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20SilverGT00
03-05-2007, 07:30 AM
My '00 GT's mileage has steadily fallen off for the last 6 months. I was getting roughly 22-24 mpg around town for forever, but then it started falling. I didn't think my driving habits had changed in particular, but to prove my point to myself, I drove very conservatively for the last tank and got 16.5 mpg. What happened? I replaced plugs and wires around 30k ago. I really don't think it can need that again. I've run Techron injector and system cleaner, every 3000 miles since new. The only thing I can think is maybe something wrong with the MAF. The only mod it has is a Dr Speed Stage 3 CAI. I would appreciate any advice as far as where to look.

bballr4567
03-05-2007, 07:32 AM
When was the last time the fuel filter was changed??? I changed mine when I started to get some bad MPG on tanks and I got back the mileage I lost.

Also run a few bottle of fuel system and injector cleaner through the system. You prolly have over 100k miles and it might help too.

20SilverGT00
03-05-2007, 07:35 AM
Yeah, I have 130k on it. I have run techron injector and system cleaner on a regular basis, however, the fuel filter hasn't been changed in quite a while. I'll definitely do that. Thanks for the advice!

bballr4567
03-05-2007, 07:45 AM
If you tackle this the easiest way to change it is to PUSH the green clip all the way up on the side that is closest to the rear of the car. I would of taken pics of it but I was covered in gas so I just wanted to finish. Forget about the correct technique. Just be careful not to break the clip.

20SilverGT00
03-05-2007, 08:38 AM
My car is a 2000 and I believe it has a different clip on the filter, does anyone have any advice on removing the clip?

gectek
03-05-2007, 02:54 PM
they are all the same, and what you do is take a really good look at it, one piece of the tab will show a part between it, so then take a knife or screwdriver and put it in there and pry the tab out, then push it backward and that will release the fit it has on the filter

slickjoe
03-11-2007, 12:37 PM
I get bad gas mileage, but that's because I drive fast lol.

Panacea
03-11-2007, 08:53 PM
You have noticed that it's winter out, right? I avereaged 17 to 19 mpg (more towards 17mpg) in the winter time in my 99 GT. Wait till summer and then see what your mileage is.

cardude007617
03-11-2007, 08:55 PM
ya winter does affect the mileage u get, id do that too^^

gectek
03-11-2007, 08:56 PM
dont forget rain also

aaronandchris
03-14-2007, 07:54 PM
If you have replaced the fuel filter and still get bad gas mileage you may want to look at the fuel pressure regulator. My gas mileage started to get worse and worse and I had almost perfect car maintenance. I replaced the fuel pressure regulator and my gas mileage improved dramatically.

20SilverGT00
03-15-2007, 02:07 PM
What should the fuel pressure be when the engine is idling?

Mirage85
03-15-2007, 02:09 PM
I havent seen this mentioned, but hows the air filter? Perhaps try putting the stock filter back on and see if your mileage goes back up?

20SilverGT00
03-15-2007, 02:39 PM
I have put my stock intake and filter back on, mileage is still poor. I also cleaned the MAF and the throttle body.

Sprucegagt
03-15-2007, 07:01 PM
What should the fuel pressure be when the engine is idling?

At idle with the vaccum line off the fuel pressure regulator you should have 51 - 55 PSI of fuel pressure

WeibSunder
03-15-2007, 07:24 PM
This might sound totally retarded, but my dad was having the same problem with his little S-10 2.2 4 cylinder, and someone told him to change his thermostat and he did and POOF, his MPG went from 12 mpg back to the 25 that he use to get. You could always try that, worth a shot.

gectek
03-15-2007, 07:46 PM
well rule of thumb is that if you are running a colder t stat then you will lose a little MPG, due to it richening up the regualr mixture

drumking1721
03-24-2007, 08:30 AM
yea so i changed my fuel filter yesterday....i was 93 octane or higher...and yea when took off the old one and dumped out the gas...it was coming out black...so i think i found my bad MPG problem haha

Mike Jung
03-24-2007, 10:06 AM
yea so i changed my fuel filter yesterday....i was 93 octane or higher...and yea when took off the old one and dumped out the gas...it was coming out black...so i think i found my bad MPG problem haha
^
You are lucky you didn't have mis-fires, from lack of fuel flow lol

Mike Jung
03-24-2007, 10:11 AM
This might sound totally retarded, but my dad was having the same problem with his little S-10 2.2 4 cylinder, and someone told him to change his thermostat and he did and POOF, his MPG went from 12 mpg back to the 25 that he use to get. You could always try that, worth a shot.
^
Might of been a stuck open/failing thermostat.

If the engine does not get up to normal operating temps.
& runs cold.
It will run richer until it does.

With a stuck open thermostat:
The engine coolant goes to the rad to early (by not being closed) & cools the engine coolant down before it gets to the normal operating temps.
So it would take forever to get warmed-up to the normal operating temps.

drumking1721
03-24-2007, 12:35 PM
^
You are lucky you didn't have mis-fires, from lack of fuel flow lol

yea no ****.....i took it off and the end where it comes out,or goes to the engine...it came out on my arm and the gas was black and iw as like wtf........

B0000rt
03-24-2007, 05:37 PM
^
Might of been a stuck open/failing thermostat.

If the engine does not get up to normal operating temps.
& runs cold.
It will run richer until it does.

With a stuck open thermostat:
The engine coolant goes to the rad to early (by not being closed) & cools the engine coolant down before it gets to the normal operating temps.
So it would take forever to get warmed-up to the normal operating temps.

Not necessarily, 12 to 25mpg is a HUGE jump.

Our CTS right now has a thermostat stuck open, it's throwing the P0128 (waiting for the weather to warm up to take it out, as you gotta remove the intake manifold to get at the t-stat on the 3.2L DOHC) Anyways, we're been getting the usual mileage since we bought the car, ~10L/100km and less.


As for restoring gas mileage it could be a billion and one things,
Fuel Filter like someone said,
Air Filter like someone said,
Plugs,
Plug wires,
FPR like someone said,
PCV,
O2.. etc

Just go for the cheap fixes for now, like air filters, PCV and see if it improves. I just did my Fuel filter & Plugs & wires today, hopefully I'll get back to the 500km a tank I used to get..

pauljs75
03-27-2007, 06:41 PM
What's funny is considering the problems poor fuel flow can cause, that nobody has figured out a way to monitor fuel pressure as part of a diagnostic system. With all the stuff going to a computer, you think they'd have that by now. *shrug*

drumking1721
03-27-2007, 08:04 PM
I need a fuel pressure gauge,i put that adjustable one in it and the gauge it came with broke on the fuel rail...so yea idk what the pressuure is right now

gectek
04-08-2007, 09:57 AM
What's funny is considering the problems poor fuel flow can cause, that nobody has figured out a way to monitor fuel pressure as part of a diagnostic system. With all the stuff going to a computer, you think they'd have that by now. *shrug*

it isnt that no one has done it, its just that gm didnt do it then. ford has a sensor for FP on their fuel rails, and the new PWM fuel pumps will use that as well. also if he had a t stat problem it would show up as a SES light with code P0128...did u find the cause for the bad MPG yet...and drumking did the new filter help you out any

tenspeed
04-09-2007, 12:07 AM
Yeah, I have 130k on it. I have run techron injector and system cleaner on a regular basis, however, the fuel filter hasn't been changed in quite a while. I'll definitely do that. Thanks for the advice!

With 130K on it, your top O2 sensor is bad. That makes the car run on a open loop like when the engine is cold.