Hi. So it's regarding my 2001 x308 4.0 XJR (supercharged - obviously) 176000km.

Perhaps someone has had a similar issue at some point and can guide me?

She drives beautifully (like a normal V8) but the supercharger doesn't "engage". Twice it did for about 0.5 seconds ... just enough to hurt ones neck! LOL.

Regarding the word "engaging": I know that there is no clutch and that the SC is permanently engaged. What I am referring to is the the bypass valve not closing, meaning that the supercharger has no effect.

My findings so far:
- When the car is off, the actuator has no vacuum working it, so the bypass valve (in its resting state) is closed.
- When the car is started there is a good vacuum on the actuator ...
- ... the bypass valve is opened by the vacuum activating the actuator, keeping the bypass valve open.
- The SC, while being permanently engaged, does not push air into the engine because the bypass valve is open and allows the air to escape (I don't know where to). But that is of no consequence right now, the bypass valve should be open when the car is idling or under only light load. So everything up to here is fine.
- When I now drive the car, there is no supercharge at all. Just a normal V8-feel.
- When I disconnect the vacuum pipe, the bypass valve closes and one can hear the air being pushed into the intake. I take that, when the engine is under load this is exactly what is supposed to happen ... vacuum is reduced in the actuator and the bypass valve is closed, forcing the air into the intake manifold.
- To test whether the SC is really working, I plugged up the vacuum pipe. This means that the bypass valve is closed and that if the SC works the car will have air forced into the engine at all times. This is indeed the case ... when I drive her with the actuator disconnected and consequently the bypass valve is closed, she is a beast!
- Conclusion is that there is nothing wrong with the SC.
- I then attached a hose to the actuator and sucked - the actuator opens and closes and doesn't loose air. Conclusion is that the actuator works fine also. But that was evident in any case because it keeps the bypass valve open at all times - a defective actuator would not do so.
- My understanding is that something must control/cause the actuator to loose the vacuum so that the bypass valve closes. It doesn't.
- When I get it to close manually (either by disconnecting from the vacuum pipe or by using a screwdriver and force it closed) it closes and the compressed air moves into the engine freely, as it should.
- My question now is this: What controls the vacuum? Ie, what controls that the vacuum "stops" sucking when the engine is under load and is supposed to have the supercharger do its thing?
- In short, my problem: The bypass valve does not close. Or put differently - the vacuum is not broken, so the bypass valve remains open? Is this controlled by the ECU? I don't think so because I can't see any electric/sensor wires going into the actuator.
- In any case, I am stuck.
- Any input as to where I can continue in my search to solve this issue would be immensely appreciated.

Check all your vacuum hoses, below is a link to Jaguar's parts site and a diagram that shows what to look at...

https://parts.jaguarlandroverclassic.../brand/jaguar/

Thank you, Sean. Yes, I saw that. But here is the thing ... if there was a leak somewhere, wouldn't the bypass valve close!? Mine doesn't close. It remains open. When I disconnect the vacuum hose (representing a big leak in vacuum pipe) the bypass valve closes and the effect of the SC then being able to do its thing is very evident.

Bypass valve is there so the sc won't make any boost and therefore load the engine. Valve passes the boost back to the sc intake. System works like "crazy loop" as called here. When accelerated vacuum in the hose drops and valve closes. Vacuum hose should be connected to the port what comes to zero vacuum when throttle is opened more. I think the port under throttle valve is correct.

How long have you owned the car? Did it work but has now stopped working - or don't you know?

Hmm interesting that the SC is disengaged at low throttle, I didn't know that.
Not to hijack the thread but my car occasionally has slightly slow days so I wonder if it's a similar thing. I shall keep reading the advice here & have a think about it.

If there are no leaks in the vacuum lines and the routing of the vacuum lines is correct, I can envision two possible approaches for moving forward.

1. Check the pressure/vacuum conditions in the intake manifold:
   When accelerating, the vacuum should be significantly lower than it is at idle. If everything is working properly, you should be able to observe the actuator closing when you pull on the throttle cable at the throttle body.
2. Mechanical cause:
   It’s plausible that one of the moving parts along its travel path of the actuator mechanism is getting (intermittent) stuck. For example, the diaphragm of the actuator could be the culprit. Likewise, the mechanism of the small butterfly valve that the actuator drives might be sluggish, or the butterfly valve itself inside the intake elbow could be binding. You can find all kind of deposits there...mainly from the EGR (or if your engine is burning oil or if the engine breathers aren't working properly). This stiffness or sticking might only occur during normal (vacuum‑driven) operation and not when you manipulate the mechanism manually.
   I’ve just got the entire assembly on the workbench, completely disassembled it, and cleaned it. Haven't had any issues with it but because replacing some injectors I did a bit of TLC to it. So just in case you're going down that route I can provide some details on that.

Have only had her for 6 weeks now and spent time with cosmetic stuff. Now getting to mechanical things. But to answer your question: It never worked properly since I got it ... driven only about 200km.
Edit: Sorry, am still finding my way around here - wanted to delete this message but don't know how. So pls ignore.

I don't know. I only got her 6 weeks ago and have only driven her 200km odd. I drove her another 100km today but with the vacuum pipe disconnected - she's wild!

Danke Herr xjr2014-de!
1) Yes, I have done this ... the actuator moves slightly when I pull the throttle cable, but doesn't come near to closing and immediately pulls back (ie keeps the valve open).
2) Thank you. This is exactly the point where I am busy with now also. It seems that the valve is "stuck" ... ie, it doesn't just close smoothly but seems to have some kind of resistance it needs to "click" over. It's not easy to get to it without dismantling the inter-cooler ... but looks like that's what I will have to do next. I won't have time to do it right away but will keep posting.

Thanks for eveyones input. It's very much appreciated.

There’s no need to remove the intercoolers. In fact, you won’t be able to take them off unless you first remove the relevant intake elbow together with the bypass actuator—a necessary prerequisite. 😉
So have a quick search around the forum and download yourself the workshop manual.
To get to the intake elbow and its complete bypass‑actuator mechanism, you need to remove the components in this order: intake piping, throttle body, EGR valve, and finally the intake elbow.

As I'm too lazy to open the manual tonight, can someone say where this bypass valve is on the car please?

Above of the intake elbow and under the "boost" pipes to the ic's.

Thanks, I'll have a look at some point as I'm curious now.

Hi! A have the same problem on my XJR 1998. This week I’m going to connect the boost gauge to vacuum line from the fuel pressure regulator to check the boost. I’ll post the results. Can you please let me know if you will solve the problem?