BAD MPG, No Codes?!?!?

[LawnGnomeHitman]

Ok guys, here goes nothing. I've got a 01' Chevy Prizim, same thing as an 01' Toyota Carolla. In the last 8 months the MPGs have gotten steadily worse, we are down to sub 20MPG averages now. No OBD codes have popped. Built my own fuel pressure tester adapter and am getting between 40-44psi at the fuel rail, exactly what the OEM specs say it should. Pulled and read the plugs, no bad gas smell or heavy carbon deposits from running rich. No Exteral leaks, inspected all lines for fuel and emissions found no cracks and tight connections. Pulled the MAF and did a bench test resistance reading and it comes up wayy off, reading over 5kohms when it should be well below 2kohms in 80 degree weather. If the MAF thought lots of cold air was heading for the injectors, would it cause it to run rich? And if this is the case, wouldn't the Air Intake Temp sensor or Primary O2 Sensor throw a code?

[Buurin]

I see you're not a complete noob, good.

Now that you found the MAF bad, search this board for how to clean it. I'm not sure if your engine still has an IAT sensor - the MAF is supposed to replace MAP and IAT from older 1ZZ-FE engines- but if there's one, it's a plastic probe plugged into the airbox. Test it - you should already know how to determine its health. If cleaning doesn't help, bite the bullet and replace it.

Once you think you have these sensors sorted out, you need something that gives you much faster feedback than pen and paper at fillups. Get a ScanGauge - plugs into OBD2 port and tells you a whole lot.

[LawnGnomeHitman]

Thanks for the feedback, I tried running a few more tests, the MAF is shorted out internally or something I think, as it has the opposite reaction to cold air as it should. I cleaned it with some high grade electronics cleaner and it's still behaving the same way. This car does not appear to have an Intake Air Temp Sensor, one is listed on most parts suppliers, but they only have component locations and diagrams for the MAP sensor cars. I'll be running down to the Zone later, lists there for 80 and change, but I get a discount so it won't be too bad. Thanks for the heads up on that scan tool, thats a nifty device.

[LawnGnomeHitman]

Update, it's not the MAF, turns out my multimeter was on the fritz, tried testing it with some brand new resistors and it read them all wrong. Ran the tests again with a borrowed brand new Fluke multi meter, and it checked out on both the new resistors and the MAF worked flawlessly. This is staring to drive me crazy. Anyone know what the resistance range is for the O2 Sensors? Any other ideas would be appreciated, I've eliminated the pump and regulator, their fuel pressure checks out. There still are not any OBD codes coming up, the light comes on when you put the key in the ignition start position, but goes out as soon as the car starts, and my Scanner comes up PASS, no codes found. Checked other fluids tonight too, all full and good in color.

[ilovecorollale]

well all i could say is good maintainance, i clean my maf with maf cleaner bought at autozone for 14 bucks i think...put in short ram...and use lucas oil and now getting 32-33 mpg.was getting below 30 mpg before all that...just want to let you guys know of my situation.sorry nothing goody info here for you.

[EdW]

How many miles on the car? How's it run/idle overall?

[LawnGnomeHitman]

121k Miles, Starts on the first crank, idles smoothly. The exhaust definitely smells a bit rich, but the plugs are clean, none of the excessive carbon deposits you would expect to see if it were running rich. As for the maintenance, it's got a new air filter, fresh clean oil, and all the other fluids are full and clean. The contact cleaner I use has the same active ingredients in higher concentrations than the CRC MAF Cleaner. I thought for a second that maybe someone had been siphoning off gas from the tank, but the car has no external release on the fuel door. There are no tool marks on the locking mechanism or fuel door there would be if someone was popping the door with a screwdriver to steal gas. I got some resistance range readings for the O2 sensors and am going to give them a try later today. My buddy used to have a pretty advanced OBDII scanner, capable of pulling soft codes from the system memory, I'm going to try and borrow it and see if I can find something in the computer, my basic code scanner can't find anything.

[huangxin]

if i remembered it right, you need to check the voltage output for the o2 sensor instead of its resistance.

i think u need to backprobe a wire on the sensor harness and watch the voltage produced by the sensor with the engine running. I think it suppose to be 0.1 to 0.2 volt on a cold engine and changed between 0.1 to 0.9 volt when it get in closed loop.

now i assumed u r checking the pre-converter o2 sensor. the one after the converter should act the same but it the voltage doesn't change as fast.

hope this helps

[EdW]

Or grab your Buddy's scanner and take a look at the output waveform from the O2 sensor. Much eaiser that probing the leads and more accurate as it'll show smaller voltage issues. But if there's no code, I have a feeling all is well there.

Just a long shot thought, have you checked the brakes on each wheel for sticking?