Misfire Cylinder 3

[mealmond]

Hi

 

I have a 2006, Fabia 1.2 htp.

 

It has been running well, then today when driving along, the engine light came on, carried on a bit to pull over, stopped the car, started again and it was fine. 

 

I was out so I could not just go home. 

 

Checked for error messages and it mentioned a misfire on cylinderer 3.

 

On the way home tonight it went again, went into limp mode and the engine light came on and I had to pull over. Stopped the car, restarted and it was fine, not a long drive home though.

 

I have just ordered a new coil pack , so I will have it tomorrow.

 

Coil packs were all changed two years ago and spark plugs changed last year.

 

I can only hope it's that and nothing more serious.

 

As I stand looking at the engine, which is cylinder 3, is it left to right or right to left.

 

I thought it was worth trying this first.

 

Thanks

 

Martin

[Breezy_Pete]

Left to right 1, 2, 3 as viewed from front bumper looking into bay.

Compression test if new coil pack doesn't fix.

[mealmond]

Thanks Wino, fingers crossed it is just a coil pack, it runs really well so it was a surprise when it went. More worried as I know the coil packs are not that old.

 

When it last did this it stayed in limp mode so I had all the coil packs changed but this time just turning it off and on clears the engine light and the car runs normally, so I hope that is

 a sign it is the ignition and not a compression problem.

 

Martin

 

 

[mealmond]

Changed the coil pack and cleared the error code, 50 miles in and ok so far.

 

Trying to use the car as normal and see what happens over the next few days, fingers crossed.

 

Could not see anything wrong with the old coil pack, but I presume it would be difficult  to see anything wrong.

 

Martin

 

 

[Breezy_Pete]

Sounds like good news.

[VWD]

Point of interest. What size is the plug.  Asking as I had an old Vauxhall Victor ( last of the Bedford engines ) with th smaller plug and I learned to keep a spare as these fouled up regularly ( even with electronic ignition). Last Astra I owned had same size plugs and tended to a foul up and management light /clear .

But mealmond, at the internal volts in a coilpack you'd never see a problem, just the result. I always wondered WHY car makers moved to coil packs when the old coils lasted longer than the cars. I've only seen one or more old fashioned coils fail and that was when an idiot wrapped the body in tape and insulated the body from earth, meaning that the spark had to jump from the internal to the car earth, burning out the internal connection.

[mealmond]

Not sure about the size of the plugs, when I changed them in February they had been in three years and looked ok.

 

All three coil packs were changed about 18 months ago so I was surprised it happened again, I have only changed the one this time.

 

I can only hope that was the issue as it's been a good car and is not worth very much now.

 

I will see if it happens again, hope not.

 

 

Martin

[sepulchrave]

Some pattern coil packs are no good and don't last, buy cheap, buy twice as the saying goes.

[TMB]

3 hours ago, VWD said:

I've only seen one or more old fashioned coils fail and that was when an idiot wrapped the body in tape and insulated the body from earth, meaning that the spark had to jump from the internal to the car earth, burning out the internal connection.

 

I don't get that. The body of an old-skool ignition coil doesn't need to be earthed.

[sepulchrave]

It overheated more likely, coils used to burn out if you left the ignition on when the points were closed and got hot in use, so wrapping it in tape would insulate the aluminium can preventing is from cooling.

 

Maybe you misunderstood what was being insulated?

[VWD]

perhaps I misunderstood, but from the circuit diagram it's a transformer where the primary (12v side, -12 for older cars ,+12 for newer cars ) is fed a 12v feed to the coil and the other side gets an earth from the distributor as the points close.  The secondary has one visible output -the HT CABLE fed to the HT side of the distributor. WHERE does the other side of the circuit go ( AND it needs two sides for a circuit to be complete- simple electrical theory😀) -the case of the coil, clamped onto ( usually the inner wing) which is bonded to the other side of the battery ( in the case of Reliants and other Plastic cars, a return wire to earth), (+v  on older cars, - on newer cars). The return path is through the spark plug ,which again is earthed to the engine block . In the case I mentioned, where the coil was covered in tape - there was no circuit from the HT side to the can, but on those replacements I gave the bloke, there was a path, and on replacing the coil with a new one, his car started immediately.

