Posted: Fri Mar 04, 2011 3:06 am Post subject: Why does my mileage suck?
Right now I am getting 17-18 mpg. Mixed driving. I recently had a tuneup, using acdelco parts. I have a 3.4l. I don't drive the car hard, if I really baby it I may get close to 20 but never more. Something has to be wrong. I have a 65mm TB, a Milzy PCM, and intake (short length for winter). I talk to people with stock 3.8s and there mileage is way better than mine. With gas prices nearing $4.00 I can't handle the horrible mileage. Any tips? Solutions? or is it just the winter temps, and gas? Thanks!!
Yes it was part of the tuneup, and it was horrible, it looked like coffee grounds and mud were coming out of it!
If you have not done so already, consider replacing the upstream oxygen sensors if your car has over 100k miles with the original sensors.
I am like 700 miles from the big 100k. I should do both of them right? Also I am guessing they are the same part. This is probably a good idea since (unfortunately for me!) I have to go in for a DEQ test in June. Hopefully I will pass. None of the Upgrades I have done should interfere with me passing I don't think.
I wouldn't worry about the rear oxygen sensor if it isn't throwing a code since it doesn't really effect the way the car runs. The upstream sensor on the other hand is worth replacing if it hasn't been done. Make sure you get an AC Delco or Denso sensor. You car won't play nicely with other brands.
Protip: If you don't know what you're doing, don't do it.
In the spring time if i dont swap out my snow tires and rims, even with warmer temperatures, the car still has the resistance with snows on, it still gets lower mileage. As soon as put on the summer tires, mileage is regained, and I dont need as much throttle for starting off.
Are you getting ethanol mix fuel?
Colder air in=needs more fuel -summer will be better.
What's the best mileage you've ever gotten?
Are you just assuming you should have better mileage?
If so then one variable will be engine size.
3.8 while being a bigger engine will probrably get better MPG if babied then a 3.4.
This is because engine is under less load pulling that heavy a$$ car,lol.
What are you using as a baseline and to calculate actual mileage?
instant mpg guages are an indicator and don't account for alot of things.
And don't use EPA ratings as a baseline.
Every car gets different mileage due to different variables like driving style,altitude,fuel condition etc..
Some suggestions for anyone would be to clean your MAF,it gets coated with dust/dirt. Should clean every oil change.
Also keep tires at correct pressure to reduce rolling resistance.
Keep fuel filter fresh,don't get walmart gas.
A proper on car injection/topend cleaning can realize not only lost MPG but also power usually.
Verify your TC is locking up at cruise also.
While cruising keep your foot on the gas and tap the brake slightly,should see a very momentary blip upwards in the tachometer as the TC disengages.
Keep the short ram intake under the hood/unshielded for better mileage.
Hot air intakes suck for power,but great for better fuel mileage.
Hope something in there helps.
Would be nice to have it on a scanner just to check a couple things out and make sure it's not an issue that hasn't flagged yet.
I wouldn't change 02 sensors until you put it on a scanner.
Those things are expensive,and while the resolution does dissipate over time on a scanner the waveform can be monitored to see if it's choppy or smooth and if the switchpoints are correct.
If they are 5-10 years old,swapping in some new ones certainly wouldn't hurt though if you got money to spend.
Last edited by DefEddie on Sat Mar 05, 2011 11:44 pm; edited 1 time in total
just curious, have you ever sea foamed your engine? If you haven't that might help a bit, i know mine runs a bit better, a mile or two per gallon more, when i sea foam it after a long time of carbon build up.
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