Yes I think it might be a vacuum leak, but...
Bought the one owner GLH-S 53,000 miles at Mecum last month. Took it to a mechanic who rebuilt the CV axles (boots were torn), replaced the plugs, replaced the thermostat,confirmed timing is correct, checked the fuel pressure, replaced the pcv valve hose (was a patch job leaking). I have run two complete tanks of premium gas though the car in the past month both times using injector cleaning additive. Initially car did 8.33 0-60 with older bald tires (after tune up) and 16.33 1/4 mile at 86.19 (two different runs on same day).
Bought BRIDGESTONE 205/50R15 POTENZA RE-71RS tires and took it to a local alignment shop that does older cars and race cars. Had them look the car over as well and looked for vacuum leaks and did a smoke test but did not find any leaks. Did a compression test and all 4 cylinders were at 120. Air cleaner was not replaced but looks new. Two codes were present 42 and 47. Mechanics said the auto disconnect was working and the low voltage might have been from kissimmee where they disconnected the battery while it sat and reconnected when they moved it.
When cold, car idles at 1,200 and is sometimes a little rough. After it warms up it idles between 1,500 and 2,000. I installed a harbor freight boost gauge just before the map inside the car (for diagnostics). The boost gauge never goes above 10 (I believe it should go to 12) and that might explain the slower 0-60 and quarter mile times. After the tires the 0-60 is consistently 7.8 seconds. I understand it is hard to make a stock car match what car and driver or Motortrend list as their time of 0-60 in 6.5 seconds, but thought I should be closer to 7 seconds than to 8 seconds especially with modern performance tires. Launches were on asphalt not concrete.
I have repeatedly tested the vacuum system for leaks with soapy water, propane, brake cleaner etc with no leaks discovered. I removed, inspected and reset the Idle Control Valve and attempted to adjust the idle with the screw but could only get the idle to go up not down (the valve is the newer one not the one with the 3 gears and looks to be original). I have traced each vacuum line and have contemplated replacing it all with new vacuum block and hoses but the car is a nice survivor and trying to keep it as original as possible. I have checked for leaks around the intake and valve cover with no luck.
One peculiar note. If I remove the cap from the factory vacuum harness assigned to a boost controller (which was not installed in this car) creating a vacuum leak the idle barely rises 50 rpm and only for a quick second. This is why I continually suspect there is a vacuum leak, but not sure if the idle should jump higher if I do this. There is vacuum on the line when uncapped.
The car is fun to drive and pulls hard but it seems like it is below where it should be. It's a cars and coffee and car show car not a daily driver. A far as I can tell everything is stock and the car sat in a Dodge Dealership car museum for 20 years plus but was driven during that time. There is one spliced wire repair going to the AC compressor but nothing out of the ordinary. The valve cover does have a very small oil seepage above the number 4 plug but not able to detect an actual leak. The mechanic both thought it was not causing the issue.
Any guidance is greatly appreciated. I have searched the board and tried just about everything I have seen before posting for help. The only thing I have not tried is replacing the fuel injectors, but the car does run strong except for the high idle and possibly the boost being slightly low (which might be the cheap harbor freight gauge).
Here is a video of a cold start. Temp here is 61 Degrees on You Tube:
It actually starts rough and drops to about 500 rpm before returning to 1,200 to 1,400 rpms. It has started rough a couple times previously but usually does not so possibly a symptom.
Here is a shot of the 1/4 mile dragy result:
Idler valve (AIS I believe):