On my 5.3 with the Marelli ignition, how important is the gasket JLM1910 that goes under the distributor cap?

IE, there was none when I got there. Should there be one when I leave?

Thanks

Steve, aka 71MKIV

 

Find a diagram or parts online to see if you need one. My 88 V12 has one, but my XJ6 did not. The dizzy is high and dry on the v12 but under a coolant pipe on the xj6, go figure.

 

Forget about it. I did on Grant's advice, years ago.

 

Important, keep it!

The Marelli has a fire history. If you don't fit the gasket, you can have a vacume leak from around the foot and therefore have a likelyness of combustable gasses (crank case) build up which aren't removed and combusted in the engine.

Would you want to risk a dizzy fire with all the extra costs because you save on a gasket which costs low single digits?

 

Marelli = dunno, they got a cap venting system, same as Lucas, so maybe the Italian shaft seal is better/longer lasting , than the Lucas predecessor. Not convinced on that one.

Lucas = leave out, and ensure the venting system works PROPERLY. Replacement Lucas caps are pretty much crap these days. ALSO, leaving that gasket out on a Lucas allows the cap to sit a tad lower, and the rotor to "spark" in the centre of the cylinder probe, as appose to the bottom edge with the gasket in place, crap parts again, sadly.

Warren, AKA Warjon, will know the Marelli answer.

\\

 

Daim
Unless you are meaning another gasket, I cannot see how the Marelli dizzy breather system can be ineffective or less effective just because a dizzy cap gasket (3 below) is not used.

https://www.jaguarclassicparts.com/u...-5-3-6-0-litre

 

Hey Greg,

due to the 'great' quality of the aftermarket caps, they don't sit flush. So, it will draw air from under the cap instead of through the filter under the left side diagonal strut. Also the air leak can be bigger (the inlet of the filter is tiny) making the engine run lean er etc. blah...

People tendiere to use silicone there nowadays. Reults: corroded distributors etc.The gasket is about 0.5mm thick and enough to make the seal work better.

 

NOPE, and RUBBISH.

That venting system on either Brand cap is NOT vac related.

The inlet is a pissy lawnmower filter stuffed up the orifice of a hose that attaches to ONE spigot of the distributor.

The other hose goes FROM the 2nd spigot of said distributor, to a 90deg elbow in the intake DRAFT hose of the AAV.

There is NO vac present at all in those hoses at ANY time, Cold or Hot.

It is a "draft venting" system, old as internal combustion engines themselves.

The reason for that filter, SIMPLE. The draft air is being drawn form the "clean air" side of the LH air filter, so without a filter of some sort, the risk of inducing DUST into engine is very real and high.

 

You sure there Grant? Especially since the connection of the Marelli vent tube is directly to the AAV hose...

-> Filter -> Tube -> Dizzy -> Tube -> L-connector -> AAV bypass hose

That is the way it is laid on the Marelli. If there is no reason, why is it connected to the intake and why wouldn't they have just left it open instead of investing and fitting parts for no specific reason?

My manual even states regular changes of the filter and not to ever run the engine with the little filter disconnected or removed...

 

The AAV hose and thus the dizzy vent, is upstream of, ie outside, the throttle butterflies. There is no vacuum there, merely, at best, air movement (ie draft venting). Thus at best a very small current of air might be generated to flow, and even then probably only when the AAV is open at cold and the throttles are closed.
The Great Palm expands upon this matter I seem to remember (though I cannot find the reference so I could be wrong) anyway, maybe the dizzy vent system could be improved upon by direct connection to the airbox. Even then, there would be no vacuum, only draft, and it is debateable whether any real flow would occur down the vent tube when one considers the far easier airpath into the airbox through the inlet trumpets.
In any event, it is hard to see how the dizzy vent system could compete with the PCV system in terms of pulling oil vapours out of the crankcase. So the question comes, what does the dizzy vent actually do? Well, given the number of cars running about with the vent tubes either broken or blocked or disconnected, probably not much. I suspect that just leaving the spigots open and forgetting the tubes would be just as effective in preventing vapour build up - if there is any in fact!

