can I run with vacuum removed from FPR

My 30 second hot soak rough idle problem returned after several months, the new IAC apparently wasn't the issue. I put a second (new motorcraft) IAC on and the problem is still there. since pulling vacuum from FPR makes a smoother idle, why not just leave it off?

Lately, I have been reading a lot about long term fuel trims, mine is about +15 at idle, so may have a vaccum leak. the Fuel pressure is 30 with vacuum line on and 40 with it off. But why/how could the vacuum leak get so bad as to sometimes stall engine for that 30 seconds of rough running after hot soak? New intake manifold gaskets were put on at about 100k miles, have 200k miles now. not loosing coolant. Verified rough idle with plugged vacuum tee that sits atop manifold. have to wait for warm weather to come again for symptoms to manifest again. Also I need to check LTF when symptoms occur.

You probably understand this, but the reason for the vacuum line on the FPR is to insure that the pressure drop over the fuel injector is constant regardless of the manifold pressure. This insures a consistant metering of the fuel to the combustion chamber, since it is determined from the time the injector is turned on. (i.e. FuelMass = FlowRate * TimeOpen) In closed loop the O2 sensor feeds back to the PCM the amount of O2 in the exhaust. Too much O2 means there needs to be more fuel (increase the trim), too little O2 requires less fuel (reduce the trim). If the pressure isn't staying constant (high pressure at idle, lower at WOT), then the PCM is shooting more fuel into the engine during idle conditions than expected.

As you've mentioned, the high trim values point to a vacuum leak, as does the improved operation with the MAP feedback to the FPR disabled.

Vacuum leaks are due to "holes". If the hole is somehow temperature dependent (expanding gap with heat, or cool) then that will confuse the PCM.

Also, I have a hot soak issue that only happens when the ambient temperature is in a certain range. It happens after the car has sat for 10-30 minutes after warmup, then when I turn the key on to restart I can hear a bubbling sound from my gas tank. The bubbling sound stops when the fuel pump shuts off. I can cycle the key a dozen times before there is no bubbling when the fuel pump is running. I do the engine will idle OK. If I restart without extengiushing the bubbling noise, then the engine will misfire at idle. The trim numbers go off scale (i.e. +15%) while it misfires. If I run the engine until the bubbling stops, then the misfire stops and the trim drops back down to normal levels.

I re-read my post, and I wasn't clear in several points.

(1) The reason for the vacuum feedback to the fuel pressure regulator is to maintain a constant pressure over the fuel injector, in order to keep the flow rate through the injector constant too. This way the PCM only needs to monitor the time the injector is open to determine the amount of fuel delivered to the cylinder.

(2) Disconnecting the vacuum from the fuel pressure regulator causes the fuel pressure over the injector to vary with throttle conditions. Since air pressure in the manifold is low at idle (lots of vacuum) compared to an open throttle (less vacuum), the pressure over the injector is more at idle. At idle, this causes the PCM to deliver more fuel than expected, which would help to compensate for excess oxygen in the mixture.

(3) Since you're saying that the trim levels are high at idle when the vacuum line is connected to the fuel pressure regulator, this indicates the PCM is reading high O2 from the sensor. This is, as you said, as strong indication of a vacuum leak, especially if the trim is closer to zero at higher throttle settings.

(4) A leak could easily be dependent on the temperature of the engine, given the thermal expansion of the materials involved.

Hopefully this is useful, helpful, and clear.