Ignition coils. Part 1.

Replacement of ignition coils for direct injection engines is very usual operation. It goes without saying – as refilling the window washer fluid. The engine is shaking again? Let’s change ignition coils – and done!

If the person, who performs diagnostics, knows, how to use correct test in ISTA D, it’s possible to see, which cylinder is “suspect” by the opinion of MSD80 self-diagnostics. In most cases, all ignition coils is changed, and done, no fuss! Yes, there is a result, but it’s very short-lived and unstable.

A while ago I watched “What Went Wrong” on Discovery. So this time I decided to do investigation on my own, because something in this story about ignition plugs wasn’t sticking well for me…

Current type of ignition coils is already 10 years in production, already several generations changed. In the beginnings BMW used ignition coils supplied by BOSCH, after – by Delphi from their factory in Italy. Both of them are huge and respectable producers. And, to become a OEM supplier for BMW, very strict and serious tests and quality requirements.

Ignition coil, produced by BOSCH (officially not offered anymore):

BMW N series Bosch ignition coil

Ignition coil, produced by Delphi (official offer):

BMW N series Delphi ignition coil

Ignition coils for direct injection engines are not “regular” coils. Spark plugs, which are used in this type of engines, have ~2 times wider air gap (it means, 1,5-2 mm instead of 0,8 – 1 mm). To get through such wide air gap, the voltage has to bee at least two times higher than before.

Ignition coils of direct injection engine produces voltage of approximately 40 KV. Primary current is significant – 8 .. 10A, the factor of transformation – 1:100, the voltage in primary convolution accordingly 400V. If we compare it with regular coil, the voltage is doubled, energy of spark is 3 to 4 times higher.  In the same time, size of ignition coils stayed practically the same – it means, that every,  even seemingly little issue, could become the spot of inter-shortening.

To help to understand, where is the problem, I made such a test tool for ignition coils testing:

BMW ignition coil test equipment

My goal was to find source of the leaks (spots, where the voltage creates punctures). I choose different test methodology from which was seen before: I didn’t used source of discharge – instead I stabilized the maximum voltage of ignition coil (to avoid overshooting of coil’s driver or coil itself), trying to identify spots, where the spark “jumps”.

This is the test of normally functioning ignition coil:

And this demonstrates typical defect of coil:

As you can see, the spark jumps, when the grounded wire is put close to coil’s “neck”. It was discovered, that the inside of “neck” is covered with brownish residues (obviously it dries out oil and dirt), which conducts electricity…  It is obvious, that the spark will jump from coil’s “neck” to spark plugs body, not via it’s electrodes!

As the significant test group was checked, I concluded, that BOSCH-produced ignition coils are well-made. Insulation don’t disrupts even after long-time exploitation. Situation with ignition coils, produced by Delphi, was worse. There is a problem with “neck” – part of coil, which  covers spark plug’s insulator. What’s it wrong, what is not as it should be? Where the problem lies?

Even slightly used Delphi ignition coil didn’t pass the test. In the same time BOSCH ignition coils even after 100’000 .. 150’000 km run very often were in working condition. So my first conclusion is: only BOSCH ignition coils have to be used in N43 & N53 engines.

Comparison of prices:

  • ignition coil by Delphi – EUR 45 .. 50 (local BMW dealer, discount  20% included);
  • ignition coil by BOSCH – approximately EUR 25 (Bosch distributor).

Incompetency of service centers also is a problem.

Typical problem of N43 & N53 engines (and also for many other engines) – oozing of valve cover and oil filler plug seals, and also a work culture in process of changing ignition plugs (plugs and coils are taken with greasy hands, plugs been laid on greasy surfaces, coil pits aren’t cleaned, instruments used are also greasy etc.). Due to all of theses seemingly insignificant issues oil gets on spark plugs and slowly seeps down the body till groove. In the first moment we think – what’s so special? The oil is an insulator after all! Service centers also perceived it in stoic peace – in their opinion, greasy ignition coils are just cosmetic defect.

No, greasy coils is the cause of the problem! 

While oil accumulates only on outside of coil, nothing happens. But, if this greasy coil is taken off and after imposed on spark plug, the oil gets on plug’s insulator and also on the inside of coil’s “neck”. Differently from fresh & clean oil, used oil (together with dirt) is a very bad insulator. By time, this layer of dirt is been punctured by current, and the coil is been permanently damaged.

In order to solve the problem with ignition coils, first thing the cause of problem has to be solved:

  • oozing of valve cover seals has to be eliminated in order to exclude the oil accessing spark plugs;
  • oozing of oil filler plug seals has to be eliminated in order to exclude the oil accessing spark plugs.

After solving problem with oil oozing, following procedures has to be done:

  • all spark plugs has to be taken out, insulation parts has to be cleaned with cloth soaked in petrol, wiped with dry napkin; after performing this operation, DO NOT TOUCH insulators of spark plugs with hands and place the plugs in dry and clean napkin;
  • wipe all spark plug shafts;
  • clean outside of ignition coils and also inside of coil “necks” by using single-use cotton pads – each coil with separate pad to avoid dirt “traveling” from one coil to other etc.;
  • ignition coils has to be tested with tool shown before: if the test results are not satisfactory (the insulation of coil is damaged) – the coil has to be replaced with new one.

Also remember, that all tools used in the process (especially the tool used to replacing the ignition plugs) has to be WASHED in petrol, wiped (especially inside part of the tool) to avoid dirt getting on insulators of spark plugs.

And once again I would like to remind: DO NOT TOUCH spark plugs by insulators!

And in the end a provocative question: which service center changes spark plugs in such way? None! Even if there is no oil oozing, typical service worker takes spark plug insulators and ignition coil “necks” with greasy hands. And we as customers pay for “result” – actually damages done to our vehicles…

P.S. How to visually distinguish ignition coil with damaged “neck”?

BMW ignition coil

From outside the “neck” of damaged coil can be dirty, with marks – different kinds of lines, circles.

BMW ignition coil

If the inside of coil’s “neck” is rough, as worked with sandpaper (in the picture you can see scratches in direction from electrode to end of “neck”) – the coil is “tripped”. Scratches are marks from high voltage discharge. New ignition coil “neck” inside has no such marks – it’s smooth (both BOSCH and Delphi).


Related entries:

Ignition coils. Part 2

N54 engine misfire faults

Spark plugs