help: sudden mpg drop

[BallerMaro13]

about the last two weeks i've been getting awful mileage in the 'maro and can't pinpoint the problem yet. normally, i'd have no problem seeing 30mpg unless i'm on the backroads getting on it hard, and the last two weeks i'm lucky if i'm seeing 15mpg. i just changed the oil, filters, cleaned and recharged my k&n, and the usual maintenance, so this is baffling me. she's throwing no codes, plugs and wires are new and not toasted, runs strong, and isn't giving me any other problems except for a blown header gasket which i've had for a while and never really did much for my mileage.

i'm thinking maybe a faulty 02 sensor, but i'm getting no codes, and just replaced one before the winter. all filters and fluids are new and maintained, but i am noticing a bit of oil dripping from the cutout. any ideas on what could be the culprit? help and suggestions are more than welcome.

[pace2006]

Re: help: sudden mpg drop

Mine did the same about a month ago and I've hopefully narrowed it down to a bad O2 sensor (or two). Sure sucks getting just barely over 200 miles per tank.

[whitdog54]

Re: help: sudden mpg drop

you said you did the simple stuff. Buty after you recharged the k&n you should really spray the hell out of you MAF with some MAF cleaner. I just recharged my filter too. Ill prob put about 50 miles or so on it, get a couple WOT pulls, then blast the hell outta my MAF with some cleaner.

is your cutout pre or post cat? You said you had a bad header gasket, that might also be your problem.

[BallerMaro13]

Re: help: sudden mpg drop

you said you did the simple stuff. Buty after you recharged the k&n you should really spray the hell out of you MAF with some MAF cleaner. I just recharged my filter too. Ill prob put about 50 miles or so on it, get a couple WOT pulls, then blast the hell outta my MAF with some cleaner.

is your cutout pre or post cat? You said you had a bad header gasket, that might also be your problem.

last time i recharged my filter i cleaned up the tb and MAF, but i just did this a few days ago after all my gas mileage trouble had already happened, so i know that isn't hurting anything --- basically just did it to see if it helped.

my cutout is post cat right before the rear axle, but once again, i've had a blown gasket for a while now and my mileage never dropped before. i just got in the SCE copper gaskets steve posted about so i'll be fixing that asap --- but i doubt that will correct my gas mileage much, especially not up another 15mpg where it was.

[BallerMaro13]

Re: help: sudden mpg drop

Mine did the same about a month ago and I've hopefully narrowed it down to a bad O2 sensor (or two). Sure sucks getting just barely over 200 miles per tank.

exactly what i'm hoping is the problem and not something i'll have to keep digging for --- i just washed up the 'maro yesterday while it was nice and put a half tank in to cruise on for the day, and ended up burning it all in just 109 miles :eek:

when she's up and running correctly, i can get well over 400 miles, if not almost 500 miles per tank --- 20 bucks of gas used to last me forever haha.

[whitdog54]

Re: help: sudden mpg drop

yea man, that just sounds like a bad 02 sensor...hopefully. It isnt throwing a light is a little wierd, but I had a fried sensor in my ps header and that never through a code, cheanged it and my mpg jumped from 16mpg to almost 20 in the city.

Im in the process of just reaplcing things one at a time (it sucks) to fix my cars hesitation...I realy hope its not fuel pump.

speaking of that, maybe fuel filter?

[BallerMaro13]

Re: help: sudden mpg drop

yeah that's what's throwing me for a loop too -- no codes. definitely gonna get the sensors on the headers checked out to see if that's the problem, and my only other guess would be a bad fuel filter like you said. funny thing is, the car is tuned so it doesn't run so rich, and i'm seeing no drop in performance other than the slight bit of power lost from the header leak.

i've done a full tune up as of the fall, all new exhaust and intake upgrades with bolt ons, and i keep her maintained really well since it's not my DD anymore, so this is very odd. until now the only problem i've had is just the passenger gasket.

[Rob Bell]

Re: help: sudden mpg drop

A dying O2 sensor may not necessarily throw a code. It's only going to throw a code when it gets so bad that it exceeds the parameters in the PCM, allowing a code to be set. When they get old they start to get lazy and don't switch as fast as they should. It may still be switching fast enough to avoid the code, but not fast enough to run at the optimal AFR. IMO a sudden drop in mileage and old O2's means it's time to replace the O2's. Try replacing the old one and see if things improve. You're not wasting any money anyway since it's probably time to replace it anyway. The only way you can test it though is with a scan tool so you can see how fast it's switching. To have that diagnosis done, it's probably cheaper to just replace the sensor.

[Leonard 3.8L]

Re: help: sudden mpg drop

Could be your fuel pump or MAF dude.