Ok so pretty much about 6 months ago, i started to get really bad gas mileage on my '02 S40. A little while after that, CEL came on, got it scanned, came up with bad mass airflow sensor. Bought a new MAF, replaced it, had good gas mileage for a few days, then started to go downhill again.

Now here is the thing, back when the original MAF was installed, the car wouldnt stall much (maybe twice in a month) but it would hesitate quite a bit, but just to get from point A to B i would disconnect the MAF and drive around like that. When disconnected, the car ran like a dream.

The new maf though is a little different. So as i said, at first it gave much better gas mileage (almost doubled) but now its difficult driving in the city without the engine stalling at least twice. There is no hesitation, but LOTS of stalling. So i tried the old method of disconnecting the MAF untill i could take it back to a mechanic (forgot to mention, changing the MAF never cleared the CEL) except now when i disconnect the MAF, the engine STILL idles rough and stalls.

As for a little background history, about 1 year ago the CEL came on, and i got it checked and they said it was the catalytic converter. Using my $0.02 i figured the status of the catalytic converter is determined by the output of the O2 sensors. So now im wondering, is it possible that the O2 sensor is the one thats at fault or does the cat have nothing to do with this and its all the MAF? (Im a broke college student so i never got the catalytic converter fixed, i just told the guy to clear the CEL and i would deal with it later)


Another thing is, just a little while after i started to get lower gas mileage the first time, i also change the air filter, i know everyone is telling me to stay away from the K&N but i think i used STP (i seriously dont remember) the problem is though, the air filter never really fit in the box... I went back, checked and rechecked with the store that it was the right model, and they said yes, so i just stuck it in really tight, but it looks like a wave now because its bent so it can fit (there are no gaps though, so 100% of the air is going through the filter). Not sure if this had anything to do with it, but i thought it might be important to note (also the filter is not the oil drenched kind, its just a plain dry filter)


Scheduled another full diagnostic for this coming monday which will cost me another $90 ... anyone have a clue as to what might be going on?

 

Spend your money on an OBD-II scanner from Harbor Freight. Just a cheap one for now. Then you can check your trouble codes at anytime and clear the check engine light.

I'd buy some MAF spray cleaner, remove the MAF, and clean it. Then clear the trouble code and see if it comes back. Make sure it's not bad wires on the connector too. Use some dielectric grease on the connector.

The easiest way to avoid the cat convertor trouble code is to use an O2 extender on the bottom O2 sensor (the top one is for fuel control feedback only). You can buy a pre-made one at IPD for $30. Oxygen Sensor Spacer (CEL Boss)
or go buy two 18mm spark plug anti-foulers at the local parts store for less than $10. You will need to drill out one large enough to fit the O2 sensor (slightly larger than 1/2 inch). That in turn screws into the 2nd untouched one. Then both are screwed into the bottom O2 hole below the cat. If you do not have tools then buy the pre-made one. What these do is simply back the O2 sensor out of the exhaust stream so it's not so "sensitive" shall we say? As always use anti-seize on the threads, same as spark plugs.

 

clean the throttle body and iac for the stalling it can cause the maf code sometimes. what is the code?

 

...you need to find someone which a computer that can watch the O2 values with the engine running, blip the gas hard once or twice and the sensors should drop a lot (.60 +)

if they don't there is your dead O2 sensor.

check the ECT (engine coolant temperature sensor) located just under there the thermostat is housed. if you see corrosion on it (it would be greenish) replace it.

Check all vacume hoses, make sure the flame trap hoses are connected. Better yet, with the engine warm idling, pull the dipstick and look for smoke.

Plugged crankcase ventilation system on Volvos - YouTube

plugged flame traps will throw 3 way cat codes.

 

Thank you all for the responses, I ended up not taking the vehicle back to the mechanic for a $90 diagnostic, instead i took it to get the oil changed since its all i could afford... but for some reason while i was there, i decided to try the unplugging of the MAF once more... so i unplugged it, it ran fine, then i plugged it back in and they started changing the oil. Once i drove out of the driveway, the CEL had cleared, the engine hasnt stalled/hestitated since and the CEL has never come on and its been over a week. Im crossing my fingers that it was a bad connection at the MAF and when i kept unplugging it i set it right. For sure i know if i get that light again or it stalls, im going to replace the wiring since it would be cheaper to do.

Never the less, i did follow the steps you all had suggested just yesterday, found nothing wrong with the ECT sensor, no smoke from the dipstick slot, nothing unusual, hickman123 I apologize but i dont remember the code that was given out

And Hudini, thanks for the help with the Cat Converter, i havent done it yet, but i am going to very soon!

Again thank you all so much, very helpful!

 

What a relief, after weeks of dealing with stalling max, she purrs like a kitten again. The fix took 10 min, by simply running extra grounding wire from MAF to the chassis.
All stalling when coming to stop , shaking at idle, stalling in reverse and running rich is now gone !!!