Engine vibrating after warm-start

[babulio]

Be wary of counterfeit spark plugs - even direct from Jaguar!

[XJ4081]

Update: plugs, leads, rotor and cap have been successfully replaced! Unfortunately I did not check out that the Marelli V12 has two coils, so I had ordered only one. Will replace them both when the second one has arrived. Ordered both at SNG, so I hope they are of equal quality, though original Marelli parts seem to be out of stock. But as I have learned, cap and rotor are mostly responsible for the infamous 'Marelli fire', so I should already be on the safe side.

But there is one mysterious thing: my V12 did not have the air filter (DAC4062) in place, so we left it away, too. Is that maybe related to the country spec the car was built for - or did a previous mechanic perhaps leave it away for simplicity?

BTW: This is an interesting site. They make suggestions on how to prevent the known ignition failures, has anyone ever implemented one of these measures?

[Brake buster]

firstly

be wary of aftermarket items for the V12 ignition , especially Marelli replacements ,

if you dont understand the way the V12 Marelli system works you will end up throwing lots of parts and money at the engine without knowing if and what you may have fixed , and will be none the wiser

a GOOD condition Marelli system is a perfectly good ignition system , the so called ' Marelli fire' issues come from incorrect spark plugs, not gapped correctly and poor HT cables , and poor maintenance , high resistance in the system ( from bad spark plugs and poor HT leads ) causes the Distributor cap to burn out, causing running issues and unbent fuel entering the exhaust onto a hot Catalitic converter, hence the fire,

btw UK cars do not have a CAT , and we dont get the fire ( just back fire , lol )

one thing to note, the spark plugs out of the box need re-gapping, this will improve and provide consistent combustion , i have original Brass Genuine Marelli distributor caps if you need to buy one ,

i presume you mean the modification to the top of the rotor arm , this is a known worthwhile modification to prevent issues should the ignition system develop problems ,


the distributor intake filter you mention needs to be there, the dizzy body has a vacuum pipe from the manifold, and then a pipe to this filter, this draws air through the dizzy body to the intake manifold, this prevent the build up of Ozone gas within the dizzy which corrodes the metal parts, Ozone is created from lightening , such as the electrical discharge from the posts to the rotor arm even time the cylinder fires , any small filter type device will suffice , as this air does go into the engine unfiltered otherwise

hope this helps

BB

ps, i have been running and maintaining my own V12 marelli system for 15 years

[XJ4081]

Got the car back onto the street this week and had some test drives, I tend to say that the warm-start behaviour improved a little bit after replacement of plugs, leads, cap and rotor. Besides that we found that there one of the vacuum hoses was a little loose, maybe that was also one of the causes. Idle is now between 600 and 700rpm and the trembling doesn't feel as strong as before, after approx. 10 Minutes of driving again it almost completely disappears.

@Brake buster
The goal is actually not to get rid of as much money as possible but to get rid of the lack of maintenance . Unfortunately I was not able to find an original marelli cap, about rotor I am not sure, it was at least coming from Jaguar. But I think new aftermarket parts are still better than worn-out Marelli parts. So at the moment I don't need a Genuine Marelli distributor cap, but it may be worthwhile to have one as spare, at least once my purse has recovered from the last bleeding . What price do you ask for it?

[XJ4081]

After some more rides the behaviour has changed, or at least my perception about it. After cold-start it runs normally with increased idle, no suspicious trembling or shaking. But as soon as the car gets warm, it's getting nervous on idle, when I brake down until idle, I can cleary see (and feel) the passenger seat or seat belt shaking (when no one is sitting there). But that's not always the case, sometimes it idles the way it should, or it's shaking at first and after a minute or two it calms down, or the other way around. As soon as I touch the throttle and increase RPM by 100-200, trembling goes away.

Anyone an idea? I also had another close look at the injector wires with damaged insulation, it's not only the outer insulation that is damaged, at least on one wire the inner insulation is damaged, too. As far as I can see that's not something that can be replaced easily, it seems to belong to the wiring harness.

