Hi,

During the last month (2500 km) my car developed a knocking noise which seems to come from the engine (I'm not saying it's the engine, I don't know yet). (note: it is the supercharged version)

If I try lo locate the sound from the top of the car it seams to come from under the supercharger. When I'm under the car, the source of the sound seams to be somewhere in the back of the engine, or the transition between the engine and the gear box.

1) The noise is the loudest when the engine is cold, when I start it in the morning. As the engine warms up it fades away, but doesn't disappear completely.

2) It is load dependent. By the same rpm it is much lauder if I'm demanding power from the engine. Two examples: with cold engine I start the engine and wait few second for the idle rmp to stabilize. Then without pressing the throttle I put there a gear (R or D, doesn't matter). When I do so, the rpm are the same, but the noise is much lauder. second example: I'm driving 100km/h and there is no knocking, I press the throttle: the knock sound appears immediately, I release the throttle: the sound dissapers immediately.

3) the knock is rpm dependent.
4) I tried to remove both driving belts (both supercharger and accessory): knock still there
5) tried to disconnect the cylinders one by one by disconnecting the wires to the coils: No change of the knock by any cylinder.

any help will be appreciated! Suggestions?

one thing I read about is a test for piston slap: to pour about 50 ml of motor oil in each cylinder, than start it and if it is a piston slap, the knock should disappear for shot time. I haven't done it yet because I'm not sure if this test is a good idea on modern engines ... Is it a good idea or not? How the cats will take it?

thanks in advance for any help!

 

If you can feel the knock when touching the engine, it may be a bearing knock on the crankshaft somewhere. You need a competent mechanic to look at it to be sure. Going by sounds requires you be standing next to the car to be sure.

 

Mines the same, 50k miles, took it to top independant Jag specialist and was told it was piston slap, nothing can be done apart from major engine rework, but was also told that it really dosn,t affect performance...... but on saying that it,s a noise that shouldn,t be there and is annoying.And in my experience once you know its there you listen for it every time you start the car! Drives me mad!! May go to Doctors to get some "Chill out" pills
Bob

 

"oldmots" is right. You need an experienced engine mechanic to listen to your engine. I hate to bring a negitive tone to the subject, but piston slap is just not a possability. If you actually had piston slap your engine would be so worn it would be using excess oil, and have so many other side effects that the car would be seriously sick.

 

Have you replaced the tensioners?

 

Just relaying what I was told by specialist. Performed a compression test myself and found at cold No4 cylinder right bank was slightly down on the others but when the engine was warm it tested ok.
There lots of interesting reading regarding "Piston slap" if you Google it.
Bob

 

Thanks I will read up on piston slap. I am only stating what I now from experience. I haven't heard a real case of piston slap since the flat-head ford days. Thats when engines really wore out bad before they were ovehauled. Most times you could also hear some wristpin chirp. Sometimes I hear some of the dambest stories on these forums that I cant believe my ears. I have rebuilt a lot of engines, but not recently, and I always sent the block and heads out for machine work. I always measure everything to make sure I get quality machine work. My opinion is worth what I charge for it. Nothing! I just hate to see a guy lead astray by someone who thinks he is an expert because he has worked on a few Jaguars. I mean the best...good luck!

 

I had an ominous engine knock from the front of the engine on the RH side.
I replaced the water pump and serpentine belt idler pulley and the knock has gone away. Neither component were obviously worn out. It was PM at 142k mi.

 

Martin, I know you mean the best and the beauty of these forums is that others can benefit from people with experience like yourself as you say "for nothing".
As the post from Dan R illustrates anything that moves on a car can produce an unwanted noise, and trying to diagnose by post is always conjecture, like me trying to discribe to you the music I,m listening to at the moment without telling you the title.
Would be useful if shl 1980 posts the outcome of his problem in the future though.
Good luck to you too
Bob
Ps The music is "Smack my bitch up" by the Prodigy

 

Can you have someone record a short video near the top or bottom of the engine and post it to youtube for us to hear?

 

I had a knock in my XJ6 sometime back. Did it at idle only. Took it to a friend who had a lift and listened underneath. We were convinced it was the transmission torque converter. Took it to another shop for them to service the transmission which they did, but also found the source of the noise. The alternator was loose.

Now my point is it may not even be the main part of the engine or transmission. I would suggest go buy a good stethscope and do a lot of listening around the front of the engine. And also around areas where the engine might be touching someting. If the engine runs good and doesn't burn much oil, and you have always taken good care of it I really don't believe it is the main engine or transmission.

EZDriver XK8 XJ6

 

thanks for all the suggestions.

