A few years back I got a beautiful 72 XJ6 from an estate for a good deal because it had issues. I won’t bore you with the details but whoever did the head gasket forgot to tighten the upper chain adjuster and it came apart… after fixing that and finding most of the pieces (just found the rest in the pan) I found that there was an incorrect thermostat installed without a bypass foot never letting the car warm up. After that I found that the same genius horribly over tightened the cam cover nuts cracking the covers and bending the studs, after that etc… etc… I’ve driven it very little since getting it needless to say.
One of the issues it had was great oil pressure at start up but when it eventually did warm up the pressure was low - low enough for the idiot light to come on at idle. I come to find out now that the PO installed the canister-type oil filter with no spring or washer to hold it against the filter head so it’s just been flopping around in there, nothing inside the canister but the long bolt and a filter… he apparently thought all that other stuff in there just wasn’t necessary. This is really for my own curiosity because the engine is out and I’m starting the rebuild, but if the filter is effectively left out of the equation, would it result in low pressure?
Anyways, my Pressure relief valve stuck open twice in the last month, and sympoms are exactly like you described: good oil pressure cold (40-50 psi according to gauge) and as it gets hotter and the oil thinner, pressure drops close to zero and the warning light comes on at low idle. More rpm give more pressure, but never more than, say, 15 psi, I did have to drive like that but it hurt (me, not the engine I hope)
Debris from the pan might have gotten stuck in the valve? It’s also part of the oil filter housing, maybe you find something still stuck in there… I did.
Maybe your oil is too thin as well, but you certainly changed that - when the oil is cold, the PRV is the limiting factor for pressure, and when it’s hot and too thin, it’s actually too thin. I changed from PO 15-40 whatever to 20-50 and readings got way, way better!
DPOs do the dumbest things! Its the kind of thing none of us would try intentionally, but yes, I would say running with effectively no filter would reduce the pressure measured at the top of the housing, or wherever it is measured on an XJ6. I don’t know the details of the 1972 filter head, but on my '51 XK120 there is an adjustable bypass relief that, when open, dumps excess oil back into the sump, it other words when the pump is putting out more than the bearings can take. In such an event the pressure measured at the top would certainly be reduced.
Obviously you should look for the right missing parts for yours, or find a whole filter head.
Ian is right. Immediately before my rebuild, at a last ditch effort to try to correct an oil burning problem (hoping that it was because of stuck piston rings), I changed the oil and then I dumped a can of some snake oil (can’t remember what it was and it doesn’t matter, it didn’t work) in the engine and ran it for a hundred miles or so. Since I was going to drain that mixture out, I didn’t put a filter in the canister. No change in oil pressure with or without a filter.
Thanks for the replies guys - Sorry for the delay in responding. 4th of July Holiday was a little hectic.
David - I kinda feel it probably had something to do with the relieve valve especially in light of the missing filter spring. Like I said, the upper adjuster came apart and although I did find all the pieces when I pulled it apart I didn’t find all of the spring…
Rob - This guy was the King of all DPO’s He even installed a leaper on the hood crooked. Not just a little crooked either - the poor little chrome Jaguar was leaping off to the right by a good inch! Drove me nuts the few times I drove it. Needless to say I took it off.
Ian and Mark - I kind of felt that it shouldn’t be affected by the missing filter and you’ve confirmed it. I though possibly that the restrictiveness of the filter might act as an accumulator that would raise pressure slightly inside the canister but I guess not.
Well the engine is stripped down now and ready to go to the machine shop anyway. Going to replace the filter head with a series 3 XJ spin on version since this canister is missing stuff anyway.
He even installed a leaper on the hood crooked. Not just a little crooked either - the poor little chrome Jaguar was leaping off to the right by a good inch! Drove me nuts the few times I drove it. Needless to say I took it off.