Hi,
does anyone know how the oil temperature is controlled in sedans and XJS with V12 engines? Is it thermostatically controlled or does it rely on oil pressure? Unfortunately, I haven’t been able to find any information about this anywhere so far.
Cheers!
Some had full flow cooling. All oil goes thru the cooler. Others had bypass cooling where only some of the oil was cooled. The exact details of the bypass elude me at the moment.
What year, model, and original market?
Cheers
DD
Hi Doug,
it is true that the V12 sedans and XJS have two different oil cooling systems you mentioned. It is astonishing to me that Jaguar apparently operated the oil cooling without thermostat control until the V12 engine was discontinued. I once heard that the oil cooling could be controlled via the pressure relief valve. This would mean that the higher the oil pressure, the less oil flows through the oil cooler. But I haven’t received confirmation of this yet.
Cheers!
The background to my query is that I run a 1993 XJ81 V12 engine, but in my 1972 E-Type. After I continue to use the oil cooling from the XJ81 system, i.e. pure oil cooling using air, I only get oil temperatures of around 50° C (122° F) at cool weather and normal driving style. That’s actually too cool. On hot days it is just around 80° C (176° F), which is ok.
I’m now thinking about installing a thermostat. Still, I’m wondering why Jaguar decided not to do this.
Has anyone retrofitted an oil temperature display in their XJ12, XJ81, X305 or XJS 12? It would be interesting to me what oil temperatures prevail in the XJ/XJS.
That’s the way it works on the six cylinder (Series 3 XJ6) and I believe on the bypass style coolers fitted to V12s. The pressure relief valve (located in the oil filter block) will open if the pressure drop across the cooler is relatively high, allowing flow to bypass the cooler. This higher pressure reflects the higher viscosity of cold oil. This bypass valve is different from, and in addition to, the similar bypass valve that dumps oil into the pan, bypassing flow through the bearings, etc, when it is cold.
I have an oil temp gauge on my S1 XJ6 and can affirm that oil temp helps determing oil pressure–when the oil gets hot, the pressure goes down. So a bypass valve using a linear spring serves as a pretty good thermostat IMO. Not as good as an actual thermostat of course. There are some threads in the archives about thermostatic oil cooling on the V12, IIRC.
I use oil cooling in the main flow, which is probably why the cooling depends on the pressure relief valve. Intervention will therefore only be possible via a thermostat. Fortunately, today’s high-performance oils are able to provide sufficient viscosity for bearing lubrication even at temperatures as low as 40° C (104° F).
In the 1970s, however, the oils aren’t as powerful as yet. Jaguar obviously saw no reason to use a different system, e.g. with a thermostat.
Basically the lubrication quality of modern oils do not change much with temperature; the basic reason for the cooler is to help oil from becoming excessively hot. The engine oil certainly also play a vital part in cooling the relevant engine parts…
The ways of controlling the oil cooling is usually rather rudimentary, but somewhere there is probably an oil thermostat that will do the job for the discerning or worried?
One way to reduce the oil cooler efficiency, and raise oil temperature, is to screen it from the air flow, which can be manually or temperature controlled - but I cannot really see the need. The main hazard remains excessive temps - though in winter condition we indeed want initial rapid oil heating, and an oil cooler is then counterproductive…
Frank
xj6 85 Sov Europe (UK/NZ)
As an alternative to using a thermostat, you surely can also partially cover the oil cooler in the fall to increase the oil temperature.
But that doesn’t change the fact that Jaguar saw no need for this during the entire construction period. If you look at it soberly, Jaguar’s concept work, even if you viewed it critically like I did.
The V12 engine is generally considered to be very robust. Most engine failures resulted from lack of or no maintenance and, primarily in the southwestern United States, from operating without antifreeze (Glysantin). As we now know, limescale deposits clog the water channels, which usually caused the rear cylinders (6 and 12) to overheat.
Maybe there will be another XJ/XJS/X305 V12 driver here in the forum who has retrofitted an oil temperature display in order to be able to make a comparison to my temperatures.
I would also like to rule out the possibility that my oil cooler, which has about the same radiator mesh size as the original sedan oil cooler, is too big.
I thought of this video when reading the post. I find these guys quite informative regarding V12s.
A really interesting post. The difference with my situation is that I’m using a 1993 6.0L engine with mainstream oil cooling. My oil pressure doesn’t drop below 40 psi even when idling, regardless of whether the oil is cold or warm. On cool days it is usually just under 60 psi when idling and the oil temperature is between 50-60 °C (122-160 °F). On hot days I always have at least 70 psi oil pressure while driving and the temperature has so far been a maximum of 100°C (212°F). The reason for this was a loose contact in the relay that switches the fans. In this situation, the pointer of the water temperature instrument was off center, which fortunately I noticed in time. Normally the oil temperature is around 80°C (176°F) on very hot summer days. These values made me wonder if an oil thermostat would be useful for me to maintain a higher oil temperature on cooler days. But as we know, Jaguar saw no need for this. Their success proves them right.