NaughtyGeek

I have an 04 GT with ~ 40,000 miles. I have experienced a very sudden loss of fuel mileage to the tune of 4 to 5 mpg. It was so sudden, I really toyed with the idea that someone was stealing gas out of my car at night. Anyway, what I've noticed is I seem to also have an intermitant performance problem as well. It seems to exist more than not. No rough idle or anything along those lines, just a perceived lack of umph. Every now and then it seems to come "back to life" but for the most part it just seems to be dragging a bit. I would think fuel filter if it wasn't for the sudden drop in fuel efficiency. I read another thread where someone seemed to have a similar problem and both the MAF and TPS were mentioned as possible culprits but again, the fuel mileage problem wasn't an issue in the other case.

First, if I had a bad plug, would I get a check engine light? Second, would any of the items above lead to the fuel problem? Third, would it be of any benifit to have the injectors cleaned? It seems that at this mileage, I shouldn't have to be worried with them yet.

I just wanted to get as many ideas as possible before I dive into it this weekend. Also, could anyone point me to a diagram that could show me the connection points to check the TPS and MAF with my Fluke?

99GTvert

Have you replaced your fuel filter? I would replace that if I were you.

Have you checked your air filter?

NaughtyGeek

Yeah, forgot to mention I did put a new K&N air filter on it in hopes it would help.

oxfordgt

Was it the kind of filter you have to oil? Sometimes over oiled filters will mess up the maf and cause the problems your having. Try cleaning the maf with some electrical cleaner.

NaughtyGeek

No oil for the filter

cliffyk

Front O2 sensor(s)... [/align][/align]When they are dying their output still cycles however the voltage drops,telling the PCM (in closed loop mode) that the mix is lean--the PCM makes the mix richer (using more fuel) until the O2 sensor output look right.This will also make the engine "boggy" at partial throttle and lighter loads. [/align][/align]It switches to open loop mode at higher loads and WOT, however after a while the long term fuel trims ("learned" from short term trims) willget messed up and affect open loop operation as well. Open loop uses a combination of mapped values and the learned LT fuel trims.[/align][/align]

NaughtyGeek

Any way to check the O2 sensors for proper operation other than replacing them? What you've said makes perfect sense but I hate just throwing parts at a problem. Or, are you 90+% sure it's the sensors?

cliffyk

Youcould usecheap narrowband "lightshow" O2 meter ($40 at your FLAPS) oran oscilloscope to check the output waveform, or take them out and bench test them as described here. [/align][/align]With the cheap meter the readout would tend to be on the lean half of the display (if the sensor is bad itwill bedisplaying the low O2 sensor output, not the actual A/F ratio); an oscilloscope would show the waveform as switching about a voltage lower than 0.45V as it should be.[/align][/align]The O2 sensors pretty much run the show as far as fuel mixture goes,another possibility could be the coolant temprature sensor (if the PCM thinks the engine is cold it will richen the mix).[/align][/align]How many miles do you have on the car--if it's over 65k then I'd oth bet on one or both sensors...[/align][/align]

lewistk

fuel filter probably or 02sensors i would say.change fuel filter first.

lewistk

do you have an exhaust leek by any chance? my friend had the same thing happen, the 02 sensors were getting tricked by the leaking air and caused the car to run really rich