This is in regard to a 2002 Daewoo Lanos 1.6L Automatic.
I have owned this car for approximately two months. It ran great for the first month, I even had a professional mechanic change the timing belt on it. Now, for the past few weeks, it doesn’t run properly. It has a very rough idle, feels like it’s misfiring and going to stall, but it never does. If I put my foot on the accelerator to raise the RPMs, it smooths out. When the car is moving, it has decent performance and acceleration. The only trouble code stored in the computer is P0442–evaporative emissions system small leak detected.
I took the car back to the mechanic who did the timing belt, they double-checked everything, confirmed that the mechanical timing is correct, the engine has proper compression on all cylinders, proper fuel pressure at the fuel rail test port, measured resistance of the fuel injectors, etc. They claimed that their scan tool provides only basic information for this model, and cannot access the diagnostic functions provided in the real factory scan tool, but based on all their tests they don’t see a problem. They even did a smoke test, but the only thing that found was a hairline crack in an emissions check valve near the fuel tank.
I looked into getting a real Daewoo scan tool, but at approximately $3000 for a used one, that would be insane. So I bought a decent low-cost OBDII scan tool online, which provides live data and sensor values if supported by the particular vehicle.
So with my scan tool, I see that the short-term fuel trim constantly fluctuates from -0.8 to 3.6, as the engine warmed up at idle the long-term fuel trim went from -6.3% to -11.7% to -14.1%, the engine RPM constantly fluctuates from 857 to 948 RPM, the ignition timing advance for #1 cylinder constantly fluctuates from 2.5° to 13.0°, oxygen sensor B1S1 constantly fluctuates from 0.160V to 0.825V, and oxygen sensor B1S2 only fluctuates from 0.845V to 0.865V.
The fact that the fuel trim trends more and more negative the longer the engine runs would tend to suggest a fuel pressure problem or fuel injector problem. Is there any way for me to test the fuel injectors other than measuring their resistance, which was already done? Also, I have heard that this model is known for a defective camshaft position sensor (unless already replaced) and this can cause serious drivability problems without setting a misfire code.