I'm considering bypassing my throttle body heater, and running the throttle body heater hose
that connects to the rear heater manifold directly to the coolant expansion tank from the back of the motor.

I would also buy the JLR plug for the water outlet pipe that's attached to the top of the motor under
the intake manifold.

This would eliminate the flimsy plastic thin tubes that go on the top of the motor entirely.
This would also result in lower throttle body temperatures and potentially more power.

Has anyone done this? this seems like a way more robust setup.

 

proposed routing, note the pink tube is actually a black rubber hose connecting to the throttle body with a ear clip
which requires cutters or a flat screwdriver to remove.

 

Yes, it's not needed unless you're expecting a frozen engine in winter...

 

Is there any potential issues from plugging the front coolant pipe?

 

this is a great idea and i love everything about it, with one caveat:

don't forget there's vaporized oil in the blow-by returning to the intake that you want to remain hot enough to stay vapor. this change probably won't make a difference, but if you continue dropping intake temps you may want to start sending that air somewhere else.

 

@tarokyama , thanks for inspiring me to do some new research.
I am left with the question … is there a risk of an airlock if you plug the throttle body heater hose connection… and would it matter on the throttle body alone. I’ve tried to trace the flow (workshop manual and engine technical training guide). but I can’t come to a definitive answer. It does look to be independant from the upper cylinder head cooling flow.
@jahummer , @jons your thoughts?

 

Having just had the XF AJ126 3.0ltr SC in bits to add Alloy forward CW pipes and replaced the rear CW heater header line, Due to prolonged CW leakage that I could not find.

(which eventually turned out to be the CW water pump seal leaking)

I tried to remove as much of the small bore plastic pipe as I could and go direct to upgraded Rubber/Silicon pipe.(some of the small bore pipe just shattered at the touch)

While the 3ltr SC AJ126 is not exactly the same, but similar to the 5ltr SC AJ 133.

Your aim of reducing heat into the air at the Throttle body, and secondarily reduce future small bore plastic pipe breakage is commendable.

The CW pipe flow(s) / connection restrictions and the CW system venting must be taken into account. In order that the bleed from the CW water pump and the upper CW header pipes, bleed to the expansion tank.

I cut out the small bore plastic pipe and replaced with the original connections and directly piped with re-enforced silicon hose.

I intend to do the same to the XKD-R when I replace the CW front headers with Alloy pipe. (and hopefully the rear header)

This is just a suggestion and I may be totally wrong, but keeping the hose connections the same and replacing the small bore plastic pipe (which is easily damaged) with silicon hose increases reliability in the CW set up and keeps the CW flow/ venting the same through the bleed line to the expansion tank.

Keeping the Throttle body by-passed with a correct diameter connection/orifice ensures the design flows are the same and the system venting/ bleeding from the bleed lines the same as original and there is no CW flow dead heading of the CW bleed line. From either the water pump or from the upper CW header line.

You could run into issues of dead heading the CW bleed line from forward CW header and the water pump in having the Rear heater header rail piped directly to the expansion tank. I could be completely wrong as I have only gone into this on the XF AJ126 engine.

While I have not heard of the valley pipes failing on the AJ126/AJ133, (please let me know if you have) these being the same size/ bore from Aft CW heater header to the throttle body. I did replace it with HD silicon hose on the AJ126 engine.

I expect to go through the same on the XKD-R AJ 133 at some stage.

The XF AJ126 having the CW pump and the forward CW header pipes all replaced under warrantee for the same type by Jaguar during the maintenance contract.These all being replaced with alloy and silicon rubber where possible.

Please do let us know how you get on.


Some photos of the XKD-R engine bay, with a Plethora of small bore plastic pipes which I would like to remove.

 

UPDATE: My plastic coolant overflow pipes cracked and leaked for the 2nd time, lasting only 3-4 years..

Soo I finally decided to bypass the throttle body and run
400psi hose from the heater manifold behind the engine
Directly to the coolant reservoir and plugged the hole
with part # listed above.

Success..! Engine appears to be happier and stronger.

400 psi trans oil hose

 

Outer diameter of the coolant expansion tank is slightly larger than the heater manifold, was not easy getting it on.

 

Thin nylon hose has no business running 100c coolant through it...

 

Thanks for the update — it’s useful to hear a real-world outcome after living with repeat failures. Replacing the small-bore plastic lines with higher-pressure hose will certainly address material durability, and it’s understandable after multiple leaks.



One thing worth keeping in mind for longer-term observation is coolant venting behavior. On these engines, the throttle-body heater branch also participates in bleed flow back to the expansion tank. Rerouting that path directly to the reservoir changes how air is purged from the system, even if things look stable initially.



If you’re able, it would be helpful for others to know whether you notice any changes over time — cold-weather behavior, coolant level stability after heat cycles, or evidence of trapped air. Short-term success is encouraging; longer-term data will really determine whether this approach is robust.



Either way, appreciate you circling back with results — that’s what makes these threads valuable.

 

Yes, it would be great @trkyam if you could provide a quick updated on how things have been since you made this change over a year ago. Have any issues or concerns arrived since then?