97 malibu ?

[blazinzuk]

Now I haven't really checked this out much. Want some opinions of what to check.

Car used to get 30 mpg now only getting 25-26(decrease was rather sudden)

Replaced a bad 02 sensor didn't help. I have run it on a diagnostic machine(a good one at a real shop) and the only thing it showed was the 02 sensor bad.

Its a 3.1 its got 219000 miles on it runs great. No loss of power it seems just worse mileage.

Things I have thought about, the leaking intake manifold gasket, could that cause this?

plugged catalytic converter.

any other ideas would help.

My plan is to run some sea foam through it and replace the intake manifold gasket. I know it needs replaced

 

[lectric80]

With the mileage on the engine I would suspect that the cat is pretty restrictive at this point. It would have an effect like you are stating, especially when combined with the leaky intake gasket. The intake gasket could allow extra air to be pulled in, but because it is after the MAF sensor, the O2 would be telling the CPU that there is a lean condition. I would say start with the intake gasket, then look at the cat. I get about 25 out of my Lumina with a 3.1, but I know that mine is the cat and lack of tune up.

 

[badmix]

IF the cat was truely plugged, it would smell like bad eggs and run like crap. Get someone to hammer the throttle in park and smell the exhaust. Otherwise, probably jus a wore out motor, nothing more. Try putting some RESTORE or stuff like that in it. Couldnt hurt. Also check your tire pressure, this will dramatically effect gas mileage. Also if tires are wore out (they do get smaller as they wear out).

 

[JBo214]

Yes, I feel it would be a waste of money to try and chase a condition like this. That vehicle is old and fuel mileage is just going to decrease at this point. I dont think that replacing those parts is going to help your situation much. A local exhaust shop can check backpressure on the catalytic converter and tell you if it is restrictive.

$.02

 

[Jason4x4]

a tune up and some top end cleaner is never a bad idea though, and maybe a fuel filter with it.

 

[blazinzuk]

Yes, I feel it would be a waste of money to try and chase a condition like this.

this is true unfortunately I simply will not be able to afford a new car in the next 3 to 5 years. I need to keep this thing running well for at least that time period.

As far as tuneup goes I have replaced the plugs and wires.