So I took a mech. stethoscope and tryed to locate the knock. i spent quite some time on it, touching all kinds of components. The result is:

1) on the upper side of the engine the loudest knock comes from the supercharger inter-cooler, left side. (the aluminum thing with "supercharged" written on it). I tried to touch the compressor itself and the engine block close to the inter-cooler, but the noise there was much lower. Correct me if I'm wrong, but there are no mechanical parts inside the intercooler? Can anything there make a knock noise, or it is just a resonance from somwhere alse?

2) From the bottom it seems to come from the torque converter. Engine block - low noise, gearbox - low noise ...

any suggestions?

BTW: is the torque conv. from XJR MY 2000 the same as in XKR 2001 ?

 

You may check that the convertor to flywheel bolts are tight. Also a cracked flexplate (flywheel) can produce knocking noises but changing as it warms up would be a new twist - for me. The supercharger could have an issue, and as it warms up its clearances change, The charge air cooler may enhance the sound as it is basically a hollow chamber.

I've not seen everything but as this is my 41st year of being a professional mechanic I've seen a lot. The last case of piston slap, that I would consider audible to an average owner, was in the early '70s. It just doesn't happen that often. I'm not saying it isn't possible but don't let a bunch of internet yayhoos get your anxiety up. Kind of like getting upset over diagnosing your own health by spending an evening on WebMD...

 

... just to add for the benefit of the OP that sometimes the front drive mechanism can make what some might call a "knock" noise when it is on the way out. The kits to fix this can be found on ebay, and can sometimes be installed without removing the S/C depending on the vehicle.

It resides inside the casing just behind the drive pulley.

edit: sorry, missed this bit:
4) I tried to remove both driving belts (both supercharger and accessory): knock still there

 

Just a thought, I remember a similar situation, which turned out to be a failing cat. converter. The internal monolith was breaking down and rattling on the cannister. The location you describe are where the cats are situated, rear of block, not under the car. You said the noise seems left biased so i would check that one first. If it is indeed a cat, you might see what looks like wash powder granules in the tailpipe.

 

What year car are you talking about? It has been my experience that an engine knock that appears when the engine is cold and disappears when it is hot and reappears when under load it is most likely a wrist pin. One way to tell without additional harm is to have the car on a lift, drain the oil and allow the old oil to drain then refill and be listening when you start. In most cases of a bad wrist pin or baring you will hear it and know where it is coming from. You should have and use a stethoscope like this http://www.chain-auto-tools.com/engi...ls/AMR199L.htm to identify the exact spot. Did you ever run low on oil or fail to change it on time?

 

Thanks for all the replies.

I tried to tighten the bolt on the flexplate, no improvement.

changed the oil, no difference (BTW: since I own the car I change the oil regularly and no problems with the level. Before I bought the car ... Well who knows ...)

I have issues with one of the cats, code P0420 Catalyst System Efficiency Below Threshold (Bank 1), but that should be the opposite side. Well a place to check with the stethoscope ....

the car is a MY 2001 XKR (supercharged 4L V8).

Since I have my own stethoscope now, any suggestions where to listen? Any special spots which will help to pinpoint the problem?

Thanks.

 

As for the P0420 do this, fill the tank with hi-octane fuel and give the car a run a few times. I had the same on my 3.0 s-type and it cleared the problem after two tanks. It could be many things and that would include a bad sensor. For now try this. As for the knocking when cold have you checked the tentioner and belt? Take a look at my tentioner video and see if it is the same Link Belt Tensioner If you have a stethoscope just probe around until the noise is loud or pronounced and you will have a close proximity of the problem. Just be careful not it poke it into mechanical or electrical areas that could cause a problem. Let us know what you find.

 

I too, have always thought that a regular knock on start up that goes away on warm up is piston slap. Pistons that slap can still have normal compression since the rings expand to the bore and keep the seal. I had a beautiful restored 1965 BMW R69S that was famous for piston slap just due to the clearances designed into it. New ones did it. Piston slap is not necessarily a big deal if it stops at some point during warm up. If it keeps doing it all the time, it's rebuild time.
As far as knocking, it is an experience thing, I can generally tell if knocking is a loose part or trouble inside the engine by the way it "rings" and the regularity of it. You just have to have heard some doing it to have the feel. I have a hard time telling if its a main bearing or a rod bearing though, just not enough experience.

 

I have a similar issue, and have seen a couple other threads trying to address the concern. Though mine is on a 4.2 SC, and is much more pronounced when warmed up / hot day & or after driven aggressively, and not noticeable at all when cold.


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