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2001 Subaru Impreza RS P0130 O2 Sensor Rich Mix Issue After Valve Repair and Timing Belt Replacement

Model: subaru Fault Code: P0130 Posted: 2007-12-12 16:02

I own a 2001 Subaru Impreza RS with a 2.5L engine, currently at 49,049 miles. It has a manual 5-speed transmission. The vehicle was involved in a collision that caused the timing belt to rupture. After repairing and re-timing the engine, I discovered bent valves in cylinders 1 and 3. As a result, I removed the cylinder head on that side and replaced the valves. Since then, the engine runs rich and continues to trigger O2 sensor fault code P0130 (Bank 1 Sensor 1). The upstream oxygen sensor is returning an unusually high voltage of 7.9 volts, indicating a rich fuel mixture. In contrast, the post-catalyst O2 sensor behaves more normally, fluctuating slightly but stabilizing around 0.7 volts. The engine also reports random misfires in cylinders 3 and 4. I performed a compression test: cylinder 1 at 190 psi, cylinder 2 at 190 psi, cylinder -3 at 180 psi, and cylinder 4 at 195 psi. Compression appears adequate. The misfires occur only above 2000 RPM, not at idle, which suggests the issue is not related to idle timing or low compression. I expected the ECU to shorten fuel pulses in response to the high O2 sensor reading (indicating a rich mixture), but this does not appear to be happening. What could be causing this persistent rich condition? Given that the engine has been re-timed and valves replaced, is there another factor—such as camshaft timing drift or vacuum leaks—that might still be affecting fuel delivery?

Related fault codes
P0130
Comments (4)
Anonymous 2007-12-12 17:02

Check camshaft timing and look for vacuum leaks — these are common causes of rich mixtures in Subaru engines after valve work.

Anonymous 2007-12-14 12:32

I tested the vacuum system and found a steady 19 inches of pressure. I also inspected the timing belt covers and confirmed that the marks align perfectly with both camshaft and crankshaft positions. To test, I adjusted the timing belt by one notch — this improved cylinder compression to 205 psi in cylinder 1 and 193 psi in cylinder 3. However, after this adjustment, the car now backfires under load. I suspect that the original timing belt may have been too loose, causing slight camshaft misalignment. Could even a small amount of timing error lead to fuel delivery issues?

Anonymous 2007-12-14 19:25

Possibly — minor timing errors or vacuum leaks can cause erratic O2 sensor readings and rich mixtures.

Anonymous 2007-12-15 16:20

I replaced the timing belt and tensioner, but symptoms persist. The upstream O2 sensor still reads a very rich mixture (7.9V), and the car exhibits random misfires and backfiring under heavy throttle. I've driven it for about 50 miles with no improvement — I'm now unsure of what to investigate next.