Was sitting waiting in a drive thru line and noticed that my 2000 XJR wasn't idling completely smoothly. I drove home and sitting in the garage I can hear a repeating sequence of clicks coming from near the throttle body. It sounds like a valve or solenoid opening and closing, perhaps releasing pressure, mp3 attached. It's there every few seconds at random intervals. I hadn't noticed it before but then I haven't had the car long. Anyone got a suggestion on what it could be? Is it a normal noise for an XJR?

 

Almost sounds like the supercharger bypass butterfly cycling. You could remove and plug the vac line going to the control pot and run at idle with no worries and listen for it again. They open and close in normal operation very fast

 

Is that the windshield wiper making that chirping sound in the background??

 

Yes, I was thinking waste gate. Should there be enough boost at idle to trigger it? I'll check with the vacuum line removed. The wiper motors not running.

I'm running a tank of gas with Seafoam added at the moment, I'm wondering if that's related.

 

the seafoam is fine. If anything it can foul a plug or make an o2 sensor wonky for a bit. The charger bypass is open by vacuum at idle and light throttle then when manifold vacuum drops on heavier throttle the spring in the bypass snaps the butterfly closed for boost. It is a go or no go system. Open or closed. That is why unhook the vac line only at idle because the charger will create boost all the time

 

There is no wastegate. The boost level is controlled by the supercharger bypass valve, the opening of which is controlled by the vacuum in the intake elbow. At closed or partially open throttle positions, the high vacuum in the elbow keeps the bypass valve open which causes the air from the supercharger outlet, passing through the charge coolers, to be directed back to the supercharger inlet so the boost is just circulated through the supercharger and practically no boost (or very little) is delivered to the engine cylinders. With progressive opening of the throttle, the vacuum in the elbow is reduced and this causes the by-pass valve to gradually close directing some or all of the boost to the engine. The boost is continuously variable, from zero to max, depending on how much power from the engine you request by opening the throttle (power-on-demand).

The pulsing you hear could be from the supercharger bypass valve which would open-close if, for some reason, the vacuum in the intake elbow fluctuates from max. to very low. Disconnecting the vacuum hose from the bypass valve will hopefully give you some indication as to where the problem is.

 

That's very interesting. So high vacuum at idle should be holding the bypass valve open. I was thinking the opposite and the bypass valve would open to dump excessive pressure at high revs not low. It does sound like it's cycling so maybe not getting enough vacuum to keep the valve open at idle.

Thanks for the input, I learned something new today!

 

When it is open the cylinders feed off that little pipe. When closed it is supercharger showtime and the closed valve prevents the boost pressure from back filling the induction elbow and throttle body. Open you have a n/a engine and closed a boosted engine

 

Yes, and the spring on the valve will close it when there is no vacuum. So, the vacuum is fighting the spring. This diagram may help you troubleshoot:


If you positively establish that the "clicking" you have is from the bypass valve, first check the valve (vacuum) actuator. If good, then the possibility is that your EGR valve, which is fed from the same vacuum line, is faulty causing vacuum fluctuation in this line and causing the bypass valve to open-close. You could disconnect the vacuum line from the EGR valve and plug it hen see if it solves the issue.

 

The cylinders are always fed in the same way - from the main air stream pumped by the supercharger that goes through and exits the charge coolers and is distributed by the manifolds to the intake ports of the heads. When the bypass valve is open, the supercharger cannot build-up the boost at the manifolds as the chamber between the supercharger outlet and the charge cooler core is open to the supercharger inlet side via the two small pipes (hoses) and the open bypass valve. In simple words, whatever boost (the pressure above the normal air flow) the supercharger could deliver at its outlet is sucked back into the supercharger. When the valve starts closing, the boost will start building-up at the manifolds (and the cylinders). With the valve fully closed, all boost is delivered to the cylinders.

 

i meant from the pipe to the outlet then charge coolers. Just trying to keep it simple as to what the valve affects. Another easy way is look at map reading. At or below baro pressure no boost. Anything above baro pressure boost

 

Hmm, this doesn't look good - right side intake elbow to supercharger bolts loose. Not sure if this is related to the noise or not but I'm glad I looked. From what I can see the throttle body has to come out to be able to access these bolts.

 

That would definitely cause a wonky idle and fluctuating vacuum. The induction elbow is where the vac signal originates to the bypass valve i believe. Good find. The gasket there is pretty stout so unless it is cooked and brittle you should be ok. You might be able to get a ratchet wrench in there. Lucky you have socket head bolts and not the torx head. Gives you more tool options. If you have to remove the throttle body it is not as daunting as it looks

 

I'm going to take out the throttle body. The bolts on the other side look to be tight but I think I should check and I can go through the vacuum lines too. Finding loose bolts makes me wonder about other work done on the car. The previous owner of 10 years said all maintenance was done by Aston Jag in Naples and he could only remember a new battery and regular maintenance. He didn't have any service records but clearly the supercharger area has had some work. Hopefully they replaced the coolant pipes and tightened everything else up when done.

 

Removed the throttle body and tightened up the bolt. Managed to break the log tab on one of the electrical connectors but otherwise easy enough. Still hearing the clicking noise but the supercharger/induction noise is much louder now so it's not so noticeable. I haven't been able to find my old mechanics stethoscope but I think that's what I need to pursue this further.

 

How about the idle issue. You could hook a vacuum gage to the bypass vacuum line and see how it reads

 

The idle seems to have smoothed out. It's a little faster, maybe 600rpm now instead of 500.

I'm surprised by how noisy the supercharger is now but I'm trying not to obsess about it. I didn't have time to hook up a vacuum gauge but maybe this weekend. The lines need to be rerouted so I'll have to get in there again. Someone's been in there before me but I can't tell what they did.

 

if the charger noise is a whine, sit back and enjoy. If it is a marbly or clacking sound, that needs attention

 

Unless there is aftermarket solid coupler. Then it is blender sound.

 

rarely happens

only people who have problems with the $9.99 coupler post here