High fuel consumption, 12.1 mpg on vehicle computer in city, hesitation on acceleration.


Asked by Oct 23, 2016 at 12:33 AM about the 2002 Pontiac Grand Prix GT

Question type: Maintenance & Repair

No service of check engine lights. 130,000 miles, recent spark plugs and
cables changed, both manifold and valve cover gaskets replaced, new
transmission, changed fuel filter about 5000 miles ago. However, I did put 4
bottles of fuel injector cleaner over the course of about 6 fill ups. I have not
had throttle body serviced, or mass air flow sensor checked.? I am
concerned if all the fuel injector cleaner may have damaged fuel filter, or
loosened tank deposits, that may have restricted fuel filter. I believe air filter
was checked not too long ago. Also, I began noticing, than if I take off from
a cold start, sometimes it tends to hesitate a little, but only until it warms up.  
I keep my car maintained, and for the 5 years I have owned it, never has it
let me down, or stranded. A little help please.???

8 Answers


Changing the fuel filter is no bid deal so you may want to go ahead with that but it would not cause a huge drop in gas mileage. Check for fuel leaks. Do an actual mileage test by filling the car up, writing down the mileage and then again at the next fill up. Calculate the mileage after a couple of hundred miles to make sure the computer is not off for some reason.

1 of 1 people found this helpful.

Thanks for the response, but I failed to mention that fuel consumption has been a problem for a few years now, and nothing I have done seems to help. I will check for fuel leaks, but what about the hesitation, this is something new, and it has never hesitated. That is why i was concerned about mass air flow sensor being dirty, or the throttle body. ???


Maybe your fuel injectors are leaking or the mass air sensor is off. I would have a mechanic check to see if your emissions show that the car is running really rich. What did the spark plugs look like when you changed them? Sooty?

1 of 1 people found this helpful.

Sounds like you've lost overdrive and torque converter clutch (TCC) lock in. Very common with these cars. At cruising speed (55 or so MPH) your tach should be around 1500 RPMs. My '00 GT, with 155,000 miles on it has lost overdrive and TCC lock in. At 55-60 MPH the tach is at 2700 to 2900 RPMs. This kills your gas mileage. To be sure of your mileage figure it out yourself. Don't rely on the car's computer. They're not always 100% accurate. HTH. -Jim

1 of 1 people found this helpful.

I just had transmission rebuilt, because I had lost overdrive, was doing 3000 rpms at 60 mph. Now, 15-1600 rpms. TTC was defective until 2003 when manf. corrected. The spark plugs still looked good, like they did not need to be changed. It has always passed smog on first test. I am going to clean the mass air flow sensor, that I have not done. Thanks.

1 of 1 people found this helpful.

You're welcome. Glad to help. -Jim


What trouble codes came up when you had it scanned?


Have not had it scanned. Drove a 150 miles yesterday on freeway, hesitation on acceleration, almost seems like it is starving for air or fuel. Okay once I get to the desired speed. I averaged about 20 mpg. Thanks.

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