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[Webby the Bear]

Hi guys, doing a mini assignment for school and as ever I like to use my car as a case study where possible. mine's not leaking but for the purposes of the assignment I have to say why leaking CC gaskets causing oil in the plug wells would/could cause misfires.

whats the reason? is it simply that the oil gets in to the ht lead/spark plug connection and severs the electrical contact? or is it something to do with the extra heat? 

[chrisgixer]

Oil eats the rubber that insulates the spark. Spark then escapes outside, via the porcelain insulator on the top. A classic fail will see black etch marks in the porcelain leaving a track down the side of the spark plug. There's pics on here of one somewhere.

[Webby the Bear]

Got ya, Chris. Thanks. So essentially the failure is ''voltage leak''.

[Webby the Bear]

oh one more q...sorry lol

once you have a misfire from the oil in the wells.... is a ''clean up'' of the ht lead good enough or is replacement recommended... im thinking replacement based on the fact they've brken down

[chrisgixer]

That's certainly the case ime.

Assuming plugs are not damaged...Once a missfire starts, just cleaning up any oil doesn't cut it. Water will clean up and dry out, provided the iron core isn't damaged/rusted and swelling, but once the oil has eaten the rubber and a miss is present, new leads or boots are needed.

See what others say though, I can't provide viable answers on voltages etc. There's others here far more qualified to help you.

[Marks DTM Calib]

Right.

For one, you dont get 'voltage leaks', the only thing that flows in an electrical circuit is current so the 'current' will track down the side of the spark plug.

In essence, what happens is that the oil gets around the spark plug insulator, the voltage is raised across the plug and the current then takes the path of least resistance.

As the oil is not as good an insulator as the ceramic insulator, the current tracks down the side of the plug rather than jumping the gap on the plug.

The question of if you can re-use the leads etc is very dependent on how long its been in the condition, oil will over time attack and damage most rubbers so catch it early and degrease the rubber well and it will be fine.

For info, voltage is the difference in potential (PD) between two points....

[TheBoy]

In laymans terms, the 2 reasons this will cause a miss are (as DTM says, explaining it technically):

1) The oil (partially) shorts the HT to the head (ground/earth/0V)
2) The plug leads (or plug caps on those with coil packs) get damaged over time by hot oil, causing a high resistance (which tends to arc, and will eventually burn the cable

[Marks DTM Calib]

In laymans terms, the 2 reasons this will cause a miss are (as DTM says, explaining it technically):

1) The oil (partially) shorts the HT to the head (ground/earth/0V)
2) The plug leads (or plug caps on those with coil packs) get damaged over time by hot oil, causing a high lower resistance (which tends to arc, and will eventually burn the cable

Corrected 

[TheBoy]

In laymans terms, the 2 reasons this will cause a miss are (as DTM says, explaining it technically):

1) The oil (partially) shorts the HT to the head (ground/earth/0V)
2) The plug leads (or plug caps on those with coil packs) get damaged over time by hot oil, causing a high lower resistance (which tends to arc, and will eventually burn the cable

Corrected 

Earth bit went in because some peeps insist on calling it that. And for your benefit

Lower resistance? Are we talking about the insulation (in which case I agree) or the conductor (in which case I don't think I do)?

[Marks DTM Calib]

In laymans terms, the 2 reasons this will cause a miss are (as DTM says, explaining it technically):

1) The oil (partially) shorts the HT to the head (ground/earth/0V)
2) The plug leads (or plug caps on those with coil packs) get damaged over time by hot oil, causing a high lower resistance (which tends to arc, and will eventually burn the cable

Corrected 

Earth bit went in because some peeps insist on calling it that. And for your benefit

Lower resistance? Are we talking about the insulation (in which case I agree) or the conductor (in which case I don't think I do)?

Insulation, the conductor wont increase in insulation (although its length may get compromised as it errodes) but either way, tracking is down to finding a lower resistance path .

[Webby the Bear]

Thanks for the info boys, very well explained. 

[Toledodude1973]

i'm learning on here all the time, thanks for info

[chrisgixer]

Yeeeep, they lost me at the start of ....   Well, the electrical bit. I'm still on "electricity flows, like water"

[Marks DTM Calib]

Yeeeep, they lost me at the start of ....   Well, the electrical bit. I'm still on "electricity flows, like water"

Sort of, consider the flow as the current, the difference between the top and bottom of the pipe as the voltage (potential difference) and the bore of the pipe (e.g. the resistance to flow) as resistance

[Entwood]

Yeeeep, they lost me at the start of ....   Well, the electrical bit. I'm still on "electricity flows, like water"

Sort of, consider the flow as the current, the difference between the top and bottom of the pipe as the voltage (potential difference) and the bore of the pipe (e.g. the resistance to flow) as resistance

That brings back memories !  When I started my flight engineer training way back in 1970 I knew absolutely nothing about electronics, it had not been taught to me at school (despite doing A level Physics) in any way .. and I could not get my head around it ...  one of the instructors sorted me out in 2 hours .. by comparing every aspect of electronics / electrics to basic plumbing .. and it really works ..

Battery - header tank
Voltage - pressure head
Current - flow rate
Resistance - pipe diameter
Variable resistance - hose pipe being bent
Capacitor - automatic toilet cistern
Switch - valve
Diode - non-return valve


etc etc etc ...

Might not make sense to many .. but it got me through the course  ..