Marelli V12 idle stumble

[davidr]

My 1989 V12 with Marelli ignition still has a slight stumble at idle on the right hand bank. It isn't a consistent misfire but it's a bit "fluffy" and occaisionally misses. An irregular exhaust pulse can be felt at the right hand tailpipe. The left bank is regular and fine. I have checked or replaced the following
The plugs and leads have been replaced
Both fuel pressure regulators have been replaced
The engine has been "Terracleaned" which should have cleaned the injectors
Distributor cap and rotor have been replaced with genuine ones
I have checked the inlet manifold gaskets for leaks by spraying brake cleaner around them and listening for a rise in revs but they were fine
I have listened to the injectors which are all clicking regularly
I can't find any other air leaks
Have pulled the injector connectors one by one and this obviously makes a consistent misfire on the relevant cylinder but didn't give any clues as to the misbehaving one.
Otherwise the car runs and pulls well (although twelves are very good at smoothing out and disguising problems)
Im running out of ideas as to why one bank should be doing this and the other runs fine. Any ideas?

[Brakebuster]

There is a running thread on a US forum where the owner has a similar problem, he has even managed to time it to every 32 seconds or something

he is also having a very hard time finding the issue after replacing almost everything

my only thoughts would be a dodgy injector loom , or a high resistance in one of the injector plugs loom, not forgetting the power resistor plug behind the right front headlight , this is known to ' go green ' raising the resistance enough to cause trouble

just thinking out loud ?

BB

[davidr]

Thanks BB - the stumble is more frequent than that.
I wondered whether one of the coils was perhaps breaking down but I guess it would be more apparent at higher revs when there's more demand? When I get some time I'll swap the coils round and see if the stumble moves to the other side.

[Brakebuster]

highly possible and worth a try


BB

[BigCatXJS]

I had a similar issue with my left bank last year / earlier this year (you may recall me mentioning this at Kemble). The issue is pretty much resolved now. Unfortunately there is a list of things that were revised at the same time so can't attribute fault to single item.

New HT leads (originals replaced)
New distributor cap (old one had a very small crack)
New spark plugs
New intake manifolds with tuning pipes (AJ6 Engineering)
Revised ECU fueling
Timing checked and adjusted (found to be out)
New intake gaskets

Are the two coils on the Marelli system (DAC4608) located next to each other in front of the throttle cable pulley and then the two ignition amps (DAC4607) located on the radiator cross member? Just want to be sure...

Ta, Rich

[steve_m]

Mine is running a bit rough since the throttle rods were changed, I also have to give a touch of gas when I start it which I never used to.
It is also running a bit rich and is a bit uneven on the right bank although it is hard to tell with the noise of the new exhaust set up.
Comparing throttle actions with Vince's at Kemble a couple of weeks ago it looks like the micro switch is acting before the butterflies are opening.
Anyway, I have the car booked in at KWE next month to get it set up properly and the fuelling adjusted so I'll report back when it is done.

[davidr]

BigCatXJS - yes the 2 coils are on top of each other between the distributor and the throttle capstan and the amplifiers are mounted on the radiator cross rail.
Thanks all for your suggestions. I will report back when I've tried swapping the coils.
SteveM - I'll be interested to hear how you get on at KWE

[oldtimer]

Hi Davidr,
You may recall I had a completely dead 3A cylinder which turned out to be one of the ferrite rods had slipped part way out of it's location in the periphery of the rotor. I presume that part is still incorporated in the Marelli set up, if so is it worth considering.

[Brakebuster]

Mine is running a bit rough since the throttle rods were changed, I also have to give a touch of gas when I start it which I never used to.
It is also running a bit rich and is a bit uneven on the right bank although it is hard to tell with the noise of the new exhaust set up.
Comparing throttle actions with Vince's at Kemble a couple of weeks ago it looks like the micro switch is acting before the butterflies are opening.
Anyway, I have the car booked in at KWE next month to get it set up properly and the fuelling adjusted so I'll report back when it is done.

i would also be interested in the results , and the cost if you would want to share ? as any pro tune up has got to be better than guessing ? and at least you know they have set up everything ?

BB

[davidr]

Hi Oldtimer - no such system on the Marelli I'm afraid but thanks for the suggestion. All was replaced by electronics!

[davidr]

Steve. I'm interested in your observation that your micro switch was activating before the throttles had moved. What do you mean by that?

