louder while the truck was running rougher an rougher. I spent a couple hours trying to track down the problem but I'm not finding anything disconnected, cracked, torn, or otherwise allowing a vacuum leak. The only thing I do notice is the oil cap on my truck is very loose as if someone lost the original and just stuck one in there. Would this be a vacuum leak source?

I should note that the HVAC mixer and cruise control are still working, which absolves those vacuum lines.

 

Under normal circumstances, there is a slight vacuum on the crank case, so, having the correct oil cap on there IS an issue. (don't wanna suck crap INTO the crank case.....) Shouldn't be hard to find the 'right one' at the parts store though.

Try taking off the air filter housing, and see if the sound gets louder. Have you cleaned your throttle body in recent memory? (the usual source of the sucking sound.)

 

It Sounds like it maybe your idle air vac motor. It allows more air to bypass through the throttle body under certain conditions. When it does this it sucks air from in the engine compartment and makes a loud hissing noise. It may have built up carbon. There is a procedure on this website for cleaning it out. It's pretty simple though just remove the screws pull it out and use some Carburetor cleaner to thoroughly clean out the cavity and clean the piston. As I recall it does it used torax screws.

 

Oh yea, when they bind up, the engine runs poorly.

 

Hmmm, very helpful, thank you. I wonder if this is what causes the undertone "suck" noise that I've referred to in the past?

 

I may just as well replace it. Not too bad of a price really.

http://shop.advanceautoparts.com/webapp/wcs/stores/servlet/product_Idle-Air-Control-Valve-BWD_18480233-P_757_R|GRPFUELAMS_____#