First, specs. 1999 Ford Ranger 3.0 V6 Vulcan Flex Fuel. No mods. 113,7xx miles or so.

So, the problem. At highway speeds, when the engine is at 2,000 to 3,000 RPM (assuming I have either the AC or Heat on, something blowing out the vents) it will change direction. Say it's blowing towards me. The engine turns at 2 to 3 thousand rpm at 50 MPH or greater and it goes down to the floor until the engine slows down or the vehicle slows down.

This is the only symptom I experience during these conditions. The engine doesn't feel like it's going to die, no spluttering, no stalling, nothing else.

My father believes that the vacuum ball isn't holding enough vacuum during these conditions, causing the symptom. I'm inclined to believe that, but I want to know what else it could be before I take my fender off; something I do not want to do at all. I'm considering the purchase of a vacuum gauge to splice in and route into the cab so I can see what the vacuum is doing while this is taking place.

 

I am a bit confused by what you are saying, do you mean the air is blowing out of the dash vents and then under these conditions it begins to blow from beneath the dash?

 

The direction changes from the front vents (towards you) to the floor vents.

 

I would say you're probably gonna be taking that fender off...

 

$25 vacuum gauge is good to have in any case.

Could just be a leak in a hose going into the cab, I would check that first.
Vacuum reservoir has a check valve on the Source hose(from intake manifold), but not on the device hose(s).
So a leak in a device hose would deplete stored vacuum fairly fast while accelerating(Source vacuum level drops)

 

Initially I thought it was a leak, but I don't hear the classic hissing/sucking noise of a vacuum leak. I'll likely run to harbor freight sometime this week for a vacuum gauge.

Since I've never really worked with a vacuum gauge before, where would be the best place to splice this gauge into? I'm thinking after the check valve.

 

Yes, you can get a couple of joiners and "T"s of different sizes and some vacuum hoses of different sizes when you get the gauge.

If check valve is a separate part, not on the reservoir, then yes you could just shut off AC/Heater system so valves are closed, and then just suck on that hose to see if it holds pressure, lol, no gauge needed

 

Sounds like a cheap and simple way of doing it. I like cheap and simple.
Oh, hello Cheryl. What are you doing here?

In all seriousness, though, I'll have to give that a try after I get my bed back together. Currently in the process of putting down primer, undercoating and fiberglass to stop the rust, and now fixing a master cylinder (not on mine) after my screw up