Ben Stafford had another problem with his 420. There was generally very poor oil pressure of only 20 to 25 PSI when hot and cruising on the open road that dropped back to almost 0 at idle. The engine did not sound "clapped out" and the oil consumption was quite reasonable. The electric oil pressure gauge was suspected but substitution of a hard line gauge only confirmed that the electric gauge was telling a true story.
I suggested that before he did any thing drastic like an engine overhaul he should check the condition of the oil pressure relief valve. I have had two occasions in Jaguars where low oil pressure was traced to relief valve problems.
Ben found that the oil pressure relief valve spring had been bent. This resulted in the valve itself sitting at an angle on the relief hole and a groove had been worn into the face of the valve. He used a lathe to clean up the face of the valve and subsisted a straight spring for the bent one
The results were fairly dramatic with an improvement to 45 PSI on the open road and 15-20 PSI at idle when hot. In his own summation of the situation he "was surprised that such a small gap in the relief valve had led to such a dramatic loss of oil pressure."
Whither goes thy oil pressure? (continued)
MY Mk2 3.8 Auto has had a slightly low oil pressure and I was a little concerned that the engine may have been "loose" i.e. getting worn. I took a trip to Ben Stafford's to use his hoist and get easy access to the pressure relief valve.
Upon removing the relief valve it became immediately obvious that the valve face was not seating properly. A quick trip in the lathe to face off the valve resulted in an increase of 10 pounds per square inch over all of the operating range.
However I now believe the real problem lies in wear on the upper flutes of the valve which allows it to sit in the bore of the valve body at an angle and the final solution will be to replace the valve itself with a new part and possibly even re-sleeve the valve body.