Bad mpg

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

cat. converter i m sure of it

could very well be

Air filter? Dirty oil? Lotsa stuffs

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.

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

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

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.

what ended up being the cause of your p0420?

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.

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

cat. converter checked out ok

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

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?

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

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.

The Intake Manifold Plenum and gasket was replace the Intake Manifold Gasket was not replaced

Why were they replaced?

I blew it I when I spray starting fluid to get it started when the fuel pump was out of it I spray two much it backfired one spray good one big spray BAD

I need help 12-14 mpg

Todd, do you have access to a scanner that can watch live data streams? I would take a look at the long term fuel trim numbers, what are they?

Also, if you havn't yet, you may want to check fuel pressure to make sure it is within spec, and not bleeding off too quickly as if a fuel injector or fuel pressure regulator were leaking.

No way to watch live data all fuel pressure are good could my trouble that i driver to work 2-3 miles to work

yes, turning your engine on and off every 2-3 miles will certainly affect your mileage in a negative way.

My daughters 97 GTP was getting 17 to 19 mpg on the highway, did all the tune ups, (plugs, plug wires, air filter and cleaned the MAF sensor) still didnt help.

I bought a reprogrammed PCM (stock for GTP with all current pontiac updates) on ebay, day after I put it in, took the car for a 200 mile road trip and was getting 28 to 29 mpg by the time I got home.

Made me happy. (I buy her gas)

Is there something i can do for my car because of my 2-3 mile drive to work beside walking to help with mpg thank you

Is there something i can do for my car because of my 2-3 mile drive to work beside walking to help with mpg thank you

bike

Todd, are you basing your bad MPG off the DIC alone? Or are you calcing it out the old way at the pump?

You would be suprized how in-accurate the DIC really is.

I do it at the pump the old way. I do have a bike i ride to work

No drop in miles. It ben like sent i got the car 8 mouth a go. it runs grate and ldi good and fast beside that some times it crank 5-6 times before it starts. Iam happy with the car and if that the best mpg i can get ok will be happy how about some good mods thank you all for your in put

Went on a good road trip today 120 miles round trip on the highway use less then a quarter tank of gas is that go?

120 miles per 1/4 of a tank sounds pretty darn good to me.