Peugeot 206 | Misfire

Peter
May 22 2013

Fascinating article, thank you very much for posting. In tech, they taught us the primary mirrors the secondary voltage waveform, but, as you say, being able to see the secondary in decent resolution reveals those small discrepancies that we are looking for. Noticed some confusion on resistances in comments. A multimeter reading O/L in ohms scale is Infinite resistance. Not ‘No resistance’ as No resistance indicates zero resistance or a direct short. E.g. there is ‘no resistance’ from a point on plug body to another point on plug body. There should be ‘infinite’ resistance from body to centre electrode on a healthy plug, and usually a K ohm resistance between terminal to centre electrode to counter radio frequency interference.

phil j
January 12 2013

Regarding scope settings wouldnt it be clearer if the four channels were offset rather than superimposedf over each other on the primaries?
personally i find it difficult to separate them dislayed like that.

Don Smithson
November 04 2012

I really do not believe what I am reading spark plugs with a resistance reading to earth and still firing. So a electrical voltage going to earth before crossing the internal air gap in the spark plug and still firing across the spark plug gap in the cylinder showing a high voltage on the scope. You learn something new every day. REALLY

Stuart Duff
November 01 2012

An interesting and thorough approach to a common issue with modern coil packs, the process quickly tracked down the offending plug and gave the right and most cost effective repair for the customer, yeah we could swop plugs and coil packs and see if the fault moved, but its good to have a methodical process of analysis and elimination especially when the fault was only when cold.
Great post.

Robski
October 30 2012

@ Marny
These vehicles are prone to coil packs, should I have thrown one on just to see or pinpoint the actual fault in hand ?
Originally the first fault was an injector on the same cylinder due to the well known washer jet/pipe leak issue, I thought it more interesting about the shorted spark plug.

Of course I took them out to have a look which turned out to be a fruitless exercise as A) they had not been in long & B)if the problem is internal to the plug what am I going to see ?
so back in they went & on the scope went.

I wonder how many would have fitted a coil with no testing ?

Darren Cotton
October 30 2012

Nice work…. Not sure if the previous posters have understood what they have read….

I have never given it a thought about plug resistances, But when I read this I got a plug and checked the resistance. This is from the terminal tip to the centre electrode mine was 5.2kohms obviously terminal tip to the body of the plug has No resistance as the plug has a ceramic insulator..  Nice approach.

A visual inspection of the plug does not mean its OK, Its takes experience to know what to do with the tools you have and with no fault codes many would have either thrown some parts on and got lucky or given up! I don’t see anything wrong with your approach, you have proved and identified the fault, with no unnecessary parts fitted…. Look forward to you next study…

Darren

Stuart Bell
October 30 2012

I agree with Darren on this. Sometimes you have to take a thorough approach to get to the root of the problem. I have just had a misfiring C3 through my ‘shop, and the frustration of not being able to test the cartridge coil pack on these (I don’t have coil/plug extension leads) was awful. I could test pretty much everything else with my 4-ch Pico, sadly not the secondaries. I’ve also learned from bitter experience that throwing new parts onto PSA engines, especially the cartridge coil packs, can just confuse the situation, as the replacement is usually just as bad the original!
Doing a resistance check on all plugs has been part of my procedure for this sort of work for the last couple of years, as I’m seeing more and more cars misfiring on “new” plugs.

Andy_R
October 29 2012

Nice find.  A quick comment regarding resistances and the interpretation.  8.8MegaOhms is lots of resistance.  The comment that the No.1 plug doesn’t have resistance is incorrect, it has lots of resistance.  the measurement is more a reflection on the limitation of the multimeter being used.  O/L just means that the reading taken is higher than the maximum reading the instrument is capable of.  I don’t know what this is, but it might be 20MegaOhms.

Robski
October 29 2012

Hi Andy, you will find i said resistance to ground (earth electrode/plug body from the nipple or nipple to the block)so with this short & a positive fired plug where will the spark go ?

Yes the plugs do have resistance from the screw on or fixed nipple at the top to the center electrode of 3.9 Kohm (the plugs fitted)
I would be interested if you could supply the specs of resistance in a typical spark plug,Cheers

Marny
October 29 2012

Why did you not do a visual inspection of the spark plugs first?  That would have saved you an awful lot of time & money to find & rectify the fault.  Sometimes & usually more often than not, experience is far more valuable than any tool!