A few days later the car develops a stumble. Press on the gas and it barely runs. Pull over and take a look at the engine. Yep it looks like an engine. Rev it a few times and everything is back to normal. The check engine light never came back. Over the next couple of days this stumbling issue happens a few more time. It feels like the engine isn't getting gas. I check the basics around the engine. Gas in the tank, air intake not obstructed, fuel injectors ticking, spark plugs sparking, spark plugs look good, O2 sensor plug dry and secure, all oil levels full, no blown fuses Over the next few days it also has several more stumbling episodes, and it throws another PO130 code which I clear out. The last few instances was really rough, engine even made popping sounds typical of super lean (low fuel) conditions, like the sputtering right before you run out of gas. Somehow I must not be getting gas.
The greatest diagnostic tool ever made is the Internet. So I hit all the usual Subi Forums asking for advice. Fuel pump, fuel pressure regulator, mass air sensor, coolant temp sensor, transmission temp sensor, and bad gas were allsuggestion. At this point I had a bad O2 sensor codes appear twice now. Why didn't I just change out the O2 sensor. Well I was under the impression that other codes told you you had a bad O2 sensor (sensor reading out of range), and if the circuit was really bad, it would constantly have the check engine light on, not occasionally (every150 miles).
Before I go any further let us talk about O2 sensors and what they do. Your car needs a mixture of fuel and air to burn. Back in the carburetor days the carburetor mixed the fuel and air. Since carbs only mixes the fuel and air to one ratio without consideration for varing conditions, fuel injection was developed. The amount of fuel the injectors spewed is based on the amount of pressure in the line and the duration the spray nozzles are open. Input signals from the Mass Air Flow/Pressure sensor determines the duration of the spray. The O2 sensors read how much fuel is left unburnt in the exhaust and fine tune the mixture. When one of the sensors is not working it sends the fuel map into a safety mode, erroring on the side of burning more gas than is needed (rich). Running rich is safer cause the car will still run and you do not run the risk of detonation (blowing up your motor with a too lean of a mixture-this is a whole other topic). This is why it waste gas when you have a check engine light on. This is also why I figured a known O2 sensor issue will simply cause the car the run rich and not generate such a shortage of fuel condition, and thus not a result of the PO130 error message.
Often times cars have two O2 sensors or more. This can add a bit of confusion, as some are desginated as bank one and two and others as front and back. In some "V" configured engines an O2 sensor might be present to monitor the left bank of cyclinders and the right. In newer cars there is often O2 sensors located before and after the catalitic converter, its purpose is to monitor the condition of the cataletic converter. I assume that it is possible to have a combination of such sensors and have four seperate O2 sensors for your car. Our Subi has two sensors to monitor the condition of the cats, and does not have sensors to monitor the left and right bank.
The problem with my stumbling problem was that it was intermittent. This makes diagnosing the issue difficult as it would not do in the comfort of my garage, only on the open road. If it was my fuel pump or fuel pressure regulator I wanted to confirm it before I replaced it. Nothing worse than replacing a part only to find that it was fine and not related to the issue. This I call the "machine gun" way of repair; fire a bunch of bullets and hope to hit the right thing. A working fuel pump will generate a constant and adequate fuel pressure. If the pump is dying I would be able to tell with a gauge.
With the fuel pressure gauge hooked up I drove it around the neighborhood for 30 minutes and no issue. Dang. The next day I drove it to work and finally the car started to stumble. The fuel pressure was higher than spec and steady. The fuel pressure regulator's job is to lower the fuel pressure to the specified amount. A vacuum actuate diaphragm restricts the amount of fuel which passes. This is also how the FPR varies its regulated pressure. As you stomp on the gas, less vacuum is available in the intake manifold, thus reducing the amount of fuel pressure reduction. To check the FPR, pull the vac line. The pressure should rise. There should also be no fuel dripping out of the vacuum port, as that would indicate a ruptured FPR diaphragm membrane.
I was now at a loss? Do I start checking other sensors like the forum members suggested. While driving home from work the car developed a stumble again. Pull over and the check engine light comes on and the stumble went away. The code read PO130, and I did not clear it. I then proceed to drive the car for 50+ miles with out a hiccup. I think I finally figured it out. PO130 means that the ECU has received a grossly out of range signal from the O2 sensor for a steady duration of time. Though often it means something is awry with the circuit, it could also mean that the sensor is completely dead. An error message is then generated and the ECU no longer looks to the sensor for fuel mixture tuning. When the error code was deleted the ECU still considered the sensor input (though grossly wrong) to mix the fuel, and did so incorrectly, causing a stumble. The car sensing that it was not running properly, reverted back to a preset safety fuel mixture.
Now to remove the sensor. First locate the connector plug on the passenger side of the engine. It has a light grey colored safety clip on it. Remember how the clip is attached cause it is tricky to get back on correctly. Undo the clip and pull apart the connector. Undo the wire keeper tab that is located a bit further down.
Most O2 sensor are a standard size, 7/8". However it is not something that you can remove using a standard socket. Due to the wire you need to use a tool similar to a flared nut tool. I guess for removing the old sensor you can simple cut the wire off and use a deep socket. Using a open end wrench only grabs the hex in two points and easily rounds out the hex. The idea is to clear the wire but to engage more than two of the hex points.