1990 Jaguar XJ6 Oil in bottom of intake manifold

I have a 1990 XJ6 Sovereign 4.0 litre engine vehicle.
The engine runs OK but the check engine light is on.
I checked the faults VCM which said FUEL FAIL 14
Under 14 in the codes something connected with the cooling system is listed. No water problem present
I checked fuel sytem Press regulator and no problem present. I removed vapour system sensor hose at cannister on upside vacuum side. 3 tbspns oil leaked out of the vapour line going to the manifold. I removed the intake manifold.
One of the nuts was only hand tight. The inside of the manifold was oily with only about 1/8 inch oil in bottom of manifold. Question:Did the oil come down through the valve guides?Did the oil come in through the overfill line of the filler pipe which connects to the Inlet Air Flow Elbow? Plennum had small layer of oil in bottom with sludge present. Does not appear that oil entered from plennum as this is lower than the acceleration valve. (No PCV valve on this vehicle that I can see) Book lists PCV hose from top of valve cover which connects into lower part of the filler pipe.
I have to eliminate oil problem in order to pass air care.
How do I ascertain for sure that the oil came from the valves and remove head(if necessary)
My inclination is that the problem is connected to the vacuum system as there is a fair amount of pressure when the gas cap is removed. I had engine running and did not notice any suction on the afore mentioned line after it had been cleared of oil.
I would appreciate any comments before I contemplate removing the head