Jaguar World Monthly Forum - Ignition service

Leo_Denmark

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Ignition service
22.01.2012 at 16:16:32

Hi all

I have owned my 3.2 1992 XJ40 for 6½ years and done 106.000 km in that period.

I changed distributor cap and rotor (and filters, oil and so on) pretty soon after buying the car, then I knew these parts were OK.
I have always (or maybe not in the very first period) in my ownership had some hesitation at kickdown when passing 4.000-4.500 RPM. After that it pulls pretty hard up to 5.800, where it shifts (at kickdown).

Then friday I did a kickdown, and things were different: It didn't pull well after 4.500, and shifting happened at 5.400-5.500 RPM Angry

I then changed sparkplugs today: No improvement, but the old plugs were pretty worn (large gap). Colour of all plugs were light brown. Could be brighter, but quite nice. My driving before changing them were 2 x 100+ miles highway cruising at 70-80 MPH.

Next: Distributor cap off. Looked pretty fine, but of cause not new after 100.000 km. Cleaning of cap and cables + some silicone on cables, and then things were back to normal.

Now I want more. The above findings indicate, that the lack of power at 4000-4500 RPM also is caused by weak ignition. This is at max torque, filling and thereby cylinder pressure at ignition is at its peak, and the spark has the worst conditions.

I guess that coil and maybe also spark plug cables are original. Should I change it all (incl cap and rotor), or will for example the coil work OK until it dies ?

I once read something about slight mistiming of distributor cap causing too large spark gap in the distributor, but I guess that is not the case here, as my current cap has done lots of miles without signs of excessive sparks inside. My cap is bright blue/green and not marked 'Lucas' as the old one. Any thoughts ?

How about CPS ? Could it be part of the problems ? I haven't touched it in my ownership, so don't tell me the break all the time... Cheesy

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Leo Jensen, Denmark XJ 40 3.2 1992 - Citroën 2CV 1975

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XJ40S

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Re: Ignition service
Reply #1 - 23.01.2012 at 00:30:25

Its a bit tricky to describe how distributor can weaken spark at certain RPM / load but it can. The timing of the distributor -when the rotor arm is closest to the plug lead terminals is fixed whereas the timing of the spark varies. The postition of the distributor cap is therefore a compromise. You can cut a hole in an old diributor cap next to No 1 plug lead and shine a timing light in. This gives you and idea of rotor arm to plug lead terinal at no load conditions. If you think the ignition is weak at 4,500 RPM try advancing the distributor a little from its present position. I would think your ignition components are a bit tired after the number of km they have done.
Cheers

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Leo_Denmark

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Re: Ignition service
Reply #2 - 23.01.2012 at 19:20:54

XJ40S wrote on 23.01.2012 at 00:30:25:

Its a bit tricky to describe how distributor can weaken spark at certain RPM / load but it can. The timing of the distributor -when the rotor arm is closest to the plug lead terminals is fixed whereas the timing of the spark varies. The postition of the distributor cap is therefore a compromise. You can cut a hole in an old diributor cap next to No 1 plug lead and shine a timing light in. This gives you and idea of rotor arm to plug lead terinal at no load conditions. If you think the ignition is weak at 4,500 RPM try advancing the distributor a little from its present position. I would think your ignition components are a bit tired after the number of km they have done.
Cheers

Thanks

A normal advance curve looks something like the solid line here:

http://www.jagweb.com/aj6eng/ignition_advance_curves.jpg

You made me think, and below is my conclusions for now... Cheesy

With less throttle (more vacuum) the advance can be (and normally is) higher without problems with pinking/overheating, and as my problems occur at full throttle and quite high RPM, the advancing must be somewhere in the middle of the avaiable advancing, so I guess that rules out the possibility of wrong timing of the distributor cap ??
Anyway, I will try it with the old cap just to rule it out. From the firing sequense I have concluded, that the distributor rotates CW and that I therefore have to modify parts to make it possible to twist the cover CCW (against more advance) to make up for eventual misalignment. Do you agree ??

I do not suppose there are no other possibilities of adjustment than modifying the cover guiding grooves ?

Regarding new parts:
What make of coil, distributor rotor/cap and leads would you buy ? What is best, what is OK, what is crap ??
Regarding coil: Is it only Jaguar (and copies of it), that will fit, or is it a generic part ?

