Turns into a thinner nylon line?

That goes to the MAP (Manifold Air Pressure) sensor on the drivers side strut tower. A thin rectangular sensor inside a metal bracket. There's a nipple on its underside for the vacuum hose. There should be a rubber boot on the other end of that nylon vacuum line to attach it to the MAP sensor.

Here's the Regina manual as a PDF:

http://oak.cats.ohiou.edu/~ridgely/regina/

-Ryan
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Athens, Ohio
1987 245 DL 314k, Dog-mobile
1990 245 DL 134k M47, E-codes, GT Sway Bars, GT Braces
1991 745 GL 300k, Regina, 23/21mm Turbo Sway Bars

I hope it is same as 240 as I have never seen what is under the hood of a 740. Anyway I think you are talking about the small vaccum hose at the top of intake . It goes through fire wall to vacuume tank inside the car. But if it is not connected then how are you operating your recirc and other HVAC buttons .
If that is the one then NAPA has exact size.

Coolant hoses can not have anything to do with bad running.But if you meant vacuume lines then you have a point.one method you can try is to plug the vacuume ports one by one with a hose stub and the one which makes the car run fine is the culprit.

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DD-1990 240 DL SW M47II FI 3.1 234 K miles