Continuing on my saga to resolve a 200rpm idle oscillation problem on my 85 HE V12, I have found an odd situation that I hope others can provide some guidance and advice on.

My said idle rpm oscillation problem was resolved by disconnecting my LHS Fuel Pressure Regulator vacuum line. Disconnected she purrs like a kitten. Reconnect the vac hose and it fluctuates.

I have no fuel leakage in the vac line so the diaphragm is fine. If I apply an external vac signal to the reg I get the same results of odd running.

I tested by connecting and disconnecting the vac line to the inlet regulator with no change either way. So the LHS regulator is definitely causing the oscillation.

It seems odd that it would run BETTER at idle with the vac line disconnected. My air/fuel ratio meters show no change at idle, meaning she's not running perceptibly richer with the vac line disconnected.

I guess changing the reg is called for, but this one is only a year or two old, I'm surprised it would fail this soon, the inlet reg must be 15yo and it works fine.

So what gives? any ideas of where to check and why a vacuum signal +/- to the reg would cause such a wide oscillation in idle?

TIA