I have seen old guys use dish soap or bubble bath & water to make a big pot of suds. Then they cover the engine intake area (or wherever) in suds and watch for any place the suds get sucked in. Never done it myself, but I've seen it work.

 

I had the same problem on my 92 4.9L. I changed the TPS and the hang went away. I noticed that there is not a positive return on the TPS when the TB closes. The position of the TPS at idle is dependent on the internal spring of the TPS to make it go to the idle position. I think that mine was gunked up and not working freely internally.

Anyway, mine works much better with a new TPS..

 

Im still convinced it is a vacuum leak somewhere, but I can't find it. I can take off the brake booster line and it doesn't change the idle at all.

I tried the propane way, and got nothing, tried starter fluid again and still nothing I took off the brake booster line and shot the torch, tried the starter fluid too, right in it, could barely discern a change in idle speed either time.

Has anyone actually tried the soapy water? Doesn't seem like it would work because it is sucking so it wouldn't bubble.

Any other ideas for finding this thing?

 

Does the idle change if you pull the line off the brake booster & plug it (i.e. bad booster)?
Maybe you have a internal leaking intake gasket? As for the soapy water trick, what you actually use are just the foamy suds, not the water. Your looking for a vacuum leak that will suck the suds in, not bubbles being created. I may not have explained it very well. Good luck, I know electrical shorts & vacuum leaks can be a major pain to find.

 

You ever sort this? My truck has the same symptoms and I hate when threads die with no resolution!

 

Nope never figured it out. Got a little frustrated with it so I moved on to a different project for a bit. Ill come back to it again when I get the motivation.

 

Did you ever get back to this and figure it out?