: 96 Fleetwood Bad Gas Mileage, Any Ideas?

11-24-10, 10:24 AM
Ive read topics on the impala SS forums about this same thing with this engine and bad gas mileage but I need more answers so here goes...

Like said its a 96 Fleetwood LT1 350 115k miles. With that mileage you automatically suspect maintenance things may or may not have been done at 100k. I have the towing package so the specs on gas mileage for this particular car are 17 city 25 hwy. Granted Ive always got about 2mpg below specs on all my cadillacs but this particular car its very low.

When I first got it a year ago I was getting 10.5 mpg in the city. I installed a cold air intake and an air foil for better breathing. That bumped it up 10% to about 11.5 mpg.

So next step was more tune-up stuff. I changed the plugs to the original platinum delcos. I checked every single vacuum line and replaced a couple of them. Changed the PCV valve. New fuel filter. Cleaned MAF. New wheel bearings and brakes just in case there was drag. Sea foam in the gas and through the intake. Throttle body cleaned. "Engine restorer" mystery oil to fix any compression issues. New fuel pump (old one broke). Plug wires are a hassle to do, I have some but still havent changed them. Well after all this I was up to about 12 mpg.

Also on the highway during these times I get horrible mileage. Not sure exact numbers but its between 15 and 20 I know. And there is no missing of any cylinders.

So after reading some forums I found that o2's can make it run rich if they are bad. Since the car has 115k miles Im not sure if they were changed at 100k so I changed them with the proper ones. I just did the upstream ones before the cat. Hoping this solved my problem I now am getting 13 mpg city and still below 20 highway.

So with some more reading on the forums its found that the manifold gaskets and connections leak on these cars. When removing them the bolts almost always break as well. Fun. But anyway if they are sucking in clean air then it throws off the o2's. This makes sense. And I did notice when I did the seafoam treatment I had smoke rolling out of the back of my manifold on the pass side down where it connects to the pipe for the cat I think. So I checked all the bolts on the manifold at the engine and at the other end of the pipe. The back bolts on the manifolds were not even finger tight. I tightened those up good but Im not sure if the gasket is still good in there? Where the manifold goes to the cat pipe those bolts were solid. AND there are no hissing or ticks coming from the exhaust that would sound like a leak.

All that said Im still not getting my potential gas mileage. Id be happy with 15 in the city. Im not sure if its worth changing manifold gaskets and replacing cats. Cats are expensive. All that stuff is super rusty though Im not sure how well it works on the inside. The o2's behind the cat supposedly just tell the computer if the cats are working or not, i dont think they control lean or rich settings.

Reading a troubleshooting guide on autozones website for this car it says the throttle position sensor can be bad and make it dump more gas in then it needs. I dont have throttle response problems though..

Also NO CODES and I dont have a scanner to read o2 readings

Does anyone else have any bright ideas? Im open to anything at this point.

Should I replace the downstream o2's? Thats the next cheapest route.

Thanks for your time

11-24-10, 11:50 AM
I can get as high as 24 MPG or as low as 12 MPG , it's mostly about how you drive the car and mine is giving out a PCM 0174 code .I had my PCM tuned when I got it and I can say I have been getting better MPG at the same driving conditions since but my main concern was to increase power peak rather than mileage .
My mileage is around 35K miles on the odometer

02-06-11, 06:35 PM
did you ever get your mileage any better? What did you do?

02-06-11, 11:02 PM
I thought tow pkg rear gears
put it at 20 hwy?