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Leo Jensen, Denmark XJ 40 3.2 1992 - Citroën 2CV 1975

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XJ40S

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Re: Ignition service
Reply #3 - 23.01.2012 at 22:32:52

You rotate the whole distributor, not just the cap. Slacken the clamp underneath the distributor body. Mark present positon first so you can get back to it if needed.
Just to clarify - rotating the distributor does not change the ignition timing at all. The distributor cap just has to be in exactly the right place so the rotor arm is near enough the HT lead pick ups at both maximum retard and maximum advance. With time, wear in the rotor arm drive can mean the rotor is further from the HT lead pick ups than it should be at maximum advance, weakening the spark. I would expect maximum advance to be at about 4500 RPM under high load.
Perhas Andy will come in on this.
Cheers

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Rolys mk10

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Re: Ignition service
Reply #4 - 23.01.2012 at 23:30:50

XJ40S wrote on 23.01.2012 at 22:32:52:

You rotate the whole distributor, not just the cap. Slacken the clamp underneath the distributor body. Mark present positon first so you can get back to it if needed.
Just to clarify - rotating the distributor does not change the ignition timing at all. The distributor cap just has to be in exactly the right place so the rotor arm is near enough the HT lead pick ups at both maximum retard and maximum advance. With time, wear in the rotor arm drive can mean the rotor is further from the HT lead pick ups than it should be at maximum advance, weakening the spark. I would expect maximum advance to be at about 4500 RPM under high load.
Perhas Andy will come in on this.
Cheers

why doesn't it?? Undecided

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1994 xj81 majestic
1990 4 litre xj 40
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http://www.youtube.com/watch?NR=1&feature=endscreen&v=Agu_vrh5deo
Don't let someone become a priority in your life, when you're just an option in theirs'

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covkid

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Re: Ignition service
Reply #5 - 24.01.2012 at 10:04:59

Rolys mk10 wrote on 23.01.2012 at 23:30:50:

rotating the distributor does not change the ignition timing at all.
why doesn't it?? Undecided

Because, ignition 'timing' is the angular relationship between the firing of the spark and the position of the piston on the firing stroke before TDC on that cylinder.

On an AJ6 engine, that relationship is determined by the EMS ECU based on information from the CPS and other factors including engine RPM and load etc, advancing or retarding the ignition (timing) accordingly.

The distributor in this case is merely that - a distributor of sparks. It does not determine when the spark occurs (i.e the 'timing'), it simply determines where to send that spark when generated. For the reasons explained by XJ40S, if the rotor arm is not properly aligned with the spark plug lead pick up for the chosen cylinder when the EMS ECU decides to generate the spark for that cylinder in its fully retarded or fully advanced position or anything in between, that cylinder will not fire.

That is a known problem limiting engine performance on an XJ40 - in particular causing the engine to misfire above certain RPM, not allowing that engine to exceed that RPM.

One final point. You will notice that the tip of the rotor arm is 'T' shaped. The angular extension either side of the centre line of the rotor is to allow the spark to jump the shortest distance between the rotor arm and the plug lead pick-up wherever the position of the rotor is in relation to that pick-up within the extremes of advance and retard of the ignition timing at the point when the spark is generated.

Simples! Grin

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Rolys mk10

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Re: Ignition service
Reply #6 - 24.01.2012 at 12:39:53

covkid wrote on 24.01.2012 at 10:04:59:

Rolys mk10 wrote on 23.01.2012 at 23:30:50:

rotating the distributor does not change the ignition timing at all.
why doesn't it?? Undecided

Because, ignition 'timing' is the angular relationship between the firing of the spark and the position of the piston on the firing stroke before TDC on that cylinder.

On an AJ6 engine, that relationship is determined by the EMS ECU based on information from the CPS and other factors including engine RPM and load etc, advancing or retarding the ignition (timing) accordingly.

The distributor in this case is merely that - a distributor of sparks. It does not determine when the spark occurs (i.e the 'timing'), it simply determines where to send that spark when generated. For the reasons explained by XJ40S, if the rotor arm is not properly aligned with the spark plug lead pick up for the chosen cylinder when the EMS ECU decides to generate the spark for that cylinder in its fully retarded or fully advanced position or anything in between, that cylinder will not fire.

That is a known problem limiting engine performance on an XJ40 - in particular causing the engine to misfire above certain RPM, not allowing that engine to exceed that RPM.

