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Engine vibrating after warm-start

Starbuck
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Posts: 681
Joined: Thu Mar 19, 2015 4:48 am
Location: Melbourne, Australia

Re: Engine vibrating after warm-start

Postby Starbuck » Wed May 23, 2018 1:42 pm

Plus the obligatory collection of old alarm bits...
Image

Loom out of the valley (sorry dirty engine bay).
Image

Best,

Starbuck
'94 XJ12 (and other toys)

XJ4081
Snr Member
Posts: 216
Joined: Fri Oct 23, 2015 12:12 am
Location: Germany (South-West)

Re: Engine vibrating after warm-start

Postby XJ4081 » Thu May 24, 2018 12:20 am

Thanks to all for your input! Your accurate knowledge about XJ40 and XJ81 is really great - in my 9 months of ownership I already had to deal with some german Jaguar specialists who claimed to be XJ40/81 specialists as well, but very soon I found out they don't have even half of your knowlegde. None of them was willing to investigate thoroughly on my idle problems, one even stated I should leave it that way and be happy that it is behaving well in all other states.

In my case, you might remember, the loom has not been damaged by heat (or at least not solely) but also by a rodent which had made itself at home in the engine bay. It relieves me a little bit that some of you were already able to build a new loom on your own - so I guess it's not an out-of-the-box part I can order at some known supplier?

I think one easy test to check for some injectors not working due to heat could be to isolate the cables again with heat-shrinking isolator (as Pim had suggested previously), if it doesn't help it should not make it worse anyway. If it does help, I can stop on searching for other causes (and put a new loom on my to-do-list).

Oliver
1993 XJ12

Starbuck
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Joined: Thu Mar 19, 2015 4:48 am
Location: Melbourne, Australia

Re: Engine vibrating after warm-start

Postby Starbuck » Thu May 24, 2018 12:58 am

Hi Oliver,

You can certainly do that, just bear in mind that to get the heat shrink over the broken insulation will require removal of the plugs on the ends of the loom. You will need to be exceptionally careful with the already fragile loom, and you will need a set of terminal tools to remove the contacts from inside things like the injector connectors. The connectors will likely be very fragile too.

Good hunting,

Starbuck
'94 XJ12 (and other toys)

XJ4081
Snr Member
Posts: 216
Joined: Fri Oct 23, 2015 12:12 am
Location: Germany (South-West)

Re: Engine vibrating after warm-start

Postby XJ4081 » Thu May 24, 2018 9:32 pm

Hi Starbuck,

I found some kind of insulation with a trench, so I simply could wrap it around the old wire and wouldn't have to remove anything. I think for a first test this could do, but it is only suited for up to 105°C, I have no idea how hot the upper engine bay can become, do you?

Thanks
Oliver
1993 XJ12

XJ4081
Snr Member
Posts: 216
Joined: Fri Oct 23, 2015 12:12 am
Location: Germany (South-West)

Re: Engine vibrating after warm-start

Postby XJ4081 » Sun May 27, 2018 11:12 pm

I found another mech who claims to be an expert for old english cars and especially lucas electric equipment - I'll check out if that one is also only a fibber like the others I met so far or if he can do a job. If it's true what he says he should be able to easily find out if the wiring or injectors are causing my issues.

I also found a supplier who has a new original loom, emission class G, GER spec in his stock for what I would call a reasonable price (~300€). I think it wouldn't be a bad idea to add it to my personal stock just for the case I really need it some day, would it?
1993 XJ12

Starbuck
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Posts: 681
Joined: Thu Mar 19, 2015 4:48 am
Location: Melbourne, Australia

Re: Engine vibrating after warm-start

Postby Starbuck » Mon May 28, 2018 12:24 am

Hi Oliver,

The valley of the Jaguar V12 gets extremely hot. On the 6.0 not only does it have all of the usual stuff cluttering up the valley like the AC compressor, coils, cruise control, throttle pedestal etc, it also has a cover over all of that to reduce airflow even further. With the engine running on a hot day it wouldn't be far off 100c in several places in the valley (remembering the thermostats open around 85c to 88c), but on hot shutdown with the bonnet closed it would be well over 100c. If you are going to try re-insulating your old loom I would get hold of some very good quality loom tape from somewhere.

If someone has an original engine loom for sale that suits your car's emissions spec it is probably a very worthwhile purchase if it is in your budget. Rebuilding a standard wiring loom isn't the most difficult job in the world, but a new original loom will likely keep your car going for its life with you.

Best,

Starbuck
'94 XJ12 (and other toys)

XJ4081
Snr Member
Posts: 216
Joined: Fri Oct 23, 2015 12:12 am
Location: Germany (South-West)

Re: Engine vibrating after warm-start

Postby XJ4081 » Mon May 28, 2018 11:12 am

Hi Starbuck,

if I knew the new loom would solve all my problems it would be in my shopping cart right away (I don't believe a car electrician could build it any cheaper though the material itself probably doesn't cost much). But of course, building or buying a new loom is one easy thing compared to bringing it into the car - that's much, much more complicated (and expensive if you cannot do it on your own :oops: ).

Have you already heard of Kapton-Tape? It is very heat-resistent and also an insulator, but I am not sure if it is really suited for use in car electrics. All the other isolation tapes I found are not suited for more than 105°C.

Regards
Oliver
1993 XJ12

XJ4081
Snr Member
Posts: 216
Joined: Fri Oct 23, 2015 12:12 am
Location: Germany (South-West)

Re: Engine vibrating after warm-start

Postby XJ4081 » Mon May 28, 2018 6:55 pm

Unfortunately none of the experts I had been referred to was able or willing to help me with my problem, at least not on short term.