Oliver

[pimgmx]

A (heat-) damaged injector loom is a known issue on the 6.0's predecessor, the 5.3 V12.
With the 6.0s turning a quarter century old I wouldn't be surprised to see this issue now appearing on the younger generation V12s...

A job already on the list for my XJ-S is to build a new loom (components in store in ye olde parts shed already) and lead the wiring as far away from the engine as possible to reduce the heat impact on the wires.

[Brake buster]

I rebuilt the loom for my V12 a few years back , and well worth the effort it was to ,

a few pics, probably an earlier version of yours but the same principles









BB

[Starbuck]

Hi Oliver and BB,

I agree with BB about the Marelli fire issue being caused by poor maintenance rather than poor design. Both our Marelli ignition system V12s had what might be called 'overenthusiastic' maintenance, and on our XJ12 the cap and rotor were original and after 130,000kms only just starting to show the faintest sign of a dark shadow on the rotor. The rotor and cap often get the blame for failing when plugs and leads have not been replaced at Jaguar's recommended intervals. Yes you can modify the shaft and fill the rotor with silicone, but in my view if the system is maintained correctly there should be no need.

As far as I am aware all Jaguar and other reputable sources state that the distributor vent is there to draw oil vapour out of the cap. The jackshaft arrangement that drives the distributor and the distributor shaft position makes the distributor cap one of the highest vent points on the block. Quite a few 1980 'Bosch injected' V12s with fuel diluted oil from rich running at cold start on repeated short trips, had 'moments' after the oil/fuel vapour in their cap was ignited by the spark inside the cap, blowing the cap to pieces and giving the owners a very nasty fright to go with the repair bill.

The vented caps were fitted to all HEs and retrofitted to the Bosch '1980' V12s, some owners even fit them to the pre-1980 V12s just to be on the safe side. The filter is there just to make sure you aren't sucking dust through the distributor into the manifold bypassing the main air filters. On the 6.0V12 the vented cap is less necessary because the PCV also draws from the jackshaft cover as well as the LHS camshaft cover, but there is nothing to be gained by removing the vent system. All three of our V12s have them.

A mechanics stethoscope will soon tell you if one of the injectors is not firing and causing your rough running. Unfortunately if you start to 'play around' with the wiring looms that have been sitting being cooked in the valley of the V12, they will be very brittle and you can do a lot of damage just moving them to inspect them. This is exacerbated on the 6.0 because of the cosmetic cover fitted over the valley.

As Pim is planning to do, we replaced the entire engine loom on our 'Lucas' ignition V12 because there were so many brittle components. The loom was relocated up out of the valley and on our 'Lucas' XJS the AC compressor was mounted on the side of the block to increase airflow through the valley and keep the fuel rail and wiring slightly cooler when running. This is planned for the other V12s as well.

Vacuum leaks will cause poor running on the V12 just as they will on any other engine - and there are lots of places they can develop leaks. Balancing the throttles will do little because the large balance tube between the plenums 'evens out' the amount of air drawn into the engine at idle and lower revs. Jaguar V12s do not use AFMs or MAFs, so uneven intake on one side does not affect idle unless the leaks are so bad the idle speed is too high even with the auxiliary air valve fully closed. Based on the idle speed you are reporting that seems unlikely.

Inlet manifold gaskets are a known failure on the V12s, though in theory all 6.0 V12s should have come from the factory with new thicker gaskets. If it looks like the gaskets are the early thin 'paper' gaskets then it is likely the manifolds have been off and replaced with the incorrect gasket type. The later 'thick' fibre gaskets are a massive improvement over the early paper type, but I have still seen them fail in service.

Good hunting,

Starbuck

[Brake buster]

oh , my ' removed ' very brittle original loom , which had already had some injector plugs ' spliced ' into it by the PO





BB

[Starbuck]

Yep, we had an almost identical pile of loom 'bits'!