Bedfordman

All the plugs same age - about 3000 miles.


One plug has corrosion to metal parts including the connector, marks on the insulator. All the other plugs in normal condition.

Had a too high CO2 reading at the emissions test. The Examiner suggested there was a slight misfire as probable cause - I had not noticed!

Pulled the Spark Plugs and found one a bit different. See picture.

OK, have ordered new plugs, and hope this will get me the right CO2 reading. BUT: What is the cause ?

Just one Faulty Spark Plug ? / HT wires failed ? / distributor ? ? Any suggestion welcome.
Peter

dr bob

A faulty plug delivers high HC (hydrocarbon) as the unburned fuel passes out the exhaust. CO is produced in proportion to fuel:air mixture proportion. CO is only produced when there is at least partial combustion. To get a meaningful reading, you must make your test measurements in front of any catalytic converters. No doubt getting all the plugs (and the rest of the ignition) working as-new will help the car run correctly. Look at the color of the individual electrodes and insulators to help identify differences in combustion. The rust on the outer shell of #6 indicates that you have water in the 32V plug well if it's a 32V car. Else it's water or some other corrosive mixture on that particular plug shell. Unless the plug is physically broken internally, combustion products never contact the outside of the plug shell. An exception is when a plug isn't installed correctly, allowing gases past the seal above the threads.

Housekeeping: Click on "User CP" in the banner line, and put some info about the car in the "signature" section there. Important stuff like the year of the car can go there, and if it's a USA or rest-of-world car. Location will give us some hints too. Otherwise you should put the year and model/market of the car in every post. Guidance for some issues changes depending on the car, engine and transmission type. Help us help you, please.

worf928

FredR

Further to what Dr Bob wrote, how long have you owned this vehicle? To have a spark plug in that state suggests a lack of knowledge as to what was in there so possibly a new to you acquisition? The spark plug well can leak oil on later models at least but not coolant [as I am aware] so the only source of water ingress to allow corrosion like that to take place is from a poor seal on the plug connector. Needless to say the ignition leads are another time based failure point- any idea when they were last changed out? Try to get the car in a dark place at night and see if you spot any HT leakage - try spraying a little water over the HT leads first. The UK weather is awful at the moment, cold, miserable and damp. If you can see HT voltage tracking it might be more obvious on the suspect cylinder[s]

It might be an optical illusion but have you checked the gaps?

The distributor covers and the rotor arms are the other obvious wear points that can cause a misfire. Remove the covers and take a look at the contact points to see if they are badly pitted or otherwise dishevelled