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Hello everybody. I have an AJ16 XJR6 engine. When I stripped the engine down I tried to take all the photos I possibly could to record what goes where, but inevitably there are gaps in the info I need to re-assemble. Notably, the throttle body (or indeed the body just in front - or downwind - of the throttle body) has four vacuum take-off points. I'm pretty sure I have correctly identified the top two, but the lower two don't seem to have an obvious place to go. I have put a tube from one to the other effectively blanking them off, but I would like to know what they were intended for. I must point out there have been a lot adaptations on this car as the engine now resides in my '68 Aston Martin DBS.
Andrew is spot on, the fuel regulator feed is as per the picture.
Your small feed tubes 3 and 4 on the throttle body are in fact for the coolant feed designed to prevent ice formation around the throttle area.
You will have a small (ID 1/4") hose from the tee on the side of the thermostat housing down to one and a similar hose going off from the other to the dreaded octopus hose.
Here in Australia this is quite unnecessary, and I have mine plugged at the thermostat end and octopus hose end. This modification has been in place many years and I did the same on my previous XJ6 series 3, coolant flow through the throttle body is just not necessary in our climate. Something less to disconnect etc when removing the throttle body.
Hi John. I think you've got the wrong inlet/outlets in mind there. The coolant goes in and out of two nozzles that are underneath the actual throttle body itself as shown here on a spare throttle body I still have:
Here is the in and out for the coolant that is feed from the 'Octopus' hose.
That is not what #3 and #4 in my original photo above are pointing at. 3 & 4 are both nozzles on the body that the throttle body bolts to (the one that is the supercharger by-pass), that feed directly into the induction air-flow and are therefore there, I presume, to provide vacuum. You wouldn't want to be feeding your coolant into either of those (and I nearly did before I realised what they were!).
Whilst looking at these coolant in/out nozzles they do seem from a logical point of view, to be a bit pointless to me. Even in the harshest Nordic environment they wouldn't do anything to prevent ice until the coolant had warmed up, and by that time I would have thought there would be so much warmth in that area that ice wouldn't be a problem anyway (but that is a side issue).
Yes, you are quite correct Andrew.
What we are actually looking at is the "Throttle adaptor assembly", diagram attached.
The parts diagram only shows the bypass actuator attached at the top spot. position 2 with 3 and 4 not used, however on my XJR (optional cruise control fitted) the "speed control actuator" vacuum hose is attached to position 3. It is very difficult from above, due to all the induction piping etc but I am able to see these two connections, but I believe position 4 may not be used.
That is very reassuring and now things make sense. Much as I would like to have cruise control I don't have it, and it would be too complicated to retro-fit, so I can blank off 3 & 4 knowing that they are not required.
However, although I am 99.9% sure the outlet on the top of the throttle adaptor (which is ignored on the diagram sadly) goes to the PCV (crankcase ventilation) but it would be great if anyone could confirm that. Although I appreciate it is almost impossible to actually see anything under there, but maybe someone has had their whole set-up apart recently and knows this...
In the absence of any forthcoming info I shall button it up on that basis. There is one thing that I find puzzling, and that is the take-off for the brake servo vacuum. It comes off at the housing elbow just before the intercooler rad plenum:
As this is a supercharged engine that area will be subject to vacuum on deceleration and idle, but high positive pressure when accelerating. Now obviously you won't be hitting the brakes whilst accelerating, and you have a one-way valve in the pipe going to the brake servo, but wouldn't it make more sense to use one of the outlets on the Throttle adaptor assembly instead as they are only ever under vacuum? Just a thought...
Last edited by AndrewCD; Jul 14, 2025 at 08:44 AM.
Hose connection number 1 is the part load breather connection.
No.3 is thr AC Rochaster valve at the back of the car that allows purge flow out of the fuel tank when the engine is running
No.4 is the connection to the purge vave in the engine bay
Hose connection number 1 is the part load breather connection.
No.3 is the AC Rochester valve at the back of the car that allows purge flow out of the fuel tank when the engine is running
No.4 is the connection to the purge valve in the engine bay
Thank you for the above info. Andy Stodart had said the same to me in an email and it's nice to have it confirmed. As the AC Rochester valve and purge valve are (I believe) part of the environmental requirements for the US market I would assume that all cars outside the US have these two blanked off?? Anyway, I have blanked these two off as I don't have either of those. Incidentally, with my conversion I have the XJR fuel tank in the boot not the original AM one as the engine needed the twin fuel pumps it contains.
Also good to know that the part load breather connection is as I thought and goes to the small branch of the PCV hose (the large section being the 'full load breather'.
I am now happy that everything with these outlets is correct now. Unfortunately we (my auto-electrician and I) are still having trouble getting an idle or any kind of run (the engine has been silent for ten years). Having tested almost everything it seems to come down to a lack of vacuum at the inlet manifold and the search for the problem continues.
There is the hidden EVAP system port on intake manifold casting bottom side cylinder 2 or 3 from the front , Evap should be closed on startup and you can use a 9 volt battery to hear it click open / closed , the banjo fitting ( valve cover ) for the brake boost hose and the brake boost diaphragm itself , run your finger along the rear end of the valve cover gasket to feel if the " Ds " have not fallen out of place / seal ( this is 2 machined Ds cutout shape at the rear that is filled with 2 Ds in the large whole valve cover gasket
The banjo fitting can be tricky to remove and can be filled with children modeling clay at the dollar stores
For an early supercharged AJ16 there is an engine item on the supercharged package that was negated during later year production that had a breather gas hose for cold climate heating / freezing up . this can be found mentioned in the very last pages of the AJ16 section of the Jaguar 801s document
There are 2 diagrams which shows the maps of both the Vac and coolant lines
The intake manifold to the throttle body round flat surface does not take a gasket of any form ( metal to metal ) , at least on the normally aspirated that I know of
There is the hidden EVAP system port on intake manifold casting bottom side cylinder 2 or 3 from the front
I don't think there is this on UK models. We don't have the EVAP system. I don't have any photos to prove that there is nothing like that on my manifold, but I have had it off and cleaned and painted it so I would have noticed I think.
Throttle Body Vacuum Take-offs
Although I appreciate it is almost impossible to actually see anything under there, but maybe someone has had their whole set-up apart recently and knows this...