 

 

There have been reports of some distributor kabooms over the years so the need for the vent system might be justified.

And I agree it's just a 'draft' type vent system.

Since I ditched my AAV entirely I have mine plumbed to the backside of the air filter housing.

Cheers
DD

 

Okay, may well be. I still think that it is at least a draft from the filter through the dizzy at least to the AAV hose. And as we all know, the standard small intake pipes of the air filter boxes reduce the air flow. So why shouldn't it at least try to draw from the tubing? Any possible source is happily accepted by the engine. When mine is running again, I'll but a vacume gauge on the end of the hoses to see what is going on there, at least on a Marelli...

 

My guess is the vacuum gauge won't show anything.

It's not really vacuum in the typical automotive sense.

Air flow across the open end of the vent hose at the AAV end creates a slight pressure drop. Air pressure at the inlet filter end is higher. The pressure differential creates air movement, from higher pressure to lower pressure, which passes thru the distributor.

Cheers
DD

 

Which is still classed as a vacume, as it sucks air from one end to the other A hoover/vacume cleaner) does exactly the same

 

well......
That escalated quickly.
Still dunno if the gasket has to be there. Sounds like I might get away with not having it there, and it's not going to hurt anything if it is there.
So we will see what the checkbook says as we get closer to the final countdown.
They are between 8 and 15 bucks around here. Fair chunk of change for a piece of (precision cut) paper I'd say.

Hmmm, wonder if a layer of the .004 Goretex would work? I can snitch the 8" piece I would need.

Thanks everyone.

Steve, aka 71MKIV

 

Daim
We all agree there is at least the possibility of draft flow. The only original point you made that I was disagreeing with was that this was strong enough to pull oil out of the engine crankcase up the dizzy drive spindle, would be sufficiently strong to overcome the PCV system's function, and that somehow the lack of the gasket would be detrimental to the vent system's function.

 

Steve,

Smart move.

Mine all went without gaskets back in the early 90's.

The PreHE never had gasket OR venting fiasco. Never blew any caps on them, and Sir Arthur's cap is now 485000kms old.

The HE cap came with a gasket, none fitted on dismantle, so what the heck, fit the gasket. Ran like CRAP, Bugga came to mind.
Back in, and cap off, 5 minute task, and noted the "spark jump marks" from the rotor tip was at the bottom edge of each cylinder probe, NOT GOOD.
2 thoughts at this time, beer getting low, raise the rotor 2mm with a fibre washer inside its guts, or remove the 2mm thick gasket. The latter prevailed, could not find my washer selection quick enough.

Vent system as designed by the boffins, still intact and working????, as best I knew.

Remember. at that time we lived 450kms West of anywhere, and sealed roads were a distant dream. Bull dust (google might help the none Aussies), was our normal conditions. Air filters replaced monthly, belts every 6 months, that "Stuff" got in any and all orifices, human and automotive.

Anyway, when we relocated to civilization, and the cars were cleaned up over 12 months (4 V12's at that time), there was NO dust past any filter, and innards of the distributors of all of them was clean. Oil leaks began soon after the dust cement was washed away, but you cannot have everything you want all the time.
Cleaning the a/c evaporator was the most fun part, NOT.

So, make your choice, but spark track integrity, and/or placement is FAR more important than a small amount of maybe???? air leak that will have ZERO affect of any ECU fueling map, coz its the wrong type of air leak to what we are always talking about in here.

 

Am I correct in thinking that the rotor position in the cap is adjustable? Looks like the thing is held in there with some sort of screw and nut affair.
Steve

 

The rotor is fixed to the shaft. No way of adjusting it. But then it wouldn't matter on a Marelli, as it doesn't fire as soon as the rotor makes a contact. The ICU has to trigger the spark first. Which is why the contacts on the rotor arms are long.

What you could adjusting though is the position of the cap/dizzy to the engine. Like that you would be able to adjust the ignition timing by a few degrees (depending on how your dizzy OS already set). Then again though you may end up causing the ignition to either miss when advanced fully or fully retarted... And anyhow, no point in adjusting the timing, as it does zilch... At least on the Marelli, as you would need to reprogramme the ICU for a better ignition curve...