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About 8 months ago I had my 2004 325ci convertible’s engine diagnosed at Autozone. They said it was likely the mass air flow sensor which was the problem. I took my car to an indy and they said the problem was my upper intake boot. So I got that replaced.
Shortly after I did, my check engine light came back on. I was broke so I didn’t take any further action until 8 months later. I didn’t even get my car diagnosed again. My light came on and went off from time to time.
7.5 months later my coolant expansion tank burst. My engine had been receiving excess coolant due to a faulty thermostat for about 1 month. I had my coolant system fixed, and 3 weeks later I had my mass air flow sensor cleaned hoping that was why my light had been going on and off. The light remained on though. So after that I got my car diagnosed again at Autozone. Here are the codes, in the order they appear on the paper Autozone gave me - P2240, P0171, P0174, P2237.
My gas mileage has been at least 15% worse than it should be, and right now it seems to be at least 20% worse. I’ve occasionally had a rough idle.
I have an M54 2.5 liter engine. I would very much appreciate educated guesses from y’all. Thanks
Member
Do a smoke test to find vacuum leaks. They cause lean codes, misfiring, and o2 sensor codes. Lower intake boot is more common than the upper.
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Originally Posted by randy72877
Thank you for responding. I just had a mechanic check for a variety of issues a couple of hours ago, most certainly including vacuum leaks.
We found one bad tiny hose, a bad lower intake boot, and a bad DISA. Also, whoever replaced my valve cover over-torqued the valve nuts and that seems to be causing a minor oil leak. I’ll update my thread when those things are replaced/fixed.
Member
So it sounds like the preventative maintenance is due or overdue.
Member
For DTC P2240 and P2237 - more information - here a similar problem: https://www.motor.com/magazine-summa...cuit-failures/
For DTC P0171 + P0174 ==> If the P0171 is combined with the P0174 code, it's very likely that the problem is caused by an intake leak. If there are no intake leaks, the next step is to replace the air filter and clean the air flow meter. If the problem persists the front oxygen (O2) sensor may need to be replaced.
Possible causes for DTC P0171 ==> 1) Intake air leaks, 2) Faulty front heated oxygen sensor, 3) ignition misfiring, 4) faulty fuel injectors, 5) exhaust gas leaks, 6) incorrect fuel pressure, 7) lack of fuel, 8) faulty MAF, 9) incorrect positive crankcase ventilation (PCV) hose connection.
P0171 is detected when fuel injection system does not operate properly. Possible symptoms: Engine light ON; Excessive fuel consumption.
BMW description for P0171:
With the Air/Fuel Mixture Ratio Self-Learning Control, the actual mixture ratio can be brought closely to the theoretical mixture ratio based on the mixture ratio feedback signal from the heated oxygen sensors 1. The Engine Control Module (ECM) calculates the necessary compensation to correct the offset between the actual and the theoretical ratios. In case the amount of the compensation value is extremely large (The actual mixture ratio is too lean.), the ECM judges the condition as the fuel injection system malfunction and light up the Malfunction Indicator Light (MIL) (2 trip detection logic).
DTC P0174
Mixture control 2 system too lean bank 2
Possible causes: 1) Intake air leaks, 2) Faulty front heated oxygen sensor, 3) ignition misfiring, 4) faulty fuel injectors, 5) exhaust gas leaks, 6) incorrect fuel pressure, 7) lack of fuel, 8) faulty MAF, 9) incorrect positive crankcase ventilation (PCV) hose connection.
DTC P0174 is detected when, fuel injection system does not operate properly. The amount of mixture ratio compensation is too small (ratio is too lean)
Jbird
Check for intake leaks and fuel pressure, lean faults can occur if the fuel pump is weak
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