Re: Fuel Mixture/idling speed

[steveb@falcon.kla.com (Steven Buchholz)]

> I'm looking for help adjusting the fuel mixture and hopefully the idling
> speed.
> I have an 86 5ktqw.  
> The plug on the mixture adjustment screw is no longer there, and the screw
> goes out of adjustment by itself.  I can tell because the idling speed goes
> up;  when I check the screw,  I can see it has moved (I had it marked).  So
> I turn it clockwise (tightening it) and I bring the idling down.  I've been
> playing this game for months, and I'm getting tired of it.
> My questions are:
> 
 <<<SNIPPED>>>
> 
> My fuel consumption has gone up substantially lately and I think its
> related to my adjusting the screw blindly.
> 
... you are talking about two different screws.  There is a mixture adjustment
screw in the airbox and there is an idle speed adjustment screw in the throttle 
body.  I have never heard of a problem with the mixture adjustment screw being 
loose, but the idle speed screw does regularly ...

If your idle speed adjustment screw comes loose it is because the rubber seal 
has vulcanized.  You can buy a replacement for a fairly cheap price, but I 
have repaired the problem by making the threads of the screw a little more 
sticky.  I used anti-seize [there ... I said it :] on mine, but grease would 
probably do just as well ... and noone will give you the "you'll screw up your 
O2 sensor" line :).  The idle speed adjustment has nothing to do with setting
the mixture, and other than the fact that your car may be idling faster will
not affect fuel economy.  

The mixture adjustment screw is usually inaccessable due to a plug that is 
installed in the airbox ... but many cars have had this plug drilled out.  
The adjustment screw has a 3mm hex head, and you are not supposed to play 
with the screw while the engine is running.  If you think about it this 
adjustment is only used to provide a useful range to the computer, which 
uses the O2 sensor signal and a feedback control signal (depending upon the 
version of the FI) to fine tune the mixture for minimum emissions.  

If you are seeing your fuel economy going down I'd look at replacing the O2 
sensor first, rather than messing up the mixture adjustment.  Bentley talks 
about all of this stuff, so you might want to use your Haynes to get that 
Thanksgiving fire going and check out what Bentley has ...

HTH!
Steve Buchholz
s_buchho@kla.com
San Jose, CA (USA)