Poor fuel mileage

So what is the question? It would appear that you have a catalytic converter issue going on. Replacement is the proper thing to do; removal is illegal in most jurisdictions. Besides, unless you want a constant Check Engine light lit, along with the possibility of very poor fuel economy, replacing the cat is the best option. Without it, the O2 sensors will not operate correctly and the PCM will default to a preset fuel usage that will decrease your fuel mileage by 20 to 35 percent.

run the fault codes and check for problem heated oxygen sensors(HO2S) and blocked cat converters

Check the timing chain. Some times the chain will shift with worn teeth and get out of synch. This will cause the engine to lose power and back fire. Sometimes with a flame through the carb. I did that one once. Easy fix if it is the problem.

Cauases or fault codes are not cut and dry, but here are the possible causes of codes P0316 and P0306 which are related codes.
P0316 - Engine Misfire Detected on Startup
Symptoms - Engine Light ON (or Service Engine Soon Warning Light)
- Engine fails to start
- Engine runs rough Possible causes - Crankshaft Position (CKP) sensor harness is open or shorted
- Crankshaft Position (CKP) sensor circuit poor electrical connection
- Faulty Crankshaft Position (CKP)
- Ignition System
- Fuel Injectors
- Running out of fuel
- Fuel Quality
- Faulty Engine Control Module (ECM) P0306 - Cylinder 6 Misfire Detected Symptoms - Engine Light ON (or Service Engine Soon Warning Light)
- Lack/loss of power
- Hard start
- Engine hesitation
Possible causes - Faulty spark plug 6
- Faulty ignition coil 6
- Ignition coil 6 harness is open or shorted
- Ignition coil 6 poor electrical connection
- Clogged or faulty fuel injector 6
- Injector 6 harness is open or shorted
- Injector 6 poor electrical connection
- Improper spark plug 6
- Insufficient cylinder 6 compression
- Incorrect fuel pressure
- Intake air leak

The specification is 44-54 Psi. The manufacturer lists the specification with the engine idling.

Also, if fuel pressure is the cause of your problem, there will also be other indicators....

First, it will most likely set "lean oxygen sensor" codes in your computer. Also, your fuel trims will be skewed - I would recommend taking a look at that as well. Skewed fuel trims, poor fuel mileage and hesitation, loss of power, can also be caused by a dirty or defective mass air flow sensor.
Another possible cause of poor fuel mileage and poor acceleration is a restricted exhaust.(Bent or crushed tail pipe or a clogged catalytic converter.)

WEAK FUEL PUMP OR STOP UP FUEL FILTER.RESTRICTED EXHAUST LIKE STOPPED UP MUFFLER OR CATALYTIC CONVERTER.LOW ENGINE COMPRESSION.VACUUM LEAKS IN THE INTAKE MANIFOLD.BAD PLUGS.WEAK SPARK OR IGNITION .

Symptoms are rough running, stalling, poor gas mileage, and especially a strong smell of rotten eggs (sulfuric acid). The main cause is old age, but poor/cheap/contaminated fuel can also be a contributing factor.

Bill B

If you connect a vacuum gauge to the intake you should see a gradual drop in vacuum while running if the cat is obstructed. sometimes you can hear a hissing noise from inside the cat as the exhaust tries to get through it as well.
Worn injectors, (they don't have to be leaking) a sensor or two that are out of range but not far enough to trigger a code and poor fuel quality (mandated alcohol in fuel) will also cause a drop in mileage. Rings and cylinder walls may have some wear as well... small things, when combined with others, will contribute to mileage problems. As a car gets older, there usually no single "magic bullet" that will fix that completely though.
Other than the poor mileage, does it run well?

Sounds like you might have an oxygen sensor problem. Your catalytic converter(s) maybe defective or dirty. That in turn can dirty up the oxygen sensor. A defective oxygen sensor will send faulty info to the computer, interpreting the fumes as cold (even when the engine warms up). That causes the computer to adjust the fuel mixture to very rich, giving you poor gas mileage and the fuel smell. Hope it helps.

There is one before each catalytic converter (usually up right near the exhaust manifolds), and one after each cat (or if you have a Y-pipe exhaust that brings the two sides together to one pipe and one converter, then there's one after that single converter). Why do you want to change them? Have you gotten failure codes or are you having poor gas mileage? Or have they just been in there a long time? They're usually good for about 60k miles, although of course they can fail early. Usually the symptoms are poor fuel economy, running rich (seen by black smoke out the tailpipes and some rough idling, as well as blackened spark plugs), or fault codes dealing with system-lean or system-rich descriptions. Those symptoms are indicative of bad primary O2 sensors (the ones closest to the engine). If your secondary sensor(s) are failing (the post-cat sensor or sensors) you'll trip a check-engine light warning you of poor catalytic converter efficiency.