Chrysler Repair: 1991 Voyager 3.3 missing at mid-high RPM, fuel pressure regulator, intake plenum

Question
Hello,
    I was wondering if you had any suggestions for me.  I recently installed a rebuilt transmission in my 1991 Voyager (3.3L, 167,000 miles).  It ran like a new car for about a week and 100 miles.  Last week it started misfiring (stuttering, popping, bucking,and power loss) at about half throttle and above.  It doesn't appear to be transmission related.  It does the same thing in all gears and in neutral.  It's very repeatable, the same thing every time without exception.  It still starts and idles perfectly, and at low power it drives great in all gears.  It has no relevant trouble codes (12 and 55 only).  It's got 40 psi of pressure at the fuel rail, and it holds steady as I run up the RPM.  The plugs were changed 500 miles ago, but I pulled them all and they still look good.  I changed the plug wires and the coil for good measure.  I did a compression check and got between 140 and 150 on all cylinders dry.  I figured I must have reassembled something incorrectly when I replaced the transmission, so I took everything related to the engine apart again and checked (intake plenum, left exhaust manifold, numerous vacuum lines and electrical connections).  All the connections seem to be clean, and all the vacuum lines seem to be good.  The only odd thing I found was what looks to be a vacuum connection at the rear of the engine that doesn't seem to connect to anything.  It's a short, round connector that's bundled with the wires for the crank position sensor and the distance sensor.  There's simply nothing there for it to connect to and I'm pretty sure it has never been hooked up to anything.  Do you have any suggestions on what to check next?  Thanks for your time.  

Answer
Hi Bob,
According to my '93 shop manual for the van, the fuel pressure is supposed to be 48 psi for that engine. This could be low due to a defective fuel pump, plugged inlet to fuel pump strainer, cloggedexternal filter, defective fuel pressure regulator, or an inaccurate fuel pressure gauge tool(?). I am not certain if 8 psi below spec would cause the miss, but I wouldn't be satisfied until I resolved that issue; and it would likely manifest itself at mid- to high- rpms.
On the vacuum fitting, I believe that could be the main issue as well. Take a look at the emission control system sticker on the underside of the hood to see if you can locate a vacuum line that would be attached to that part of the engine as shown in the diagram. I haven't worked on that engine so I can't tell you from any experience how it is supposed to be attached. You might also find a neighbor or dealer who would let you examine a van with the same engine to see how their 3.3L line is fitted in a similar vintage van.
In my manual it looks like there is a T-fitting that connects together 3 hoses from:
PCV valve vacuum supply on the intake manifold
EGR valve electrical transducer
Fuel system evap solenoid (on pasenger side inner fender)
Maybe the open connection is the T and so you will need to find the missing vacuum line that goes to it from one of those items.
That alone could solve your problem if the gauge for the fuel pressure is inaccurate and you do really have 48 psi.
Roland