Hi,
I've recently noticed poor MPG on my volvo (~25 years old) in conjunction with the check engine light turning on. The symptoms included:

• Worse MPG than usual
• Check engine light error codes:
  • 2-1-2 = faulty oxygen sensor
  • 1-2-1 = faulty mass flow sensor
  • 2-3-1, 2-2-1, 1-1-3 = poor lambda control
• Sometimes the engine is unable to cold-start; i.e. the engine turns but cannot achieve idle, in those cases I generally have to rev the gas a little to keep the engine from stalling.
• Strong gas odors coming from the exhaust, indicative of unburnt fuel--possibly over-rich mixture?
• Slight RPM fluctuations during idle; and strange engine RPM variations as I drive around.

I've went ahead and replaced the MAF and cleared the codes, and at first the engine light stayed off. But after a while of driving, the light returned and the same exact codes presented again. I have an oxygen sensor on the way and plan to replace it when it arrives, but the MAF fault code is a little confusing: I'm not sure why the ECU still thinks the MAF is broken; I've backprobed its leads and the voltages seem fine (varies around 2.4-2.8 volts, increasing as throttle increases). I'm skeptical that the wiring b/w the ECU and MAF is broken, though if that is very probable then I'd check.

I'm aware of the fact that faulty oxygen sensors cause poor engine performance, but I expected the MAF to be an independent error condition from the oxygen problems. Is it possible that, given bad readings from the oxygen sensor, the ECU is reporting poor engine performance, and faults both the MAF and O2 even though its likely just the O2 is broken?

I was planning to retry the engine codes after replacing the O2 sensor, but in the meantime I thought I'd ask around to see if anyone has run into similar issues with the MAF and ECU.

Thanks,
Sam

 

I had a time that using a too cold thermostat and the engine just throw all kind of codes at me no matter what. Once I got a hotter thermostat everything automatically solved. Might not really related to your problem but might able to inspire you little bit. Could also be timing, my timing was off by 4 teeth due to belt slip, engine runs in a funny way.

 

Did you replace the MAF with another Bosch or an aftermarket? I have yet to hear anything good about the aftermarket ones.

 

I used a remanufactured one from autozone; the sensor seems fine after backprobing the outputs.