Chevrolet Repair: 2000 Yukon / poor gas milage, poor gas mileage, gmc yukon

Question
I have a 2000 GMC Yukon that has been getting really poor gas milage as of
late. It has about 99K on the clock and is well maintained. It is a 3rd vehicle for
us and is used mainly to tow with.  The standard MPG is down around 9-10, that
is down from 15/16 I was getting ( not while towing) No CEL's, new air filter, but
I do notice a very slight miss during idle, very very faint and not noticeable on
the tach.  Any idea of things I should be looking for?  

Answer
Hi Chris

I would start by having the truck hooked up to a code scanner.  Some places will do this for free.  I would have them check the O2 sensor.  That is the biggest culprit of bad mileage.  I would just say change it, but now adays you have more than one.  You need to determine which is bad.  They are expensive.

You don't have much to adjust on that truck.  Could also be bad plugs.  Possibly a leaky injector, which could cause the miss.  The miss is slight because the truck will retard the timing to compensate for problems.  It could be your timing is so retarded that this is causing your poor gas mileage.  

Getting your truck scanned will tell alot.  Don't pay more than $50.  Might find it free (Autozone Auto Parts).  

Hope this helps.

C J S