CoupeDTS Posted November 24, 2010 Report Share Posted November 24, 2010 Im not sure how much this website can help with this topic but I thought Id give it a try. Since my 96 Fleetwood has a Chevy LT1 in it I figured most of the knowledge here is about Cadillac specific engines. But Ive read topics on the impala SS forums about this same thing with this engine and 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 Quote * 1966 Deville Convertible * 2007 Escalade ESV Black on Black * 1996 Fleetwood Brougham Black on Black V4P -Gone* 1983 Coupe Deville Street/Show Lowrider -Gone* 1970 Calais 4dr Hardtop GONE* 2000 Deville DTS - Silver with Black Leather and SE grille GONE* 1999 Seville STS - Pearl Red GONE Link to comment Share on other sites More sharing options...
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.