Poking around the driver's carpet this morning, I realised that what I had thought was just a rubber stopper for the accelerator was actually the kickdown switch - with a firm push from my fingers, it moved inwards. Today being a lovely fine day, the open road beckoned, and I headed out. And this time, once I was on the fast open road, I decided to see what the kickdown switch would do.

Well, the revs shoot up with a roar and the car very very rapidly gets well past the speed limit. All good. What is not all good is the metallic rattle that comes from the engine area on full throttle. I am absolutely not an expert, but if I were to hazard a guess, it sounds like a chain - perhaps the timing chain. I don't think it's pinging, as pinging sound higher and tinnier, and I use high-oct anyway.

In normal driving, sans kickdown, I seldom get above 3,000 rpm, so never hear it. The rattle seems to start on the far side of 4,000 rpm - I tested it three times, backing off immediately I heard the sound, and it always happened.

So... is this something horrendous that needs urgent attention for major wads of cash? Or can I keep driving normally (where the engine runs sweetly) and ignore it?
I suspect not, myself.... Probably time to get myself in the right frame of mind to see a substantial portion of my monthly income vanish....

 

Hi end detonation can be decribed as a "marble in a tin can", and is bloody dangerous to any engine.

Back off the timing a few degrees, taking note where it was, and try again, CAREFULLY.

The plastic chain tentioner does break, and maybe a chain whip at the top end is more than plausable.

My best gues is hi end detonation.

 

Back off the timing? You mean adjust the ignition timing? I thought that was computer-controlled via the ECU.

Marble in a tin can? Could be. I've been listening to videos of detonation, and it's really hard to tell, but they sound more like clicks than the rattle I heard. What it sounds like (not what I think it is) is a metal chain dragging, which is why I suspected timing chain, but I just don't know.

I've been checking my drive recorder video to see if I can post a clip of the sound it makes, but for some reason the (mono) mic on the recorder doesn't seem to catch the sound.

 

OOPS, Marelli car,

The base timing still needs to be set, then the ECU can do what it does, but if the base timing is "out" then the parameters will be stretched to do what they do.

Thats about ALL I know of this system.

A stab in the dark:

Remove the oil filler cap, the timing chain right there, use something suitable and attempt to pull up on the chain. It SHOULD be tight on that sprocket, it its sloppy then the tentioner is now high on my list.

 

Thanks. The words "bloody dangerous" make me think about taking the car into the garage sooner rather than later, even though she doesn't make that noise in normal driving. At least see what they say - and what they'll charge. Just the earth, or do they want the moon as well?

 

Just added to my answer.

Quick check you can do in 10 seconds.

I have done a tentioner and chain in situ ONCE, and it took me 3 days. I was younger and way more flexible back then.

The others I have done with the engine OUT, and its still a MONGREL task.

 

Last time the shop talked about taking the engine out (to replace mounts), they didn't even want to quote a price, just saying it would be high. Basically it seemed like they didn't think it would be worth it for the value of the car. I could press them this time, and then see what other garages would charge - this place is probably on the high end, going by what sort of machines they work on.

 

Does the engine rattle like a chain on cold start and does it rattle when warm? Can you hear a chain making shattering noises? If no, then the timing chain is less of a problem. Coming alomg with an old XJ8 where the tensioners lways fail, you get to know how a chain should sound and how it shouldn't. A little rattle upon coldstart is okay. When the engine is warm, it should be quiet... And that on all chain driven cam engines...

 

No, none of those noises. The engine starts pretty much like other XJS V12 starts that I've heard on videos. During normal driving, there's no sound other than normal-sounding whirring or humming, with a slight howl on hard(ish) acceleration - all sounds normal, and no rattling noises. This rattle only happens at full throttle kickdown, 4,000+ rpm (a region I never get to normally).

At full throttle I wouldn't call it "shattering" as such - more like metal being dragged across metal in a rattly way.

 

Almost certainly a timing issue, possibly combined with the quality of fuel used.

 

Hmm. The last time I filled her up, it was at a really cheap place. Perhaps not all hi-octane is created equal....

 

Hi Someday

I only use 99 Octane in my XJS its cheaper than a New Engine, I should burn everything that you've got in the Tank and then fill up with the good stuff and see if that makes any difference.

If not just go to your High end garage and tell them that you would like the Timing checked, rather than explaining that you've got a problem and you don't know what it is, as if so then I'm very sure that they would charge you mega bucks to fix it.

Although it might be quite easy to do it yourself.

Grant has got a Pdf of how it is done.

 

Octane ratings are never shown on the pumps here, but the legal official standard for high-octane is 100 RON. But I kinda wonder how there can be so much variation in prices if they're all selling the same stuff....
Hey, at least it won't take too long to get through a tank of petrol....

I think Grant's PDF is likely to apply to the pre-Marelli engine, as mine is all ECU-controlled.

 

Hi Someday

I don't think so, I think Grants got a PDF on Marelli but in any event I'm sure he will explain to you what to do.

P.S. had a sudden rush of Blood to the Head in a Charity Shop to-day.

Walked in and saw a really High End Gaming PC £130 had to have it (Lol)

Details on my Cherry Restoration Thread.

https://www.jaguarforums.com/forum/x...122634/page32/

 

Nice. The only computers I've seen in similar second-hand shops here are entry models people are trying to get rid of. I want a new computer soon - something that can handle Skyrim with lots of mods. My current machine is running off mobo graphics from 2011, as it won't recognise the graphics card. Oh well - at least it prevents me from wasting hours gaming....

 

It's possible the engine is rocking on it's mounts. Take a look at the tips of the fanblades and see if there is any evidence of them contacting thr shroud. You can probably see how close the fan is to the shroud and get an idea from that.

 

Hmm. The garage suggested earlier that the fairly irregular vibrations I feel through the seat when stopped at the lights were bad engine mounts. If so, it's a pretty rapid shake. I'll take a look at the fanblade tips as soon as I can.

 

Most likely cause.

I got caught in the Red Devil with a dose of our Ethanol rubbish, YEARS ago, and that engine did NOT like that at all.

Ran HOT, an aircooled VW beetle had more grunt, fuel pump played a lovely tune. I simply drained it on the side of the road, walked into a better servo, and got 20ltrs of our 98 RON, and it took a few kms for all to settle.

I know little of the Marelli system, but the basics are the basics. If its got a distributor distributing the spark, then that thing needs to be set a predetermined timing point, and YES, the Electrickery takes care af all sorts, but if the "base" is off, the rest will Domino.

 

I did this on my XJS V-12 awhile back. Was cruising about 80 and did a down shift and, wham, a bad rattle. Thought I had dropped a valve, but being a V-12 it still ran smooth. I was about 3 miles from home. Got home, changed into garage clothes, started motor and listened with a stethoscope. Found rattle in #4 cylinder. Removed plug. Plug had separated from porcelain and was rattling with each compression and decompression stroke. Replaced plug and all was well. Good luck. Try the stethoscope to see where it is coming from, then compression test.

 

Thanks for the advice, Grant and Jagfixer.

All the plugs are new, but that's still no guarantee of a bad plug, of course. Did your rattle remain even after coming down from high revs? If it's rattling on each stroke, I'm not sure why it would suddenly start and suddenly stop, rather than build up gradually, unless it takes a given amount of force to rattle it.

Actually I do remember one time before I heard the noise, very briefly, during hard highway acceleration, so it may not be the fuel after all.

One thing I would like advice on, is would there be any harm in driving the car more normally, without the rattle (it never happens normally, only on full throttle), or is this something to get seen to asap?
(Yeah, it probably depends on the cause....)