I have a 1999 E250, 6 cyl. Seem to have somewhat of an infamous vacuum problem! The thing shudders with slight acceleration, and smooths out with heavy acceleration. But then the vacuum drops and the vents switch to defrost. I found a bad plastic vacuum line and replaced it, and a lot of the rubber looked like it was deteriorating. Replaced all small vacuum rubber pieces. I disconnected the red line on the pass. side in the engine compartment, drew a vacuum and stayed steady. Used an amount of vacuum to switch vent doors, but didn't bleed off. The vacuum reservoir is good(tested) and no leaks. 2 things I saw would not let me draw a vacuum. First was egr control solenoid and the (what seems to be at least) EVAP system solenoid. Applying vacuum to those just bleeds off. Guess thats the noise you hear when you turn a ford off! Or just both my econolines are broken, haha! Anyways, my fuel mileage is 13 MPG. Also, on my scan tool, it says engine load is 94% when the doors switch. The darn thing runs at 85% to 89% LOAD AT 55 AND 70 MPH! Ideas?

 

This is a classic problem with older E series that had high under-hood heat when running. And, the vacuum systems were a bit of a mess. If you stand in front of the van, and look over to the right, just under the cowl, and if I'm not mistaken near to the trans dipstick, there is a roughly round vacuum connector that has like 3-5 hoses going through it. These econolines loved to leak vac. right there. Also, since it's older, you just can bank that they will all break when you touch them. It's best just to re-line this entire vac. system to prevent the horror of having to fix these every 2 weeks. There is also a vac. resivoir mounted to the RF inner fender that controls vac. for the engine and the A/C.
There is also another style these chassis used, where there is another vac. res. under the truck, mounted to the frame, under the RF about where the trans and eng. mount together. You might take a look. It's been a while since I've worked on one, and yours may just be up above, but I've done this so long that these chassis all run together unless I look at them.
One other thing: the EGR control valve and Evap PUrge valve are meant to bleed off Vacuum when not being actuated. This is normal, and helps prevent an inadvertant activation or "hold-open" of the system. They are designed this way.

 

Awesome info! My van has the reservoir under the right side by the passenger door.

Will check out some more stuff tomorrow and post back with my findings and results, thank you!

 

Not hijacking your thread, but my 95 model has the same problem, thanks for the advice. It will give me a place to start looking.

 

Don't worry about hijacing the thread; all the e-series had vac. problems, and most were routed about the same way.
Also, check all the PCV hoses and lines. They liked to rot out.

 

I have looked over everything and after blocking the 2 control valves off vacuum holds now. But still have a problem with how it runs, getting 14.3 MPG, and the divert to defrost happening. Maybe a product of a 4.2 6 cyl in a 3/4 ton van? But I am not hauling anything, so what the eff? I'll look again today, but the engine always running at 89% engine load would mean the throttle plate is mostly open and creating not so much vacuum in the intake.