Hello Jag Guys,

Just had my '94 Coupe about a month now and am trying to go through the whole car looking for potential problems and doing preventative maintenance items. My problems with the A/C sysstem are on hold for now so I will move on to other items.

I have read some horror stories in various places on the Internet about failures of the Marelli distributor caps/rotors.

What fails with these items? Is a potential failure visible and what do I look for?

Also, if replacement is indicated, what brand(s) should I buy? There are several different brands available. Oh, sorry, I forgot, the car has 54K miles.

OK, now for the other "cap", the radiator one. The car has the original English caps which appear to be identical and both of them are 16psi.

I have read in several places that the radiator tank cap should be 20psi and the resevoir cap 16psi. and don't get them mixed up! I am not having any cooling system problems and have drained and refilled the system with new coolant. Do I leave well enough alone?

Hope some of you guys who have owned these cars since new will be able to answer my questions, and to them, thank you for your help.

Take Care,

Bob (Spillwaybob)

 

Just like any car Dist-Cap, Look at it for wear and cracks. Make sure that your wires are not twisted and laying against metal that can wear through or cut the wires. It is a tight space if you are dealing with a V12. Check the resistance of your wires with a Ohm Meter before replacing them. Buy good wires and Cap, you will thank yourself.

For the radiator caps, make sure the seals are good. You have the correct ones on the car. I found that on my V12, you have to make sure that the fluid levels are up on the lower one first then deal with the upper one. Just like any other car, triple check the cooling system for any leaks and deal with them immediately. Dry cracked hoses are not a good idea for that V12. Do not make your hoses look pretty by using Armorall! In fact don't use Armorall on any rubber parts! Just thought I would throw that in.
Check your viscous fan (you may have electric fans). If you turn it and feel resistance, the fan should be good. If it spins freely, replace it.

Hope this helps.

 

Hi Abe,
Sorry for the delay in answering your post. And thanks for a lot of helpful suggestions for checking my 2 "caps"
I took the engine top cover off and I think I will leave the checking of the dust.Cap and plug wires to some one else----way too crowded down there and I am too short to reach.
My cooling system is in good shape. I have a viscous fan on the water pump which cuts in when the eng. gets hot and I have the new style electric fan introduced with the 6- litre eng and blows tons of air when needed.
So, as you can see, I will be in good shape after I get the dist. Cap and wires checked.
Thanks again for your help.

Bob (Spillwaybob)

 

Hi Bob

The "Marelli Failure" and I put that in inverted comas because I believe this is caused by a lack of maintenance. It is where one bank fails usually because of rotor failure. Have a look at some of these.

Marelli Cap & Rotor

I think this occurs because either the front spark plugs do not get changed and or the ignition leads fail. Simple electronic theory states that the current must flow to earth if there are plugs not firing the current has to go somewhere so finds its way down the rotor.

Do not go just replacing the rotor and cap - pull them off and check them if they are good then clean and put it back. The aftermarket rotor and caps are not as good as the original Marelli.

Definitely replace ALL spark plugs. You can check the leads with a meter these should read about 16KOhms per meter for stock leads and lower around 6KOhms/m if they are TopGun.

 

Bob
I think the 6 litre engine may be configured slightly differently as far as the cooling plumbing is concerned that the earlier models. But, if it is similar, then the centre engine fill pipe ideally needs a higher pressure cap than the one on the reservoir. In fact the system is so constructed that when the centre cap 'blows off', the coolant stays in the system and (if like the 5.3 engines) vents to the radiator via a pipe and does not vent to air at all, as you will see if you look closely at it. As your engine is fine and not overheating, nothing to worry about, but a 20 or 22 psi good quality cap is a good idea on this centre spout. THis then ensures that the lower pressure cap on the reservoir controls the system pressure without any interference.

In fact on my car I have managed after much searching to find a solid cap that just covers the centre spout top very tightly and has no blow off or sprung return inside at all. Works perfectly, and since fitting it, I have had no coolant loss whatsoever. Before, the system always needed a little topping up now and then.

