poor gas mileage

chrissbee77 · #1 01-30-2010, 04:30 AM

2001 grand prix gt 3.8 n/s 186xxx.. I used to get 330-375 per a full tank of gas and as of late ive been averaging around 205 I have a new CAT new o2 sensors,air filter, pcv valve, MASS, MAP, IAC, coils,EGR valve, TPS, IAT full tune up (ac delco plugs & wires) almost forgot ran 1 bottle of lucas fuel injector clearner thorugh the car. no check engine light or nothing just poor gas mileage, i dont smell anything like its running rich. and no driveability issues to speak of other than the poor gas mileage. My gauge bounces around sometimes but im pretty sure the float wouldnt cause bad mileage ANY IDEAS>>>>

BNaylor · #2 01-30-2010, 01:59 PM

What brand 02 sensors were installed? How is engine warmup and is engine operating temp normal? Do you still have the original thermostat?

With a scan tool with the capability to do so you can check short and long term fuel trims which may point to the cause(s).

chrissbee77 · #3 02-01-2010, 01:05 AM

failsafe thermostat,dealer installed oxygen sensors, ( gm-tech 2 scan tool, Dealer said everything checks out fine)..just poor poor gas mileage

3100 · #4 02-01-2010, 10:09 AM

winter grade of gas evaporates faster and has less energy during power stroke.

overlooked causes: dragging brakes, problem with suspension that is effecting wheel alignment, use of thicker engine oil, winter tires (higher rolling resistance), carbon on intake valves

tblake · #5 02-01-2010, 11:23 AM

Is the trans going into overdrive? The RPM needle should be a couple notches below 2000 while cruising at 55-60mph. And almost exactly on 2000 when going 70mph.

chrissbee77 · #6 02-01-2010, 04:11 PM

so i took it on the highway and here are the results...
with my foot on accelorator at 70 mph rmps(2600 ) with the cruise control on at 70 ( 2500-2200 rpm) it goes down to 2200 after 30 seconds or so ...

What does this mean and is there anyway to fix it.. also how much cash are we talking

3100 · #7 02-01-2010, 05:41 PM

Quote:

Originally Posted by chrissbee77

so i took it on the highway and here are the results...
with my foot on accelorator at 70 mph rmps(2600 ) with the cruise control on at 70 ( 2500-2200 rpm) it goes down to 2200 after 30 seconds or so ...

What does this mean and is there anyway to fix it.. also how much cash are we talking

sleeping clutches or tcc not locking up,

rickisrad · #8 02-01-2010, 09:40 PM

I would do an intake cleaning. you can run a can of seafoam through your intake. Murrays auto parts sell a 3 step cleaning kit for like $45-50. Like said above make sure your brakes are releasing properly, check calipers, clean and lubricate the slide pins. You may want to purchase new caliper slide pins, the rubber bushings bind up on the slides and prevent the caliper from releasing properly.

chrissbee77 · #9 02-02-2010, 03:57 AM

just changed the calipers and brakes as well as rotors. Flushed the fluid and ..it still does the same thing. How do i fix the TCC from not locking up or locking up and what does it do. How much money and can i fix this problem myself or do i have to take it to the dealer..

3100 · #10 02-03-2010, 02:27 AM

Quote:

Originally Posted by chrissbee77

just changed the calipers and brakes as well as rotors. Flushed the fluid and ..it still does the same thing. How do i fix the TCC from not locking up or locking up and what does it do. How much money and can i fix this problem myself or do i have to take it to the dealer..

"still does the same thing" you mean same fuel consumption or?

If you really want to see if your "D" is engaging

try driving on the same portion of the road at 55mph in gear 3, record rpm# and repeat the same for gear selector in "D". and if you see that in D range rpm is lower it is all good.

chrissbee77 · #11 02-03-2010, 03:35 PM

how do i drive in 3rd to see what the rpms are? do i put the shifter in 3 or something or do i switch to 3rd while im already driving like a manual or something? sorry for the confusion

3100 · #12 02-03-2010, 04:59 PM

Quote:

Originally Posted by chrissbee77

how do i drive in 3rd to see what the rpms are? do i put the shifter in 3 or something or do i switch to 3rd while im already driving like a manual or something? sorry for the confusion

get on the highway in 3rd and record rpm at 60mph once it stabilizes then shift into D and speed up to 60 and let it stabilize record and compare rpms

chrissbee77 · #13 02-04-2010, 03:59 AM

took it on the highway and in 3rd the rpms (27000 and in drive (2200 rpms) what does this mean .. also would a bad crankshaft sensor cause it to shift like it does. just a question..

BNaylor · #14 02-04-2010, 07:11 AM

At what speed? Steady cruising at 70 mph in 4th-OD and TCC locked you should see around 2000 rpms.

Bad crank position sensor normally does not cause the issue you are seeing.

chrissbee77 · #15 02-04-2010, 10:11 AM

At what speed? Steady cruising at 70 mph in 4th-OD and TCC locked you should see around 2000 rpms.

When its normally in drive at 70 mph it stays around 2500 rmps, here are the results from the road test

on the highway and in
"3rd" 60 MPH = (27000 rpms)
"D" 60 MPH = (2200 rpms)
If the torque converter isnt locking up how can i fix it?

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