Greetings from Seattle, WA.

My 2002 XJR with 80k miles has decided to make a knocking noise for about a second at startup. It goes away after about a second. It only makes the sound after it has sat a few days. It doesn’t do it on the second startup of the day or even the next day. I changed my secondary tensioners so they are metal. I haven’t changed the primaries or the guides so they’re still plastic. The engine oil is not low.

Here’s a video of the startup:

Please let me know any thoughts on what this sound is.

Thank you,

Danny

 

If they are plastic, they will still fail eventually. Check for anything loose in your engine bay. Check belts, pulley's, etc. Is the noise top end, bottom end, front or rear? How long ago did you change your oil, and you changed your oil filter with the oil change, right?

 

I’m pretty sure the noise Is coming from the top. I think it’s near the front but I can’t be sure. I only get one chance to listen every few days. I changed my oil about ten months and 4K miles ago. I changed the filter.

I don’t think this is related, but while driving last night and sitting in traffic, I noticed my temperature gauge needle moved just a hair above the middle line. The needle was still touching the middle line but I’ve never seen it go that high. It’s usually right on the middle line. When I started cruising again, the needle came down to the first line below the middle line. Maybe this is normal and I think it has to be unrelated but I’ve never noticed it before.

 

Almost sounds like the supercharger coupling/snout. I would take the S/C belt off and turn it by hand to make sure it is smooth/no play. If it feels off they have snout rebuild kits. Also sounds like you have a bad pulley in the SC or accessory drive. I would take both belts off and spin every pulley and component by hand to check for squeaking/grinding. However it doesn't sound super serious. Also check SC oil.

If none of this solves the issue then I would dive into the primary tensioners and chains.

 

Also running a hair above the middle line isn't out of the ordinary. My jags have always done that at times even with new hoses/coolant flush/water pump/fan fuses. Just keep an eye on it and if you have never replaced the water pump that may be your squeaky culprit as it sits near the top of the engine.

 

I agree, the frequency of the knock is high, much higher than what a knocking conrod bearing would produce. It appears that the knock has the same frequency as the normal whine of the supercharger that is heard after the knock disappears. The simplest (and sure) way to diagnose is to start the engine without the supercharger belt.

 

Thank you for the advice. I will look into this. I didn’t realize superchargers could make sounds like that and then quickly quiet down. Does the fact it isn’t noisy until the car sits a few days also make sense? I assumed the sound was oil related for that reason, which didn’t set well with me,

It could be that the gauge has always gone a hair above middle and I haven’t noticed. I haven’t replaced the water pump before so I’ll keep that in mind regarding the knock.

 

Thank you! I appreciate the input.

Danny

 

If the bearings are starved for oil or beginning to fail they may get stuck briefly if not used for long periods. If they really start to fail there will be a continuous rattling or "box of rocks" sound. However if the coupling is failing (the bushing from the snout drive to the actual supercharger gears) there can be some play and a rattle.

I would just start simple with turning the supercharger by hand to see if it turns smooth or has any play like a car with loose steering. Also check the supercharger oil level and color (use a magnet for the plug, dropping it is a pain). Some swear by changing SC oil every 30K and some say it is not necessary. To be safe I plan on changing every 30K, the oil is cheap.

It sounds like nothing to be too worried about right now, if the simple fixes don't do it you may need a snout rebuild but don't spend money until you have confirmed the problem.

 

Many thanks, Anduha!

All this advice has given me a lot of hope my issue is not that big of a problem. I listened to it again today after the car sat for three days and I’m almost certain it is coming from the top of the engine and is the supercharger.

After I resolve the issue, I’ll report back.

Danny

 

I’ve read on other posts that a smaller pulley can cause the supercharger to come into its power band quicker and make it more fun to drive at lower speeds. Is this something I should look into while the supercharger is apart or should I leave it all stock?

 

The only modification I have made to mine is the 6% or 1.5 boost pulley. It does add more midrange power and honestly is about all I am comfortable with. There are guys on here that have done lower pulleys, larger uppers, bigger intercoolers and bigger throttle bodies and claim no cost to reliability. I'm keeping mine until it disintegrates and it already has plenty of available power.

However DO NOT modify anything until you are sure the engine is in top shape. If a snout rebuild is needed many shops will put the smaller pulley on while they are at it, for a fee.

 

Did you ever figure it out?

 

I haven’t noticed the sound in quite a while. I changed the supercharger belt and pulley and then shortly afterward, someone backed into my car as I was sitting in a parking lot. The car has been laid up for a while but I’m getting it back from the body shop today. I’m hoping the knocking sound is gone after changing the belt and pulley.

 

That sucks, but glad you got the hit fixed. Hope the sound is gone! I have an 02 xjr also, and with a very similar sound on start up.

 

It could simply be due to engine oil draining down after a longer sitting and the oil pump needs a bit of time to build up the pressure. I wouldn't be concerned about it as it disappears very quickly. One way to test is to, after a longer car sitting, remove the fuel pump relays, crank the engine a few times, put the relays back and start the engine. If, in this case, there is no brief knock, then it is related to the oil pressure build-up.