03 grand am gt horrible fuel mileage

[stonemason90]

Umm what??

everyone that i know who has had a 3400 motor has told me that they noticed that around 50k miles the fuel efficency drops. they go from an average of 24-27mpg to around 15-17 mpg. ive been told its the o2 sensors but i replaced both of them on the car and it didnt improve at all.

[Skippy]

I had a lady come into my shop with a similar problem, her fuel mileage fell through the floor and then a year or so later the car would randomly start chugging real bad and then die and not re start for several hours. after tons of searching and trial and error I found a repeat problem with these cars. The crank positioning sensor begins to fail and wacks the timing out and throws the fuel curve out the window until the car gets to the point where it gets overwhelmed and dies. It wont re start until the sensor has cooled down and gets all the fuel and timing back to a normal ish area. Look into it, the sensor isnt too expensive. It will never turn on the engine light either.

[SteveCZ28]

well that is somewhat coincidental...last winter the car would take forever to start...it would literally crank for about 20-30 seconds until it would actually start up...maybe her crank sensor is acting up? i thought it was one of the 2 coolant temp sensors taking a crap, as ive had that problem before with a different car...coolant temp sensor was bad so it told the ecu it was like -150 degreees, so it would throw every last drop of gas to it and would basically flood out until it actually lit up and started

[Smittro]

everyone that i know who has had a 3400 motor has told me that they noticed that around 50k miles the fuel efficency drops. they go from an average of 24-27mpg to around 15-17 mpg. ive been told its the o2 sensors but i replaced both of them on the car and it didnt improve at all.

Well I've been fooling with and building 60*v6's for over 20 years and never heard that one.. 24mpg is close to spot one but 27??? hardly..

[SteveCZ28]

everyone that i know who has had a 3400 motor has told me that they noticed that around 50k miles the fuel efficency drops. they go from an average of 24-27mpg to around 15-17 mpg. ive been told its the o2 sensors but i replaced both of them on the car and it didnt improve at all.

Well I've been fooling with and building 60*v6's for over 20 years and never heard that one.. 24mpg is close to spot one but 27??? hardly..

yea im with you on this smittro...its not like these 3.4's have a expiration date and when exactly at 50k they just go to shit mileage wise...

[Smittro]

yea im with you on this smittro...its not like these 3.4's have a expiration date and when exactly at 50k they just go to shit mileage wise...

I looked back but can't find your mileage..? Could be a worn timming chain/tensioner happening.. The chain tensioner is "supposed" to take up the slack.. But, it gets wear grooves from the chain ridding it and it having a pretty stiff spring.. If you're by chance dropping some timming, you could experiance mpg loss.. However you will still have no trouble (aside from this and that which comes with age/mileage/duty), getting 150k out of it.. In a nut shell the motor is still most likely good and reliable, if you can live with the mileage sounds like your AFR's may be wandering a bit too..

[Smittro]

everyone that i know who has had a 3400 motor has told me that they noticed that around 50k miles the fuel efficency drops. they go from an average of 24-27mpg to around 15-17 mpg. ive been told its the o2 sensors but i replaced both of them on the car and it didnt improve at all.

yea im with you on this smittro...its not like these 3.4's have a expiration date and when exactly at 50k they just go to shit mileage wise...

Ya know the more I think about this the more I wounder if there's some truth to it.. Prolly not exactly 50k, BUT,, Gm did go to double tensioners and timming chain guides on the 3.4(truck) motors.. Ugh I hate (lol) when you guys make me pull the books.. I'll pull my books on 4th gen high features and see what we got.. You could take it to a tunner and have them see what the cam is doing.. May be something to look into if you deside to keep it a little longer.. Good luck..

[Smittro]

Ya know the more I think about this the more I wounder if there's some truth to it.. Prolly not exactly 50k, BUT,, Gm did go to double tensioners and timming chain guides on the 3.4(truck) motors.. Ugh I hate (lol) when you guys make me pull the books.. I'll pull my books on 4th gen high features and see what we got.. You could take it to a tunner and have them see what the cam is doing.. May be something to look into if you deside to keep it a little longer.. Good luck..

Nope! Prolly not your/an issue.. The tensioners were changed mid 90's..

[SteveCZ28]

the car has like 92k on it

[Smittro]

the car has like 92k on it

Maybe, try the tuner route I mentioned.. I can't see why it changed so much..