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Old 8th September 2019, 10:44   #1
Yorkshire GOC
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Default P300 misfire under load

Car is juddering at 2500 rpm - when accelerating - Reader shows p300 random cylinder misfire - i know this can be due to many reasons - first one being failed ignition coil/HT leads / spark plugs/gapping etc. Anyway whilst having a look at these things i had a look at the battery which i have always had low resting voltage readings 12.2-3 (under RAC warranty and they insist its OK) and both terminals are covered in quite extensive whitish/green deposits - never seen this before on any of my batteries. So could a defective battery produce misfires?
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Old 8th September 2019, 10:50   #2
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https://the75andztclub.co.uk/forum/a...1&d=1567936154

Picture of terminals
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File Type: jpg BATTERY TERMINALS.jpg (62.5 KB, 23 views)
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Old 8th September 2019, 12:31   #3
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Got that at the moment on our 1.8T ...

we got P303 which is P300 on Cyl 3.

And also P313 misfire ref Low Fuel ??

Going to give the plug/ Coils a good looking at ?


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Old 8th September 2019, 13:22   #4
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Had AA Man to look at battery - which i have never been happy with - tested again and all fine reading 12.4 - which i always thought was the minimum resting voltage? - but he also ran other tests and the battery is OK .

However he tested the battery with the engine on - and as he turned the ignition you could here the distinct crackle/snap of a spark - he immediately said my HT Leads are discharging to earth - and that's causing the misfire -so pleased i called them out albeit for a different reason - will get new leads and packs ASAP . One simple diagnostic to remember for the future.
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Old 8th September 2019, 21:41   #5
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Had peep today , found plug 3 a bit fouled up .and cyl 2 plug hole filled with oil
All plugs had massive gaps
All cleaned and re gapped to 0.7 , resealed cam cover , all running on 4 now, will see how it gets on next week.

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Old 8th September 2019, 22:56   #6
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Quote:
Originally Posted by Yorkshire GOC View Post
Had AA Man to look at battery - which i have never been happy with - tested again and all fine reading 12.4 - which i always thought was the minimum resting voltage? - but he also ran other tests and the battery is OK .

However he tested the battery with the engine on - and as he turned the ignition you could here the distinct crackle/snap of a spark - he immediately said my HT Leads are discharging to earth - and that's causing the misfire -so pleased i called them out albeit for a different reason - will get new leads and packs ASAP . One simple diagnostic to remember for the future.
The discolouration at the battery terminals means there is a slight acid leak coming up around the pillars themselves.

This, however, does not mean the battery is defective in action, just that there is an acid leak.

Spark plug leads should be well away from any metallic parts of the engine. Even new leads if looked at in complete darkness can show faint arcs to earth.

The spark will ALWAYS take the easiest route. If the leads are starting to age the spark will jump through the insulation. However if the lead is well away from any earthing point the spark will go to where it's supposed to go.--
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Old 9th September 2019, 09:19   #7
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There was a factory note about the plug lead contacts becoming corroded and sometimes coming off the plug. A new set of front leads transformed my 99 V6 and the old ones were coming apart where they fit to the plugs. No doubt the 4 cylinder versions would be of the same type/manufacturer?
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