Insulate the coil body and there's an obstacle in the return path. HV does not like an obstacle and if high enough will jump the gap ( spark plug a good example) , where heat is generated and eventually the wire fails.

[sepulchrave]

4 hours ago, VWD said:

...WHERE does the other side of the circuit go ( AND it needs two sides for a circuit to be complete- simple electrical theory😀)

 

An ignition coil is a Tesla coil, the coil discharges through the spark plug at the moment the points open because the internal magnetic field collapses, then the points close allowing the coil to start building charge again, it's not a transformer because it's a DC device.

[Breezy_Pete]

The earth for the HT is the engine.

The 'points' are transistors these days, no distributor to be seen.

[sepulchrave]

1 hour ago, Wino said:

The earth for the HT is the engine.

The 'points' are transistors these days, no distributor to be seen.

 

I know, I'm just trying to help VWD out and clear up some confusion, it isn't relevant to the topic because modern pencil packs are completely insulated and, as you say, solid state switched directly by the ECU for each cylinder individually.

A pencil pack is still a Tesla coil though, so nothing has really changed, the output is higher due to the longer charge time which allows for the fatter spark needed to ignite leaner mixtures.

[Breezy_Pete]

My comments about points and dizzies was aimed at VWD, I should've been clearer.

[VWD]

17 hours ago, sepulchrave said:

 

An ignition coil is a Tesla coil, the coil discharges through the spark plug at the moment the points open because the internal magnetic field collapses, then the points close allowing the coil to start building charge again, it's not a transformer because it's a DC device.

I know that from electrical theory. But on the older system, it wasn't just a 12v pulse through the coil, as the voltage output would be amplified  by a factor of dI/dT . The voltage output into the coil would be = I (Currrent through coil)( x (dI/dT))  ( where dI/dT is the instant where the points break -in this case a very small period in time).

My initial comment was that whilst I've heard horror stories on coil pack failure, I've only seen one old style coil fail. I moved away from conventional systems to one using a single coil over 40 years ago. I built the system my self from an electronics mag spec and never had a problem with it till I gave up petrol for Derv .

[sepulchrave]

1 hour ago, VWD said:

I know that from electrical theory. But on the older system, it wasn't just a 12v pulse through the coil, as the voltage output would be amplified  by a factor of dI/dT . The voltage output into the coil would be = I (Currrent through coil)( x (dI/dT))  ( where dI/dT is the instant where the points break -in this case a very small period in time).

My initial comment was that whilst I've heard horror stories on coil pack failure, I've only seen one old style coil fail. I moved away from conventional systems to one using a single coil over 40 years ago. I built the system my self from an electronics mag spec and never had a problem with it till I gave up petrol for Derv .

 

That's great, I figured you had fundamentally misunderstood the purported failure mode because you described an ignition coil as a transformer which it is not.

The most common failure mode of 'old school' ignition coils was overheating, the aluminium can was used to help conduct this heat away, many ignition coils were painted or powder coated, a good example being Sparkrite aftermarket coils which were powder coated bright red so clearly the can itself did not need to electrically earthed, only tightly sealed to prevent oil loss.

It was not my intention to start a pointless argument, rather to suggest that wrapping the coil in tape probably caused it to overheat and fail.

[VWD]

Ye olde metal can ignition coil took a instantaneous voltage ( on my home brew CDI unit it was 350v DC)  derived from the car 12v DC . To arrive at a voltage where breaking the circuit, the old system would have needed some form of inductive circuit to raise enough volts to give us old timers a decent "belt" on the primary( i.e. LT ) side . Then we need a lot of volts to create a spark to ignite fuel on the plug. Which indicates that the ( let's say 350) was changed up into something like a few or more KV. ( transformed  comes to mind).  Two coils inside a can- only way known to electrics of performing this is with a transformer. But as the voltage gets increased, the current decreases by a similar factor. ( Watts in =watts out ,minus percentage loss).

As for the connections- the primary was per terminals on the outside, the secondary was from the centre of the coil and the return was from the case.  Insulate the case and it isolates the return from the earth, heat gets generated and the connection gets broken. I've seem many instances of this overheating from TV EHT supplies to simple car bulbs. Heat gets generated from the I squared x resistance formula. For lamps it takes  less time as the connections are soldered.

No intention of starting arguments, but I started out on car electrics when cars had the +ve tied to chassis .