Spark plug - intermittent misfire

[Wobbly Dave]

My C70 started to intermittently misfire this week. This became more persistent & the usual warning light started to flash & then became permanently on.

I isolated the issue to no 5 cylinder (no 5 is on the nearside the way I count them).

I got some new iridium 8 NGK plugs as the car is due a service next week. So I have changed all of them anyway.

I assumed that as I disconnected the no5 coil pack - that the coil pack was on the blink - which is kinda disappointing as I only replaced all 5 last year.

Look at the plug pics. Unsurprisingly the no5 sparky is all sooted up. But I have noticed on zoom that there is a significant degradation of the plug cathode. Just wondering if I should try the old coil pack again & see if it wasnt faulty after all (And save myself the 34 quid).

Good news is that the car is firing very nicely now. Feels quite fresh - or maybe its just in my wobbly mind :P

[Alan M]

Nearside, gearbox end by your definition? No.1 cylinder is at the timing belt end then count across to five. The left hand picture plug looks to have some heavy oil deposits on it. Have you had a leakdown test done on each cylinder to check that they are all healthy? A misfire under load could be a symptom of excessive oil in the burn process causing your problem. Fresh plugs would eliminate the oil contamination at the plug tip for a while until the oil builds up on it again. It might be worth getting a test carried out for piece of mind.

[Wobbly Dave]

As it turned out it was not the coil pack that was at fault. The car is going in for a service this week so I will get all the new plugs out & see what is going on. The misfire to my mind is probably due to a slight breakdown on the plug cathode - its been nibbled - hence the sooty deposit - due to incomplete combustion.

The engine uses a little oil but no more than expected for an engine of 150K miles.

I tested the compression towards the end of last year - and it was holding up even & strong.

I am using the standard numbering convention - so my no 5 was the furthest away from the cambelt.

[Alan M]

Doing a leakdown test will show you where you are losing your compression. And if one particular place is worse than others if at all. No engine is perfect and every engine looses a little compression. A really good sealing engine will be at most around the 94% mark maybe a little more. Hopefully you tracked down the culprit. Genuine plugs are very good if you get bored of changing out 'uprated items'. Yes I meant to say your numbering was correct lol going by your definition.

[siamblue]

As it turned out it was not the coil pack that was at fault. The car is going in for a service this week so I will get all the new plugs out & see what is going on. The misfire to my mind is probably due to a slight breakdown on the plug cathode - its been nibbled - hence the sooty deposit - due to incomplete combustion.

The engine uses a little oil but no more than expected for an engine of 150K miles.

I tested the compression towards the end of last year - and it was holding up even & strong.

I am using the standard numbering convention - so my no 5 was the furthest away from the cambelt.

A leakdown test isn't a compression test Dave.