[steve_m]

David, you are much better asking BB about this because he spotted it and then also pointed out that one of the reasons that loads of people have to try and stick the pedal through the carpet to get kick down is because the other bit on the other side has also gone a bit gangly.
As it appeared, the micro switch was clicking before there was any movement of the butterflies; so the micro switch must be sending a signal somewhere in an expected response to a fuelling demand which hasn't actually been called for yet.
I suspect that the micro switch either actuates the heated lumbar seat or the toaster (an option in the American market I believe).
Anyway, slightest pedal movement produced the click (on mine) and then you could see the throttle actuators actually achieve their aim of actuating the throttles.
On BB's the pedal press opened the throttles and then you could hear the micro switch click; it was a tiny but noticeable difference.
Strangely enough, even though we were parked side by side we couldn't tell any difference when the engines were running with the bonnets up, couldn't hear a damn thing; everybody else at Kemble must have thought that we were both 17 years old.
A strange quirk of the 3 speed auto box also reared its unremembered head on the way home; if you are lining up to do an overtake at about 45mph don't slipstream the car in front because when you plant the go pedal it might decide (moon, tides, time of the month etc.) to drop down into first gear and then you are unexpectedly almost in the boot of the Tuscan in front of you that is out for a Sunday drive.
Driving fun, I remember that.

[steve_m]

Mine is running a bit rough since the throttle rods were changed, I also have to give a touch of gas when I start it which I never used to.
It is also running a bit rich and is a bit uneven on the right bank although it is hard to tell with the noise of the new exhaust set up.
Comparing throttle actions with Vince's at Kemble a couple of weeks ago it looks like the micro switch is acting before the butterflies are opening.
Anyway, I have the car booked in at KWE next month to get it set up properly and the fuelling adjusted so I'll report back when it is done.

i would also be interested in the results , and the cost if you would want to share ? as any pro tune up has got to be better than guessing ? and at least you know they have set up everything ?

BB

Guys, bit out of sequence on the responses here so apologies.
I've got the car booked in at KWE on the 11th of next month (if the boat doesn't steal all my money first - batteries, chargers, sinking etc.) and of course I will share; (just send me £50 in an unmarked envelope and we'll say no more).
It is just too complicated for me tune up myself, too many dinosaur electronics that I don't understand and don't have the equipment to even attempt to diagnose.
I'm not complaining, this if my 5th V12 Jag (4 XJS, 1 Series 2 LWB XJ12) so I am aware of some of their foibles; it would be nice to have one that actually worked properly for more than 6 months for a change.
Sorry, digressed. I spoke to a guy at KWE and he said it would take about an hour but to budget about £150 if they also found a problem that they could fix; as far as I understood (never been my best trait) they will make sure that all the things that should open, all the things that click and all the things that make you remember why you bought the damned thing in the first place do as they are told by your right foot. If there are failed injectors, burnt out wiring then budget another £25 to £30. Per minute I guess.
That includes reprogramming the fuelling ECU and hopefully giving a wash and brush up.
There is a definite lack of oomph at the moment when I take off which I think is down to said dinosaur brain so the car is taking off in second gear; embarrassing at traffic light Grand Prix next to the old lady in the Prius. Once up to speed, above 40 mph, she is fine as ninepence.
I miss my MGC that I fitted triple 1 3/4" SU carbs to; made my own throttle actuators and all the associated bits and could tune it with a small screwdriver and ear trumpet.

[davidr]

Rainy day today and was considering going to the JEC day at Brooklands but it was absolutely chucking it down! So a quiet day watching the Grand Prix and listening to the cricket in the workshop.
I tried swapping the coils to see if that made a difference or moved the issue to the other side. Apart from a moment when the car wouldn't start because I'd inadvertently swapped the 2 king leads around the coil swap made no difference.
The next thing I tried was resetting the throttles (again) just to make sure that the setting at idle was 2 thou as it should be.
That didn't make a difference either.
I'm all out of ideas now!

[Brakebuster]

hmm

tricky one

i to messed around with the throttle linkage today

the above mentioned switch is the ' idle switch ' and reading my ROM i discovered that the switch should click as soon as the TB opens, so it seems Steves is set quite correct and mine wasn't opening soon enough , that is now rectified

i think that it just tells the ecu that the car is at idea and not to over fuel, when it clicks its telling the ecu the car is off idle and to fuel accordingly

i did notice a lag in the linkage so adjusted that up as well

my two stainless drop links with the spring clips to the fulcrum shafts on the TB seem quite worn , and these are NLA from Jaguar , though the clips are available

will have to do some research

BB