One black spark plug - ignition or injection fault?

[maxallan]

Hello,

My '91 320 E36 started to misfire a while back. It's really obvious at tickover and not so obvious at full power on high revs. (but no warning light on dash)
So, I pulled all 6 spark plugs and number 3 was black. The rest were a fine "tan" colour. (And I found I've got the leaking oil problem while I was in there! New gaskets are next on my list to do...)

So, could it be a bad ignition coil? My guess is a leaky/faulty injector. If there is a chance it's the ignition then I can buy a new one for about a third of the price of an injector.

Is there any easy way to test which bit is faulty? I can swap ignition coils over easily and see if the "black" moves with it, but I imagine it takes quite a while to get black enough to see? Any estimate on how many miles?
To move the injector would be a bit more work having to remove the whole fuel rail etc...

Thanks for any ideas!

[David Mc]

I would be doing the gasket repair first. If the spark plug well fills with oil. it can short out the ignition to that plug as carbon-laden oil is conductive. The coils can be swapped without any problem as they are all identical in the event that a gasket repair doesn't cure the problem but I think an injector fault is unlikely.

[Big Evil]

I'm with David on that one, you're looking for something affecting only that cylinder. Its possible you may have some kind of valve issue with that cylinder also, but I'd start simple first. Most likely not an injector issue. New plugs, swap coils around, make sure they are CLEAN and some new gaskets. Might just fix it with no further issues.

[maxallan]

The first thing I'd say is don't change your spark plugs in the gloom of an evening, you might miss things, like the crack down the side of the ignition coil and the melted blob hanging out the bottom...
Once the coil was replaced, it still wasn't firing on that cylinder.
(In fact it wasn't sparking OR injecting, except it tried to inject a couple of times on startup)
After much diagnostic, we decided it would have to be the ECU. Probably whatever melted the coil blew the transistor. Pulled the ECU and opened it up and found some water damage and a burnt spot on the plastic PCB cover where the connector pin for plug 3 solders to the PCB.
Probably the ECU is being clever enough to notice the lack of spark and cutting off fuel to save the engine/catalyst.
Big thanks to Darren at DCS Bristol! I can't recommend him because if you all go, I won't be able to get booked back in to his busy schedule :-)

[maxallan]

Help!

I've replaced the ECU and reset all the codes (for example the "power lost"!)

Started it up and it still misses on one cylinder. Checked the codes and it's cylinder 3 ignition.
(I think it is anyway, does anyone have a "code book" for the cheap code readers that's got European models in ? Mine says "For an EU 320 take the nearest US equivalent which would be a 318 because they've both got 4 cylinders". Oh, really, they must have put 2 spare cylinders in my engine bay for some reason then....)

So, as it's got a new coil, just maybe the coil was DOA, so swap it to cyl2.
Reset all the codes again. Restart engine. Still cyl3 ignition fault code.

So, check the bentley manual.
Check from pin 24 on the ECU cable connector all the way through to the connector going in the back of the coil. No resistance, not broken.
Turn ignition on. Check for voltage on the relevant pin of the coil connector as in manual, voltage present.
Check the earth lead. OK
Check the resistance of the coil. OK

So, now I've got a coil with power and a connection to the ECU. And it's still complaining.
At this point I'm pretty much out of ideas, I did think maybe there was a wiring short somewhere. Checked from the earth pin on the connector to the big battery + terminal under the bonnet and got a very low resistance. Thought that was it and checked the other connectors, they were the same. So I'm guessing I was just measuring the resistance of the interior lights or something..


So, either I've got a bad ECU (it is off eBay... but it seems unlikely that it would have exactly the same fault as my original unless anyone can tell me that it's a known fault with cylinder 3) or there is something REALLY freaking weird going on!!!!

[ireestar]

what p0 code you getting mate i have the 2010 autodata with all codes and repair guide......IREE

[maxallan]

From memory :
The first code was 1A - "power supply" after being unplugged.
Then a 32 which seems to mean cylinder 3 ignition.
It's a German 1991 E36 320.

Thanks Iree