Two Tone Spark Plugs...

[James Cornell 10]

I have been trying to fault find a fueling issue on my 1.9 8v and when looking at the plugs they look white (lean) but they also have a brown/orange tinge to them but only on one side.

 

Here are two plugs and as you can see they both have the same discolouration but only on one side.

 

Is this common?

[James Cornell 10]

Plug One

 

Plug 2

 

[sub205 5]

The 8V has only one inlet valve so the mixture in the cylinder is not 100% homogenous.

I bet the darker side is pointing to the exhaust valve while the white side to the inlet.

It also gets washed by the incoming fuel.

[James Cornell 10]

Good shout that would make sense with the 8v layout.

 

What's your opinion on the plugs? Mega lean or not too bad?

[sub205 5]

looks ok to me. on EFI cars you cannot tell much about the mixture by looking at the spark plugs.

[James Cornell 10]

Really??? I though the same applies for both EFI and carbs.

 

My vague understanding is

 

black sooty = rich

White (like old fashioned dog poo) = lean

Biscuit brown = bob-on

Wet/black = oil

Blistering on electrode insulation = overheating

[sub205 5]

roughly, yes. But most EFIs tend to cause "to lean" looking spark plugs. As carbs do not offer the same as VE-tables in modern ECUs, they tend to make a richer fuelling so that the engine is never running critical lean mixture in any load area.

[James Cornell 10]

Well I am happy to report that you do learn something every day!

 

Thanks for the info sub205

[welshpug 1,622]

Thats not right at all, whether a computer or a carb is guessing how much fuel goes in makes no difference, you would still see from the condition of the plugs.

 

Granted with modern injection using lambda sensors it is less likely, but if something goes wrong youll still see it.

[sub205 5]

Yes, of course. But the ECU-controlled engines tend to look like they are running leaner than carb'ed ones!

[welshpug 1,622]

that would be because they ARE running leaner, unless they're running rich...

[James Cornell 10]

I've started a debate...

[sub205 5]

Yes, after all they run leaner and it's ok that they do! In several load conditions they do not give as much fuel as carb engines, that's the reason why modern fuel management systems need less fuel than carb cars ;-)

[DamirGTI 323]

I've started a debate...

 

Your plugs look fine ... definitely not lean , if lean they would be white or kinda "clean looking" almost without any discoloration on the entire plug tip .

 

But as said , just by looking at the plug tip you wont be able to distinguish what's really going on with the fuel mixture across the rev range .. thus tuning the air/fuel mixture by the spark plugs is a bit hit and miss , especially with today's fuel and if you don't know how to actually read plugs !

 

It's possible to read idle mixture , midrange , WOT .. roughly even the ignition timing !

 

For such procedure you suppose to have an brand new set of plugs and good magnifier glass (or cut the plug threads afterwards for porcelain part analysis) , fit the plugs then go for a spin and run the engine up to the WOT and switch it off by the key whilst at WOT and coast down until stopped without using the engine - take the plugs out , take magnifier glass (or cut the threads off) and look deep down where the porcelain part meets metal part of the plug .. there will be an black ring present on the very end of the porcelain - the height/thickness and black color shade/intensity of the ring will tell you WOT mixture quality .. midway on the porcelain part midrange mixture quality , top part around electrode idle mixture quality (by color from grey to brownish) ...

 

To save yourself a bother messing with the plugs , if you wont to read and adjust the mixture properly and precisely by yourself - buy/borrow an wideband lambda sensor along with the gauge , or at least narrow band 4-wire heated "zirconia" lambda sensor with the gauge or even if you want really cheap setup - connect it onto a multimeter and use multimeter for reading . Weld an lambda boss onto a piece of tube , screw the lambda sensor in and stick the tube with the sensor into the tailpipe , secure so that it doesn't fall off go for a spin and monitor the lambda readings on board whilst driving .. and adjust/correct if needed .

 

D

[James Cornell 10]

Thank you for the information damirgti, sounds like a plan although the lambda sensors with a gauge are surprisingly expensive...this may have to be a job for another day.