Greg

 

Hi Warren

Lots of good information in your post.......thanks for sending it. Last week I made an attempt at doing what you have suggested and got as far as removing the top cover and a large vacuum unit with a long rod sticking out of one end. Took a long look at the distributor, wires and plugs and decided that I would not be able to go any further without removing the 2 coils. I stopped there and decided to let someone else do that for me!
Question, if I should be unfortunate enough to experience a cap/rotor failure such as those shown in your attached clip, wouldn't I get some warning and have enough time to get back home?
Thank you again for your input.
Regards,
Bob (Spillwaybob)

 

Hi Greg,

Thanks for the good info about the cooling system. Since the system seems to be working normally and the 2 caps are in good condition, when the time comes, I will replace them with the correct psi ratings.
Thanks for your help,

Bob. (Spillwaybob)

 

I would imagine there would be warning what that would be I do not know as I have never experienced it, as long as you replace spark plugs (ALL OF THEM) at regular intervals and check/clean rotor and cap (as in any Distributor system) I believe you will never experience this phenomenon.

 

Hi Warren,
Got all 12 plugs replaced...boy, what a trip! The only one that gave us a problem was B1 because of the A/C compressor. I had to grind the end of a cheap 3/8" socket extension (the kind that have rounded ends to allow a little angle change) to get enough angle between the extension and the socket to get all the way down on the plug. Took several hours but I glad I did it that way instead to loosening the comp. mount.
Tomorrow I am going to take the dist. cap and rotor off for inspection. My question for you is where can I get the best price for replacement Marelli rotor/cap should I need them?
Thanks for your help.
Bob (Spillwaybob)

 

I get all of my parts from my Local Jaguar parts supplier. As you are in the US some of the other guys might chime in with local parts suppliers.

Or you could try SNG Barrett in the UK.

 

Warren,I found some extra time and to the cap and rotor off today. I'm pretty sure these parts are the original equipment (53K miles) but they do not have "Marelli" marked anywhere, only "made in Italy" I examined both closely and can't find any problems, only dust. I'll clean up and reinstall. One thing though, there is no vacuum or centrifical advance mechanisms. Do you know if they eliminated these things on the 6 litre engines? Is there anything else I shouldn't check now?
I know I am a pain in the butt, and I hope I will be able to stop exploiting your great knowledge of these cars.
Many thanks.


Bob (Spillwaybob)

 

No, the bona fide originals would have the "Marelli" stamping



Most likely the Intermotor brand replacement which, near as I can tell, is the only one available and is sold under different labels/brands, including Jaguar.

The genuine Marelli cap has been out of production for a least 2-3 years now.

As of a couple years ago, though, the true Marelli rotor was still available under the Jaguar part number




Not used at all on the Marelli cars, 5.3 or 6.0 litre. The advance is taken care of electronically via the ignition ECU

Cheers
DD

 

The genuine cap has the Magneti Marelli logo on the top. My car still has the genuine Marelli cap. My spare engine (a later 6.0L) does not have Marelli or made in Italy so I assume its an aftermarket.

 

Hi Warren,
Here I am again and confused about conflicting specs. FIRING ORDER. My Jag shop manual says the firing order of the 12 is and rotating anti-clockwise, 1A, 6/B, 5A, 2B, 3A, 4B, 6A, 1B, 2A, 5B, 4A, 3B.
Firing order on the distributor cap and the one I followed is: 1A, 1B, 5A, 5B, 3A, 3B, 6A, 6B, 2A, 2B, 4A, 4B.
The shop manual does have the 6 litre suppplement but there is no firing order listed in the supplement.
Did the firing order change since the original 12 or is one of these a mistake???
I haven't started the engine yet and have to make sure about this first.
Thoroughly Confused,
Bob (Spillwaybob)

 

Bob,

It may be that the firing order and the plug connection order on the cap are two different things. If I remember correctly, the Marelli cap did not have the plug wires connected in firing order but my memory could definitely be fuzzy

Cheers,

Allan

 

Correct the Marelli rotor has 2 outputs 180deg apart so the order on the cap is not the firing order, the one in the book is correct and the 6.0L is the same as the 5.3L

 

This is very confusing. Bottom line, please tell me when I install the plug wires on the cap, so I follow the cylinder numbers on the cap?
Excuse my stupidity, I have never been involved with an ignition system like this!

Thanks
Bob (Spillwaybob)

 

Sorry I misunderstood your question - YES the numbers on the cap go the respective cylinder.

However the numbers on the cap are NOT the firing order because of the dual rotor.

 

Warren $ Allan,
Thank you guys for straightening me out about this goofy distributor. The car runs fine and I'm glad I did it even though it was extremely frustrating and difficult.
Amen
Bob (Spillwaybob)

 

Once you get used to the idiosyncrasies of the Jag things get easy. BTW Porsche used almost the exact same distributor on their dual plug flat 6 and the Ferrari 400i used almost the exact same dissy.