[MadMan]

Ok, Captainclock. There are all sorts of things wrong with what you are saying here. *sigh*

Originally Posted by Captainclock

OK, that's interesting because everywhere I read online said that a failed rear Oxygen sensor (which is part of the catalytic converter assembly) would cause a car's fuel economy to drop off.

A downstream oxygen sensor will not (on most vehicles) mess with fuel economy, or how the engine runs. The downstream sensor exists only to monitor the performance of the catalytic converter. The upstream oxygen sensor, however, exists to monitor the fuel/air ratio at which the engine is running, giving the input to the computer so it can adjust how much fuel it injects, and also to provide a baseline for the calculation of converter efficiency, which then also uses input from the downstream sensor.

Also, it is not 'part of' the converter assembly. It's probably just rusted stuck in the bung, and that mechanic you talked to recommended just changing the whole shebang to make things easier for everyone. I would recommend the same thing.

Originally Posted by Captainclock

Also why would it be a "general rule of thumb" that cars only get 200 miles on a tank of gas

Because it's a regulation for cars sold in the USA. Any car must be capable of going 200 miles on a tank of gas. But you do realize that maybe different cars get different gas mileage? And have different sizes of fuel tanks? That's why it's a guideline, not an immuteable law.

Originally Posted by Captainclock

because like I was saying above I had read in several online forums pertaining to car maintenance

Exactly. There's your problem. You haven't talked to a real mechanic about the subject. You are now. That one guy you talked to, well, I would recommend the same thing he did, but I would additionally advise you as I am now, about this predicament, to get you completely informed about the matter at hand before you start throwing money at it. Because, if all you say is correct, you're gonna replace that converter and sensor, and the car won't run any better, or improve the fuel economy, and you're gonna think that mechanic did something wrong, when in reality, you're just not fully informed of the situation. Read what I said again:

However, if you are absolutely certain you were getting more mpgs before the catalytic problem, the reverse of what you said is likely true. That the converter is suffering because of the mileage problem. Ask a real mechanic to do a good tune up and see that the motor runs right first.

You have a converter efficiency problem (or so I gather). But you also have, with it, a fuel economy problem. Likely, what is actually happening here (as I've been trying to explain) is that because the engine is running poorly, too rich most likely (using too much gas), the catalytic converter will not be able to keep up, thus giving you a code for converter efficiency. Which, btw, I'm only guessing that's the code you had, because you never actually said, only implied. In many scenarios, the engine computer will not really 'know' that the engine is running too rich or not firing right or what have you, it simply thinks it is making the appropriate corrections, and for some reason the catalytic converter isn't doing its job. When in reality, this might not be the case at all.

Originally Posted by Captainclock

but I'm also wondering if it isn't just because the engine was filled too full of oil and because of that some of the excess oil is being sprayed out of the engine onto the exhaust manifold which is similar to what a lawnmower does when its overfilled with oil

Facehoof.

O_o That's just... I don't even. Your car is not a lawnmower. It's also not British, a tractor, or made before 1960. It's not supposed to leak oil -or- burn it. If it's overfilled, it will usually just burn the oil out, like a lawnmower, (which will damage the converter, btw). But you say it's leaking out of the valve cover onto the exhaust manifold. Which, though not terribly common on American cars, if it's bad enough, can leak vacuum as well, and make the engine run poorly.

There are all sorts of possible causes why your mpg's might be down. Converter efficiency and/or downstream O2 sensor? Not generally among those causes. 'sall I'm saying.