poor gas mileage

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>>>>

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).

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

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

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.

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

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,

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.

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..

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.

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

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

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..

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.

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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Yeah, that doesn't look good. I hate to be the bearer of bad news. Troy may know more, but looks like you may have stripped the splines off the 4th/od clutch shaft in the trans causing your trans to not go into 4th gear. Only way to repair it is to rebuild or replace.

http://i231.photobucket.com/albums/ee98/agrazela/4thClutchHub.jpg

how much are we talking to replace or rebuild and is aamco a good place to go??

Yeah, that doesn't look good. I hate to be the bearer of bad news. Troy may know more, but looks like you may have stripped the splines off the 4th/od clutch shaft in the trans causing your trans to not go into 4th gear. Only way to repair it is to rebuild or replace.

http://i231.photobucket.com/albums/ee98/agrazela/4thClutchHub.jpg


I think according to his readings that all is good. It is normal for rpms to be higher in 3rd from 4th.
Check your wheel alignment, most people overlook this when it comes to poor mpg.

how much are we talking to replace or rebuild and is aamco a good place to go??



Your trans is OK. it is going into overdrive, you can try replacing atf and filter.

from poor mpg complain you almost ended up replacing your trans, think logically, imagine bike with 5 speeds - in 3rd you have to pedal faster to go lets say 20mph and in 4th you will pedal much less to go same speed thus differences in rpm. Trust me trans is OK. that little difference from 2000 could be from normal clutch pack wear, bad wheel alignment, wind resistance, wider tires, old trans oil.

The thing with the 4t65e is that if the 4th clutch strips, the trans will shift 1st, 2nd, 3rd, not shift into 4th, stay in 3rd, then the torque converter will lock causing the RPM's to drop. The driver feels the RPM's drop and thinks the trans is shifting fine, even though its in 3rd overdrive and not 4th overdrive.

To the OP, I would take it on a trip, drive around town a little to get the motor and trans all warmed up, then take it on a ride on an interstate. Set cruise at 70mph and let us know what the RPM's are. RPM's at 55-60mph should be between 1500-2000, and at 70mph, should be almost exaclty on 2000rpm.

I don't even thing the stripped 4th clutch shaft will trip a DTC, but it might. Either way, you could take it to a trans shop and see what they think. Start in drive, go, you should feel the shift from 1-2, 2-3, 3-4, then you may not feel the overdrive enguage. So 3 nice firm shifts and then overdrive and RPMs should drop.

3100, if clutches wear, the trans will slip, not miss a gear. Either way, missing 4th or clutches slipping that bad would be time for a rebuild. Plus my car has needed an alignment since summer and I get 250-300 miles per tank even in winter stop and go and longer warm up times.

The symptoms vary with a stripped 4th clutch shaft. It may go into 4th - OD or maybe not. One of the prevalent symptoms that I have seen is it will shift to 4th - OD but then drops immediately to what feels like neutral and then the tach jumps up indicating over revving if you still have your foot on the gas.

RPM's at 55-60mph should be between 1500-2000, and at 70mph, should be almost exaclty on 2000rpm.

:iagree:

2200 @ 60 is too high I think you may have stripped the 4th clutch shaft. Now if you want to know for sure put the car in 4th D or D with a circle around it go 60 MPH again and pull it down into 3rd and see if the RPM changes. If you have access to a scanner you can check for codes if the TCC is not working correctly you should get a P1870

tblake:,

I took the car out again on the highway and with the cruise control set at 70 mph the rmps' were at 2200 rpm.

your trans is good :iceslolan With 186K on the clock 2200rpm@70mph = good.

your trans is good :iceslolan With 186K on the clock 2200rpm@70mph = good.

LOL

Since you put it that way! :tongue:

LOL

Since you put it that way! :tongue:

:icon16: