Strange engine rattle at partial throttle and under little load at highway speeds

Dear all,

My XJ Series III V12 H. E. produces a strange, rattle-like noise under very specific conditions: Throttle just gently pressed, driving at highway speeds (80 kph and above), all in all applying slight load, e. g. accelerating just a tiny tiny little bit or keeping a steady pace upwards on a gentle slope, RPMs spanning a range from 2,200 to 2,800. The noise is steady as long as I hold the car on this threshold of subtle acceleration/very light load. Most important is the exact position of the throttle: increasing or decreasing throttle just a little bit will kill the noise immediately. The other conditions mentioned are more fuzzy, but they seem to correlate with the funny noise as well.

It sounds like something was loose, but neither can I find any loose parts (e. g. heat shields), nor would it IMHO be logical for some parts to vibrate under these conditions. One would expect the vibrations of parts to be related to RPMs, not throttle position. Or am I wrong?

Throttle potentiometer works fine from 0.33 Volts up to 4.8 Volts, Voltage increases and decreases smootly while turning the pot. That should exclude the possibility of the engine running lean because of a distorted throttle signal at ECU. Bushings at throttle linkage are new and fine, linkage and butterflies are adjusted to specs, nothing seems bent, advance is set correctly, vacuum lines and system have been double checked.

Here is what the strange noise sounds like:

The rattle is audible from 00:00 to 00:08 and from 01:06 to 01:11 – compare the engine sound without the noise in-between. Sound was recorded with a mic positioned at the top of B bank under the bonnet during a test drive. Sound Quality is not great, but the rattle is easy to hear. Interestingly, when sitting in the passenger cabin, the rattling noise is quite similar to the recording, while normal engine noise is, of course, much more quiet inside the car when compared to the mic situation under the engine bay. Might be just a question of frequency response and sound insulation, but I don’t know.

Any ideas what this could be? Worn Timing chain? Turbulences at the butterflies?

I should add that this noise is nothing new to the car. I bought the XJ in 2020, and it did behave in this manner ever since. But now, since most of the other issues have been fixed, I would like to address this one as well.

And I should also add that the rattle is discernible only as a purely acoustic phenomenon. There is no loss of power related to it, no shaking whatsoever. If you could close your ears, you wouldn’t notice it at all.

All best,

That sounds a lot like Pinking… I remember this when a friend had a new 16v Mk II golf and they changed the fuel make up back then, it was under light throttle loads that it happened. He needed to have his ignition advanced to stop the pinking. It was all the rage back then!

Try adding octane booster to see if the pinking disappears… if you are using super fuel, i.e. the most expensive petrol it shouldn’t really occur… keep us posted.

Thanks for the quick reply! I was thinking about that too at first, but why on earth under light throttle load, and why is it persistent under the given conditions?

I had a similar noise some months ago under heavy loads, accelerating uphill, pedal (nearly) to the metal. However, this noise back then didn’t last any longer than a few seconds, even under full throttle conditions and steady load. It completely went away after enriching the fuel supply by adjusting the potentiometer at the ECU unit for some klicks to the rich side. CO ratio has been recently measured at MOT, between 1% and 2%, which meets the specs, spark plugs look excellent (no lean condition). – I will recheck advance settings!

PS: using 97 and 98 octane fuel, depending on the gas station, that’s the best you can get here.

Detonation; usually occurs 1800-2000 rpm and again around 2800-3000 under conditions you describe. You need to check that the advance mechanism inside the distributor is not seized (very common); requires removing the cap to check.
If it is free, I would retard the timing slightly using the cam at the front of the distributor (turn it slightly counter-clockwise about 1/8”/3mm at the major diameter). This amount represents 2-3 degrees advance- see if that changes the noise. You can always put it back to where it was.
Timing is set at torque peak of 3000 rpm, but the 18 degrees spec was based on fuel at the time and accurate timing marks- there is a lot of wiggle in the mounting of the scale and may have been disturbed over the years.

Wow, thanks! That sounds spot-on, both regarding the phenomenon itself as well as regarding the theory behind it (fuel 1983 and nowadays). Dizzy isn’t seized, advance is set roughly at 18°/3000 RPM – but I will try resetting it to about 15°/3000 RPM and see what happens then.

By the way, why is it that this pinging is bound to these very specific conditions? Is it because timing is advanced to an extent (butterflies nearly closed, some load, together producing a whole lot of vacuum?) that the spark comes so early that some of the mixture remains unburnt and detonates spontaneously afterwards?

The reason is at part throttle / higher load there is less turbulence in the combustion chamber and more heat, which leads to the charge exploding rather than burning. Probably accented by the vacuum advance system on these V12s. Plus the fuel map is lean to maximize economy at cruise ( which is where the heat comes from).

Hate to mention it. I can not listen to the recording. no sound on this machine and my ears are not that great.

loose connecting rod bearigs can rattle at ties.

and, up front the roating stff. Fan, alternator, cmprsdor, pmp and even the crank dmapener!!And yu, the timng chain. I

Had a similar issue with my 4.2 E Type; really only noticeable on the motorway, steady state, neither accelerating nor decelerating. Slightest pressure on the throttle either way and it disappeared; drove me nuts. I know you’ve checked, but in this case, the throttle rod bush had seized on the shaft and then flogged out the bracket. Easy fix, but difficult to find as it could not be reproduced unless the car was driving.

Thanks, will see to that too. And, yep, that’s why this issue is so annoying: you can’t reprocude it unless driving at highway speeds.

Checked the ignition timing yesterday. The scale seems to be correct (took cyl. 6A as a reference), timing was set to 18° advance at 3,000 RPMs, I turned it down to 16°. Haven’t been on the highway yet though.

The vacuum advance set-up for the V12HE is rather complicated,Florian - and some part may be malfunctioning, causing pinking in specific rpm/load ranges…

Your set advance, 18 deg at 3000 with vacuum disconnected is to spec - but pinking is usually a sign of too much advance for the petrol used. The usual ‘cure’ is to back off, as you intend to try - it will likely be effective, but it will also slightly reduce power. Nor will it explain fully why it it is necessary; but you may have got a bad batch of fuel, or maybe carbon build-up may have increased compression - or clogging up cat converters may be involved…?

However, prolonged pinking is detrimental both to the engine and its output so it has to be avoided at ‘any’ cost. Brief pinking under specific transient situations, like throttle applied, until the engine ‘catches up’ - and is then a sort of sign that ign timing is OK, and no action is really required…

As an aside; when petrol is ignited, it burns at a steady speed - the flames spreading out from the ignition point. As mixture is compressed, enhanced by the the burning petrol, cylinder temp increases until it reaches the ignition temp of petrol (which relates to octane) and residual petrol spontaneously detonates - the engine pinks. With early ignition the temp may rise to this level before all petrol is burnt, and ‘backing off’ delaying ign one way or another, or increase the octane, is the proper solution.

Of course, the fatter the mixture the more residual petrol is left to detonate - but mixture is in itself not the cause of pinking, though petrol in itself is a ‘coolant’ in the cylinder…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

Just remember that the scale is not reliably located, it can be moved a few degrees on its mounting bolts. Just because you see 18 degrees does not mean that is accurate. Final say on timing is no detonation, as that will destroy pistons.
Detonation at the 1800-2000 range is less of an issue, as the engine generally passes through pretty quickly. Problem with detonation at 2800-3000 rpm is that is cruising speed on the freeway and the engine operates there for long periods.