One final point. You will notice that the tip of the rotor arm is 'T' shaped. The angular extension either side of the centre line of the rotor is to allow the spark to jump the shortest distance between the rotor arm and the plug lead pick-up wherever the position of the rotor is in relation to that pick-up within the extremes of advance and retard of the ignition timing at the point when the spark is generated.

Simples! Grin

Thank you.

thats all i wanted to know.

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1994 xj81 majestic
1990 4 litre xj 40
mk10 3.8 man
http://www.youtube.com/watch?NR=1&feature=endscreen&v=Agu_vrh5deo
Don't let someone become a priority in your life, when you're just an option in theirs'

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covkid Senior Member Offline Posts: 846 England Gender:	Re: Ignition service Reply #7 - 24.01.2012 at 12:53:31 Rolys mk10 wrote on 24.01.2012 at 12:39:53: Thank you. that's all i wanted to know. Any time Roly, any time!
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Leo_Denmark Senior Member Offline Posts: 462 Middelfart Denmark. Gender:	Re: Ignition service Reply #8 - 24.01.2012 at 13:14:26 Hi guys Thanks for the explainations about the timing How about original or non original parts ? The price difference is quite large...
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Leo_Denmark Senior Member Offline Posts: 462 Middelfart Denmark. Gender:	Re: Ignition service Reply #9 - 24.01.2012 at 21:04:59 Hi I've found a set of original leads, rotor and cap at a former XJ40 owner nearby. Not used... That does NOT happen often in Denmark How about this coil: http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=160706858566&category=17407... If shipping is not too expensive I guess it's worth a try at 8£ ?
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XJ40S

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Re: Ignition service
Reply #10 - 25.01.2012 at 12:30:17

As far as I have heard / experienced non OEM ignition parts are as good as genuine so long as they are from a reputable maker.
The one exception I know about is unbranded rotor arms for E types and XJ40s. They have insufficient resistance / insulation between rotor and drive shaft and the spark goes down the rotor arm shaft instead of to the spark plug.
Anyone tried Magnecor HT leads on AJ6 engines? - expensive but I know people who have used them think they are worth it.
Cheers

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Leo_Denmark

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Re: Ignition service
Reply #11 - 05.02.2012 at 11:45:57

Update. Or maybe not. The siberian winter is in command here in Denmark now. I do my car-work outdoors in the driveway, so now I just have all parts lined up, waiting for better weather.

I delivered the car at a garage yesterday to have them make a few patches in left front wing (and hopefully not more) and then have it repainted where necesarry... Shocked

I have done sandblasting of the rusty spots before I delivered it, so I fell pretty sure no more problems pops up, but we will see... Cool

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Leo Jensen, Denmark XJ 40 3.2 1992 - Citroën 2CV 1975

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db Senior Member Offline Posts: 511 Gender:	Re: Ignition service Reply #12 - 05.02.2012 at 14:58:23 It is doing very well if that's all the rust there is. You should see mine!!
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Rolys mk10

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Re: Ignition service
Reply #13 - 05.02.2012 at 16:48:47

Leo_Denmark wrote on 05.02.2012 at 11:45:57:

Update. Or maybe not. The siberian winter is in command here in Denmark now. I do my car-work outdoors in the driveway, so now I just have all parts lined up, waiting for better weather.

I delivered the car at a garage yesterday to have them make a few patches in left front wing (and hopefully not more) and then have it repainted where necesarry... Shocked

I have done sandblasting of the rusty spots before I delivered it, so I fell pretty sure no more problems pops up, but we will see... Cool

i44.tinypic.com/14m4dus.jpg

Looks like you have been hit by a 12 bore shotgun?? Shocked

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1994 xj81 majestic
1990 4 litre xj 40
mk10 3.8 man
http://www.youtube.com/watch?NR=1&feature=endscreen&v=Agu_vrh5deo
Don't let someone become a priority in your life, when you're just an option in theirs'

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Leo_Denmark Senior Member Offline Posts: 462 Middelfart Denmark. Gender:	Re: Ignition service Reply #14 - 05.02.2012 at 19:49:37 db wrote on 05.02.2012 at 14:58:23: It is doing very well if that's all the rust there is. You should see mine!! Unfortunately I have more rust and a few holes on the other side, but nothing bad. I do not complain, the car has done 7-8 danish winters as daily driver with loads of salt on the roads.
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