Bad mpg

[toddomen]

I have a 1997 grand prix with 136.mile with 3800.I am getting 14 mpg
in town and i have put new plug wire fuel filter and new fuel pump 800 miles a go No CEL come on a po420 has came and put new o2 sensor in some time it crank 5-6 time before it start beside that it run good i have read some post and checked all vac line and have good fuel pressure
thank you

[jd-autotech]

cat. converter i m sure of it

[Shpuker]

could very well be

Air filter? Dirty oil? Lotsa stuffs

[GTP Version 2.0]

This might not apply to you, but are you leaking coolant? I had the same problem when my LIM gaskets went bad. Oddly enough I was at the same mileage with my 97 GTP. It fouled my O2 sensor and killed my gas mileage.

[tblake]

Check the vacuum hose on the fuel pressure regulator for gas during fuel pump prime.

[jakegday]

p0420 pretty much means your catalytic converter is inefficient, or not there. and an inefficient cat can certainly cause poor MPGs and poor starting.... so my vote is on the cat. instead of replacing the sensor, you should have replaced what the sensor was sensing

[tblake]

I had a P0420 code tripping on my car when my cat was brand new.

Does he have any MOD's? Maybe a high flow cat, or downpipe, or headers?

A P0420 could indicate an issue with the cat, but I'm not convinced it would cause poor MPG unless the cat was plugged. WHich if the cat were plugged would probably trip DTC for O2 slow switching.

[jakegday]

what ended up being the cause of your p0420?

[tblake]

The high flow cat and downpipe I had installed.

I just removed all the Post O2 codes from my pcm, so I could unplug my read O2 and it would not trip a code.

If you have the stock cat and you are getting a P0420, its probably best if you install a new cat.

[jakegday]

so pretty much what i said before? an inefficient cat... aka a cat that is not protecting the environment as much as the computer would like it to

[toddomen]

cat. converter checked out ok

[tblake]

Check your tire pressures. What is the RPM's when you are cruising on the highway going 60mph?

[GTP Version 2.0]

Somethings you need to consider are tire pressure, air filter, type of gas (if GTP then you MUST be using 91 octane), alignment, compression, etc. There are so many things we need more feedback.

Run some Seafoam in you gas tank and crankcase and throttle body and drive like a bat outta hell. It helped my GTP greatly. With gas, you get what you pay for. Discount gas from supermarkets and Costco, or Quicky marts is discounted because it will usually expire in a week...yes gas does expire. Thus the octane rating won't be as high which means it will burn unevenly (the higher the octane number the more side branches the hydrocarbon contains thus better burning). Shell and chevron are the best.

Also the 97 grand prix air box is pretty constrictive. Put in an aftermarket filter or intake system. Also you might want to try synthetic oil. DO NOT add oil additives as they are all BS and interrupt the chemistry of the oil itself and often leads to heavier oxidation. Also does the car exhaust have a sweet odor? Is the car running hotter? But please once again, is it leaking coolant?

[toddomen]

It is a GT I have new tire and had a alignment done I use shell or BP gas I did the seaform in all three. When you say a seet odor and no coolant that I can not see How about a bad fuel injector or a Intake Manifold Gasket

[GTP Version 2.0]

If it was a bad injector you would have a code. You might not be able to see it yet....but an intake manifold gasket that is failing will leak coolant into the lower intake manifold and make your gas mileage go to crap. It is one of the most common failures/recalls with the 3800 platform. Whenever you introduce a noncombustible liquid into the piston chamber you will either hydrolock or risk engine failure or misfiring with bad gas mileage.