If so, Robert - it must be properly adjusted and firmly attached…

Only way is to remove 1A plug and using measuring rod/micrometer to precisely set the piston at TDC - then adjust the scale to ‘0’. Then, somehow fix the scale/pointer…right?

xj6 85 Sov Europe (UK/NZ)

Short cut that will be more precise is to use a positive stop through the plug hole; near impossible to measure TDC with a scale (or even a dial indicator). Piston moves very little for considerable crank rotation. Another method is to use the camshaft timing tool to locate the cam, and thereby the crank, providing it was correctly set and not disturbed since assembly.
I stick with detonation is to be avoided whatever the cause; I suggested retarding the timing, as it is easily reversed if that doesn’t correct his noise.
Also, the timing spec determined 40 years ago, with different fuel, on a brand new engine isn’t necessary appropriate.

Just a quick update: retarding the timing a little bit might have helped, although the test drive was a rather short one. I won’t be able to thorougly test the new setting before a week or two – will keep you updated. Thanks again for your advice! Owning a Jag before this forum came into existence must have been a devastating experience.

Hi guys!

Here comes another update: I went for a longer test drive last Sunday to see if retarding timing from 18° to 16° (@ 3000 RPMs, vac. disconnected) did anything. Additionally, I adjusted the ECU two clicks to the rich side to be dead sure the engine doesn’t run lean. The test drive on the highway showed that these measures helped a little bit, but they didn’t really solve the problem: Pinging at mid-range RPMs and low throttle under some load still persists, albeit to a lesser extent than before.

I could now, of course, retard timing even further, but I think the culprit is to be found somewhere else: I took a look into a couple of cylinders using an endoscope and I should say that carbon buildup is a very subtle expression for what I saw down there. I own the car since 2020 and I am aware of the risks of too gentle a driving style. Obviously, previous owners were of a different opinion regarding this question. I tried an Italian tune-up then (accelerating to 6000 RPMs in second gear, done repeatedly four or five times with short breaks in-between). That blew out some of the dirt, but I didn’t feel too comfortable in doing so as heavy carbon build-up also means that pistons will get very hot, and a burnt piston is the least thing I hope for. It would, I guess, take maybe ten, maybe even twenty heavy runs on the highway to get all that crud out, and I sort of hesitate to take that risk.

After doing some research on the forum and via google, I figured out the following strategy: Plan A would be to use Red Line SI-1 for a couple of refills. I tend to be rather sceptical when it comes to fuel additives, but this brand got excellent reviews regarding carbon deposits on the piston heads – and even if it doesn’t do anything, nothing is lost except for a little bit of money. Plan B is to soak the pistons in ATF for a few days, as ATF is said to act as an excellent solvent on carbon deposits.

What do you think? Good or bad idea(s)?

All best,

This is a V12, Florian - I would be very careful on what I put into the engine…

The ‘Italian tune-up’ is not really about brief spurts at max revs, it requires some brisk driving over time - like driving in second gear at highway speeds. It will take some time to get rid of carbon build-up - and sudden stress on an engine lazily driven is not advisable. Long time ago, going from Norwegian dawdling to high speed Danish/German speed limits, it took more than half an hour with gradually increasing speed to coax the engine - you could feel the engine loosening up. Roadside break-downs were a frequent sight with drivers failing to take such precautions…:slight_smile:

Certainly, knocking is to be avoided, and if caused by carbon build up;take some time. It may be interesting to see the advance at 3000 rpms with vacuum connected - though it cannot be adequately tested under load. The vacuum is used specifically to counter pinging at various loads, and very comprehensive with the V12 HE - any faults may contribute to pinging.

Also, fattening the mixture is basically counterproductive; while it cools the combustion it also increases the amount of petrol to detonate - more ping for the buck. Leaning out the mixture to spec is fit and proper - and rich fueling enhances soot and carbon build-up.

Continuous pinging is to be avoided, and further ign back-of should be tried - while searching for better solutions…

xj6 85 Sov Europe (UK/NZ)

Second Frank.

Take 20 mins for heating up the engine, then increase RPM gradually from 3000 to continuous +5000 over a 10 minutes period ideally stretching the exercise to an hour. Sudden revving risks not only to burn pistons, but also to break off chips of carbon doing harm to the combustion chamber or the valves.

Good luck


75 XJ6L 4.2 auto (UK spec)