[V8] Causes of Bad MPG?

[Buchholz, Steven]

What is your goal?  What was the other car getting and what is the new one getting?  Are you comparing actual fuel economy, or what the trip computer is telling you?  Could something like different rolling circumferences of tires be fooling you?

What are the other differences between the cars ... are you running different tires/wheels?  Have you done a compression check on the engine to see if it isn't getting a bit tired?  Could you use a new set of spark plugs, dizzy cap/rotors & wires?  Could viscosities of the fluids be different?  A few little differences can add up to 5MPG without that much apparently different.  

By any chance have you checked ECU codes?  (I actually assume that you have, and that's why that wasn't my first question)  Have you replaced the fuel pressure regulator?  Pull the spark plugs to see if there are any cylinders that do not appear to be running optimally.

I have two very similar cars, and one does get better gas mileage than the other ... and I'd say it is on the order of 5MPG too.  The guy who is working on #2 tells me that he thinks it is only running on 7 cylinders ... and that's the one getting the lower mileage.  He may actually be right, that is the "new" engine I installed a couple years ago ... and it has always had a bit of a "lope" to the idle.  

Let us know what you find ...

Steve B
San José, CA (USA)
> 
> My new v8q is getting really bad MPG - I replaced the o2 and that worked
> on my last v8 - didn't here.  I'm thinking bad wire to the ECU? Bad
> ground? Runs like a champ - just about 5 MPG less than my former v8q?
> Same years so the running gear is the same.