strange one and hoping someone experienced similar and can help

V12 Marelli
Misfire bank A
Misfire only at idle and Tacho reads low at same time (say 200 rpm)
If you press the throttle the revs increase and misfire disappears (tacho normal)
Misfire comes and goes (so sometimes at idle it can disappear and Tacho reading normal)
Frustrating
What can this be?
BTW - it is ignition as fuelling normal

 

Update

Replaced distributor cap and rotor arm - no change
Swapped amplifier modules and misfire moves to B-bank (tachometer now normal - as from A bank coil)
Anyone had similar experience of a module going bad, but not a complete failure?

 

Good work

Partial failures happen all the time with with electronic modules


Cheers
DD

 

Thanks Doug. I also caught your marelli info document in response to another poster - v. interesting document.

 

If you just swapped the plug at the module that will swap the coil as well, so problem may be coil not module. These modules are Bosch and pretty robust.



To determine if issue is coil or module swap plugs at module and coil, if issue stays on A then coil is the problem.

 

Had a similar issue but somewhat worse, a bank would run intermittent due to 1 tiny wire inside the plastic plug piece that went into the coil, and being a Jaguar, the issue was hard to locate, involved tow trucks, and resulted in many incorrect parts being returned. Start the car and jiggle each individual wire until you replicate the issue this is what I did, except mine was only under hot conditions as if the wire only had partial contact due to thermal expansion?

 

Thanks for the responses - excellent forum
Did swap the coils around as a first attempt when stuck in car park - misfire remained on bank A (also, forgive me if i have misinterpreted, but if I just swap the modules doesn't the same coil still fire the same bank - so if he misfire moves bank it is not the coil?)
It was a very strange one as noted in the original email
The tachometer would record a very steady 250rpm during the misfire
When pulling plug leads i determined that I have 4 cylinders on A bank not sparking
So it was only receiving 2 out of every 6 pulses
(what initially confused me and led me to change the cap and rotor was that I did have a big fat spark from the A bank coil and nowhere could I find anyone speaking about a partial failure of the module - so I assumed this was fine initially at least)

 

Thanks for the replies - this is an excellent forum
I did try the coils first - swapping between banks and the problem remained bank A
(also the coil was firing 1A and 3A, just not 2A, 4A, 5A and 6A - and this was consistent hence the steady reading on the tacho of 250rpm - never known a coil fire some, but not all plugs)
Swapping the modules only - the problem moved to bank B
My concern was why did the module fail - was it something else that caused the failure?
Also, I was unsure why it would only drop 4 of the 6 cylinders
However, from the replies it would appear that this is just one of those things?

 

Module arrived just now - a big thank you to MOSS for such quick service (less than 24hours). Started and runs sweet - just paranoid it will happen again, but that is just me!!!

 

Well done. The module failed because your car is 35 years old or more and they get hot, cold, hot, vibrate, etc etc and fail.

 

Redoing the entire ign system in one go is real peace of mind, the only costly part is the distributor the rest of the peices are like 40ish USD or so.

 

These modules are single channel one input and one output so it is an unusual failure, I would suspect connection inside the module.

Electronics usually fails early in its life or it goes forever, but there are exceptions. I would not worry too much, just replace the 1 module and enjoy driving.

 

Thanks for all the help.
You will never guess what happened today - well maybe you will
Car running well all week
Took to show today (about 35 miles drive)
Pretty hot in the UK, but run was pleasant and no traffic
Entering show was much slower affair and it stalled when reversing (thought no more of it)
Leaving show and no ignition on bank A - just fuel (so no firing on any cylinder)
Got home and took the old module (the one that fired only 2 of 6 cylinders) from the cellar (where it is nice and cool)
Replaced Bank A module with this one and runs sweet again (changed back again just to make sure and it was the module at fault)
(note that as I swapped modules previously, the new module I bought is on bank B and the previously working module from bank B was Bank A - follow?)

So what is the problem
> my guess is that the module drop-out is heat related
> am I just unlucky to have the two module fail very close to each other?
> should I be looking elsewhere for an issue? (both failed when on bank A, but both where the original - 27 year old modules)

 

Just unlucky. Buy another and try again, say I, anyway.