Re: 89 XJ-S Marelli ECM vacuum line

[mkenrick@golder.com (Michael Kenrick)]

> Does anyone know where the vacuum line to the aneroid capsule in the 
Marelli ECM goes inside the engine bay?  Mine is just hanging 
unconnected.  The Lucas ECM in the trunk goes to the balance pipe but I 
can't see where the line from the Marelli is supposed to connect to. There 
is a two inch bit of vacuum line connected to one of the vacuum ports on 
the rear of the right manifold with a bolt plugging it. This hose is much 
larger than the hose from the marelli. Why would anyone plug it, wouldn't 
the car not run without the ignition ECM getting a manifold depression 
reading?
     
     You'll probably get more learned answers than this, but as to your 
     last point, the vacuum signal appears to trigger the over-run fuel 
     cut-off facitlty on the XJ-S -- a measure introduced in the HE (?) to 
     improve fuel economy by cutting off the fuel completely during 
     deceleration above about 1200 rpm, thus signifcantly improving engine 
     breaking.  
     
     My 86 XJ-S has now almost completely ceased to follow the required 
     cut-off sequence, suggesting a problem in this vacuum line.  However, 
     the performance of the car is unaffected, except that a dribble of 
     fuel comes through the injectors during over-run deceleration. 
     
     The strange thing is, mpg took a brief dive from 12.5 to around 12, 
     but has since bumped back up again.
     
     Michael Kenrick

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