One black spark plug

[Dr.Doofenshmirtz]

Help!
Car: 1996 Renault Clio 1.4 8v. Single point injection.
Problem:


^^ One of the plugs (cylinder #2 on the right) is going black. The above photo was taken after a short drive and idling for around 10 minutes. Engine idles quite 'chuggy' but otherwise drives fine.

So far I have done the following to try and fix the issue:
Head removed and oil stem seals replaced.
Valve seats looked fine
New head gasket
Cylinder bore looked fine (could still see the honing marks after 120k miles!

I stupidly thought it must be the stem seals and didn't bother doing a pressure check. I'm guessing it can only a worn piston ring? I will buy a compression tester and expect the worst.
The MOT is due next month and I guess this is a failure on emissions right?

[Daniel1]

i thought a black plug meant it was too much fuel?

[Dr.Doofenshmirtz]

Daniel1 said:

i thought a black plug meant it was too much fuel?

Yeah, but it's a single point injection with a four branch inlet manifold, so I would expect all plugs to be the same.

[Daniel1]

Dr.Doofenshmirtz said:

Daniel1 said:

i thought a black plug meant it was too much fuel?

Yeah, but it's a single point injection with a four branch inlet manifold, so I would expect all plugs to be the same.

maybe its not sparking properly then?

btw im not a mechanic

[Ynox]

Wouldn't a worn piston ring usually result in a load of blue smoke and an oily plug.

That looks more like it's been running rich. No idea why though.

[DAVEVO9]

You didn't consider the easy tests 1st then?

Swap over plugs or try a new one..

Swap over ignition lead or coil pack?

[Swarley]

Sounds like its missing fire to me, and pulling oil up the bore.

[Devil2575]

I would also have said over fueling or not sparking proply or at all. Have you checked the plug leads and the ignition system?

[mat205125]

DAVEVO9 said:

You didn't consider the easy tests 1st then?

Swap over plugs or try a new one..

Swap over ignition lead or coil pack?

This! Before any messing about with anything else.

If swapping coil packs and plugs along the head keeps returning a dirty plug on one cylinder, THEN do the same moving injectors about. If one cylinder's plug still remains dirtier, then consider getting into the bowls of the motor.

[NiceCupOfTea]

Racialist!

[Dr.Doofenshmirtz]

DAVEVO9 said:

You didn't consider the easy tests 1st then?

Swap over plugs or try a new one..

Swap over ignition lead or coil pack?

Done all that - swap over plugs with a good one from another cylinder...now on a new set of plugs. Tried swapping the HT leads with a good cylinder etc. Measured the HT lead resistance etc, all fine.

The black crud builds up on one side of the plug more than the other side. The crud is quite hard and crusty but comes off with a wire brush (yeah I know you aren't supposed to use wire brushes on plugs).

[8400rpm]

As above, strikes me that it's not firing properly on that cylinder!

[Dr.Doofenshmirtz]

mat205125 said:

DAVEVO9 said:

You didn't consider the easy tests 1st then?

Swap over plugs or try a new one..

Swap over ignition lead or coil pack?

This! Before any messing about with anything else.

If swapping coil packs and plugs along the head keeps returning a dirty plug on one cylinder, THEN do the same moving injectors about. If one cylinder's plug still remains dirtier, then consider getting into the bowls of the motor.

No coil packs - just a single HT coil and distributor (new cap and rotor btw).

[The Wookie]

Nothing funny on that cylinder's intake runner is there, e.g. crankcase breather or sensor or something that could be causing a difference in that particular cylinder?

I also assume you've tried swapping the plug and/or HT lead.

Looks like it's not firing, could be low compression but I'd start with electrics before ripping the engine to bi... oh

[mat205125]

Dr.Doofenshmirtz said:

No coil packs - just a single HT coil and distributor (new cap and rotor btw).

The good old days, before all this eco nonsense that effectively sends a car that is 99.9% functional to a recycling centre when a £1000 module fails out of warranty.

[Dr.Doofenshmirtz]

The Wookie said:

I'd start with electrics before ripping the engine to bi... oh

hehe - yeah, I have a habbit of jumping in right at the deep end!!
Not fixing this problem has so far taken 19 hours!! (note, it did need a new water pump which was a bh to remove, and I just fancied changing the HG for fun whilst I was at it...nothing on TV and all that

Thanks for all the pointers. I agree it does indeed look like a misfire, you can even hear the engine misfiring through the exhaust note and the chugging of the engine.

Weird eigh.

[mat205125]

Compression test and leak test?

[DAVEVO9]

mat205125 said:

Compression test and leak test?

Needs doing..

OP.. When you do the compression test do it with a hot engine all plugs out and open throttle fully on each test.

Do a 2nd test with a small cap full of oil down each bore..

[The Wookie]

mat205125 said:

Dr.Doofenshmirtz said:

No coil packs - just a single HT coil and distributor (new cap and rotor btw).

The good old days, before all this eco nonsense that effectively sends a car that is 99.9% functional to a recycling centre when a £1000 module fails out of warranty.

Funny, when I had a misfire in my old Focus I plugged in a cheap OBD reader, deduced it was likely the coil pack, bought a pattern one off GSF for 50 quid and swapped it in about 15 minutes. But then I'm sure a dizzy would have managed 100k maintenance free

Didn't even touch the valve stems