So I had another testride today and put special attention to cold run behaviour. Immediately after firing it up I noticed a slight asynchronism in engine noise and also a very slight asynchronic vibration that absolutely matched with the odd behaviour when engine is hot.

So now my assumption is the error is always there, but it does not always become evident. I lay my hand on the left hand induction pipe and then on the right hand one and I would say that you can feel the vibrations rather on the left hand side induction pipe. As conclusion I would assume that not all cylinders on the left hand bank are firing.

And as a further conclusion I would assume that either the wiring is so damaged that maybe one or two cylinders are not firing at all or it is some injectors not firing correctly. As the wiring does not look that bad, I'd rather say the second.

Or do you still think that a vacuum leak is a very likely trigger?

Oliver
1993 XJ12

Brake buster
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Location: Gloucester UK

Re: Engine vibrating after warm-start

Postby Brake buster » Mon May 28, 2018 8:07 pm

one other item to check for the injector wiring is the ' power resistor ' that the injector wiring goes to , the multiplug that connects into it can become corroded causing high resistance and therefore rough running, dont know where it is on your car, but the XJS was front right behind the headlight, i believe your power resistor is back left of the engine bay ?

just a quick reminder,
injectors are always live.......they switch through the power resistor to ground via the injector ecu , you have x4 banks of 3 injectors being switched each time,

if your wiring is flexible, and not hard and brittle then it might be ok , any hard or brittle sections means it NEEDS changing , using repair tape will not eliminate inter corrosion from being exposed to the air , causing resistance and corrosion

dont give up on the car though , but the V12 can be a tricky beast to diagnose correctly

BB

ps, you can see the power resistor in my last picture of my loom wiring, it has 'LUCAS ' written on it and a green square decal
1994 3.2S in Morocco red and Doeskin/Ruby red interior
1989 V12 XJS White with Mulberry interior
2004 2.1 X-Type Platinum with Champagne Leather interior

previously ,
1999 V8 Sovereign in Seafrost with Oatmeal and Antelope interior

Starbuck
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Posts: 681
Joined: Thu Mar 19, 2015 4:48 am
Location: Melbourne, Australia

Re: Engine vibrating after warm-start

Postby Starbuck » Tue May 29, 2018 1:00 am

Hi Oliver,

The dropping resistor pack on your car will be on the right side of the firewall, attached to an alloy plate that acts as a heat sink on which are also mounted the two ignition amplifiers. It is located behind and below the relay bank. The wiring on the 6.0 is different to an XJS and the dropping resistor pack is in a much more protected position, but by all means have a look at the condition of the plugs on all three modules.

The V12 is a very simple engine. Although the 6.0 is a generation newer than the 5.3 and has interlinked engine, ignition and transmission control modules, in many ways it is still much less complex than the six cylinder in the XJ40. For instance it has no idle control stepper motor and no air flow meters - it even has the same number of camshafts and valves as the six cylinder. It's just that the V12 has 12 of all of the simple components and therefore double the chance something will be missed that would get picked up on the six cylinder.

There are only three things that can cause a misfire - compression problems, ignition problems and fuel supply problems (including vacuum leaks, sensor problems etc) - just like any other engine.

You need to start eliminating some basics before getting into guesswork about injector wiring or anything else. If you don't have the equipment to do these very basic tests you need to find someone that will do them properly for you.

Do a compression test and get the actual figures on each cylinder. V12s almost never blow head gaskets but since your car sat around for a long time before you bought it things like sticking valves are a potential problem. The 6.0 runs smaller clearances than the 5.3 on everything, including the valve guide to valve stem clearances. I have seen plenty of stuck valves on 6.0s that have sat for long periods of time.

Next, pull all of the spark plugs and make sure each one has a good spark at the electrode. Make sure the plugs are all clean and none have black sooty deposits. V12s run relatively small spark plug gaps because of their very high compression and it doesn't take much to foul the plugs on a V12, especially if they are repeatedly run at cold start (i.e. being moved around a lot but not driven on the highway). Spark breaks down under compression (and V12s have high compression) so spark plugs and leads always need to be in very good condition on the V12.

Make sure you have good fuel pressure at the fuel rail and that your fuel filter is new.

Get a mechanic's stethoscope and check that each injector is ticking with the engine running, that will soon tell you if there is a wiring problem or a faulty injector. You can do the same test with a long screwdriver by placing the tip of the screwdriver on the injector and the handle against your ear. It is not as accurate but it does work. It would be better to use a mechanic's stethoscope though.

Listen with a piece of vacuum hose to the nose of the injector where it goes into each intake runner. The injectors on the 6.0 have plastic caps on them and these are prone to breaking up from heat and disappearing into the engine to be burned up. The injectors then have vacuum leaks around their tips.

If you do all of this and you find that one or two cylinders are still not firing properly (spark plug colour will tell you this), get the injectors cleaned and tested to make sure they are working as they should and replace them with new seals and plastic caps.

Check every vacuum hose and rubber blanking cap under the bonnet - there are lots of them. Don't assume they are okay because they look okay where they attach to the manifolds, if necessary pull them off to check their entire length. My bet is many of them will be rock hard and long overdue for replacement. The V12 has a very narrow range of fuel trim in the engine control ECU so lots of small vacuum leaks can quickly cause very lean running.

As mentioned previously, make sure the gaskets between the manifolds and the heads on each side are the correct thicker gasket. While you are there make sure all of the nuts holding the manifolds on are torqued correctly.

Somewhere in that series of tests you are likely to find a very simple problem that has been missed.

Good hunting,

Starbuck
'94 XJ12 (and other toys)