I know the gearheads has to be sick tired of them by now.

Initially, I thought it was due to the way I drove. But after many tankfuls, I concluded it is not. THe most recent tank, I got 17 mpg and that'* with about 70% highway (at 5 mph over max) and the rest in city. That was with grandma-driving style. For some reason I have not been able to get over 23 mpg combined.

I had a new O2 (Delco from INTENSE) not too long ago and the plugs/wire are only 25,000 miles old.

I'm ready to blow it up.

Remember your evap issue thread. I'm thinking it has something to do with the bad mileage.

http://www.bonnevilleclub.com/forum/...ic.php?t=86523

I think so but I wasn't sure if that would majorly affect the bad mileage.

That'* why I made a new thread.

I'm not 100% sure it is..it'* something malfunctioning on the car and it is part of the fuel system.

Would vaccum leaks play a part in bad mpg?

What brand and heat range of plug are you running. It has a lot to do with the mileage.

I honestly have no idea, they were just stock replacements, I think they're Champions but that was way before these happened.

Assuming you have already done the obvious tune up items, filters, pcv, etc.
Pull them and see how they are burning. If they are dark colored the plug is probably too cold of a heat range.

I pulled a plug, turns out it'* a AC Delco unit and the spark plug looks white turning to tan...so it'* a-okay

My mileage is close to yours. I've driven other 3800 powered cars that get great gas mileage, but my Bonnie has always been under 20 in town, low 20'* on the highway. The car runs fine, Transmission was rebuilt about 20k miles ago, O2 sensors have been replaced,plugs,wires,etc. I've had the car(97 NA 3800) for about 75k miles and it'* always been about the same. I'm stumped. I'll check my fuel evap system, but I'd be suprised if it made more than 1 or 2 mpg difference. Maybe weak coils, or partially clogged cat? If you fix yours, I'd love to hear what you find.