Oil Control: Consumption and Leaks

This is part of a posting to the saloons list. It might be of interest to the XJ crowd as well: The 4.2 liter engine burns oil like crazy. My first thought is bad valve seals. How hard are these to replace? I have not run a compression check. If I have poor compression, is this a tell tell sign of worn rings? Could it diagnose other problems?

You should definitely check compressions. Do it twice, with a squirt of oil in each cylinder the second time around. If the second reading is significantly higher than the first, you have a ring problem. Since your engine has sat for a long time without running, the rings could be gummed up in their grooves and not seal properly. Also some of them might have broken when the engine was first started after its long rest. As far as valve seals are concerned, the ones fitted by Jaguar are a joke. Suffice it to say that they appear to be made of British rubber ... It is little wonder that they did not dare use any on the exhaust side! I also happen to own a Citroen DS23 which blew a head gasket a few years ago at 160,000km. Of course I did a valve job at the same time. When I removed the valve stem oil seals, they were in excellent condition (which is why the engine did not burn any oil,with very little blowby fumes in the crancase). In fact, I have since examined the same seals from Citroen engines with twice the milage and they looked fine. Now these are teflon seals, made by Perfect Circle. Since I am in the process of rebuilding my 420G engines, I contacted them (they are part of DANA corp.): they have seals which should fit the Jag engine (5/16" stem diameter,1/2" guide diameter). The part number is D1610 ( The only problem is that they are sold in packs of 100).

Fitting good valve seals on the exhaust valves should bring about a significant blowby reduction in our engines.This keeps the oil cleaner and, even more importantly, helps maintaining the slight crankcase vacuum provided by the engine breather. As a result the pressure differential between both sides of the intake valve seals is minimized so oil is less likely to be sucked into the cylinders and the engine will not leak.

Since the subject of leaky rear oil seals has been brought up lately, I will add that, in my opinion, the problem does not lie with the seal itself but stems from crankcase pressure, caused by any or all of the following: clogged breather, bad rings, worn valve guides. I took some measurements the other day: when the oil in the pan reaches the full mark on the dipstick, it is 123mm below the pan gasket and 84mm below the lowest point of the rear oil seal. Considering that the crankshaft is designed to throw back any oil coming near the seal, the only way oil could leak that way is if it actually gets blown out.