Idle problem/misfire after spark plug change

I just had a basic service carried out last weekend on my car by a local mechanic within our industrial estate as I needed the ramp free at our garage (due to the work that was booked in).

I simply asked for the oil and oil filter to be changed along with a set of standard specification NGK spark plugs.  (all parts correctly supplied by myself)

When I got into the car at the end of the night and stopped at the end of the junction to join the main road, I noticed that whilst idling, the revs dropped slowly by about 150/200rpm, and the car started to hunt/miss a little bit.  After standing stationary for just over a minute, the hunting/miss got a little worse to the extent that the car was shaking a bit each time it hunted/miss.  However, this hunting/missing wasn't constant, after the odd hunt/miss it would idle fine, then hunt/miss again after a handfull of seconds.  But one the move the car seemed to drive ok, the problem was only occured when the car was stationary for more than a handful of seconds.  (Before this service, the car ran and idled fine)

Just took the car to a a local mechanic who I have used for the past 8 years and explained the problem.  He removed the spark plugs, and one of them was different to the rest of them. Nothing on the spark plug in question is broken or melted.  The white bit (insulator) towards the bottom of the plug that covers the centre electrode has turned a very dark shade of brown, near enough black, and the ground electrodes on that spark plug seem to be turning that colour as well (a bit like a quick build up of carbon).  All the other spark plugs were fine.

We have dropped in a new set of Bosch spark plugs, and the car seems to drive fractionally better but the hunting/miss that occurs whilst the car is stationary is still present.  I noticed this on the drive home from the garage.

Can anyone shed any light on what may have happened or caused this problem to suddenly occur?  Any suggestions for things to check/test etc would be much appreciated.

 

Sounds like a defective plug - cheap and simple to check! A leak at the air filter wouldn't cause the problem as the MAF will still see the air flow and it would affect all cyclinders - same with a vacuum leak.

BeemaBoy wrote: He has already put in a new set of plugs...

...and this was conincidental with the start of the problem.

A leak at the intake boot would cause the mixture to run weak due to the fact that it is after the MAF...

Yes, but this doesn't explained the blackened plug.

The more I think about it, the valves sounds like a plausable explanation due to you saying that the revs go up and down...try getting it vacuum tested...that would confim my prognosis

A bit of an extreme conclusion as there may be many other more common causes. I'd check that the plug lead it attached properly and swap in another plug. Costs a couple of quid and will either confirm or discount it.

b318isp wrote: [QUOTE=BeemaBoy] He has already put in a new set of plugs...

...and this was conincidental with the start of the problem.

[quote]

Bearing in mind that he had new plugs put in at the service. This was the start of the problem. He then replaced the new plugs with new Bosch plugs. Would be a million to one chance of having 2 sets of faulty plugs

Edited by BeemaBoy

Madrab - I supplied a full set of new standard NGK (BKR6EK) plugs when it went in for the service. 

A few days later when I asked my local mechanic for help with regards to the problem that occured since the change of plugs, as we had to remove the spark plugs for inspection anyway to see if they were the cause of the missing/hunting, it made sense to replace the plugs again with new ones.  This is when we fitted the Bosch ones.

(Although the missing/hunting is still present its remained much better since the Bosch plugs were fitted.  (The missing/hunting isn't as severe whilst at idle as it was with the NGK plugs).  The problem has not got any worse though and the car is going in again towards the end of the week to be re-inspected). 

Having looked at the plug in question just now, it seems like carbon fouling was occuring very very quickly on it.  Usually this relates to issues with bad ignition leads/coils, delayed ignition timing, rich air/fuel mixture and/or if the heat range on the plug is too cold.  I am hoping that when the car goes in at the end of the week, that its a bad ignition coil that may have somehow got damaged to some extent by the mechanic who initially did the work.

UPDATE folks. 

(The very first set of plugs when they were changed a few week ago along with the oil service, I found that one of the electrodes was bent when we removed the plug.  I didn't initially notice this).

The car has just been in again at my friends garage.

We removed the spark plugs today and there is no indication of a bad coil and the Bosch spark plugs fitted the other week look ok.  So we compression tested the engine at the same time, all 6 cylinders were reading very evenly and well  all the wiring etc around that area and the connectors all appeared to be in good order as well.  We have had a close look at the vaccum lines too and they all appear to be in good order as well.

But as the plugs had to be removed today for the inspection, I bought a new set of NGK plugs anyway to throw in (just incase there was a problem with a coil/plug), and when we put these in, the car started to hunt/miss worse like it was before on the first set of NGK plugs during idle again.  So we put it on a diagnostics machine and no fault codes were showing

So on the first set of NGK spark plugs - missing/hunting at idle started.  (This week I noticed whilst looking at the plugs that one of the electrodes was bent, so maybe the mechanic dropped it whilst handling it)

Bosch Spark plugs were fitted the the other week and the 2 coils swapped over and the idle was much better but the problem was still evident in a bareable manner.

Today, re-inspection showed no signs of obvious faults and a brand new set of NGK plugs was fitted.  The car has started to miss at idle again.   (The NGK plugs are the correct ones and the plug gaps are OK as well as the torque setting)

This is now really getting on my ******* nerves.

Need to look at the source of what's happening here. I would start at the point of change, the plugs. Have you checked for spark duration and HT current? As it all seems to be at that end as you can change the overall effect by changing the plugs. Have you checked for correct mixture through the rev range from idle? Compressions fine so flame front should be fine as long as not carboned up.

Rob

UPDATE for anyone who is interested.

The Mobil 1 0w40 fully synthetic that was put in a few weeks ago (when the problem started) was apparently a bit thin for my engine's liking.  So after several plug changes and no change I have cured the problem.

A fresh oil & filter change, using Forte engine flush and 5w40 (Duckhams Fully synthetic) seems to have done the trick, the idle is smooth with not hunting/missing, revs up much better, and doesn't stall and the revs don't fall/drop and cause the car to cut out as I decelerate