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Miss fire with more than 1/4 tank fuel

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Picked up a 2003 cayenne turbo the other day. Test drove with no unexpected issues, obviously with very little fuel. Bought the car and went to fill it up, as soon as I did it started misfiring. 


With just under half half a tank it was significantly better, only misfiring every now and then. Below a quarter not a problem.


i have an idea of what could be wrong but want to get opinions before I continue solving the issue.


no warning lights


Any help appreciated

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If you have a misfire then you have at least pending fault codes. Best to have a look using Durametric or another Porsche specific scanner.

Early Cayenne's did have problems with ignition coils (several design changes since new) and fuel pump failures.

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Ignition coils recently changed. Pumps changed 3 years ago and apparently tested recently. 

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Sorry for the ignorance, will a normal ob2 scanner pick up faults? Or does it need to be a Porsche specific scanner?


Much more use to older cars without scanners!!!


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If you do a search on here for 'stutter and hesitation if fuel level above 1/4 tank!' that   documents an epic! 


LewisWeller's March 27 2017 post (page 6) may be worthwhile looking at.


However, fault codes at the outset should prove invaluable.

Edited by wizard

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Plugged it in to new diagnosis machine. Stored codes confirm my suspicion it is the breather valve. New part ordered. 

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