BOV or Air Fuel Sensor Problem? - Subaru WRX Forum
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#1 Old 06-22-2014, 09:37 PM
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BOV or Air Fuel Sensor Problem?

Hi guys. I just bought my 2004 wrx last weekend. It has an idle problem that I figured I would fix myself, but it turns out that it is more complicated than I thought. It came with a Greddy blow off valve (type s I think) that I believe to be causing my car to have an unstable idle, making it run rich, and causing occasional backfire/afterfire. The bov is atmospheric and is not recirculated. I have no check engine light, but after running a scan today, I found that it does have a P0030 code, which is a code for a malfunctioning air fuel sensor (Bank 1 Sensor 1) heater control circuit. I don't know what to do because I don't know whether the blow off valve or the air fuel sensor is my problem. Since the bov is releasing the excess air, it might just be confusing the computer and making it think that the air fuel sensor is not functioning properly. I need some opinions. I'm trying to pass emissions this week so I can license my car under my name, so any help would be greatly appreciated. Thanks guys!
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#2 Old 06-22-2014, 10:05 PM
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It's the BOV, best place for those is the garbage can. I assure you, that's your problem.
You can get a stock one on eBay for cheap.
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#3 Old 06-22-2014, 11:20 PM
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That's what I've been thinking until today when it came up with a code. I've been looking for a factory bpv on craigslist and I found a guy who would do a straight across trade, but I think I can find one for cheap and make some money off the Greddy. Thank you for your input!
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#4 Old 06-23-2014, 08:27 AM
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While you should still loose the bov, the cel is for a front o2 sensor. It is the one in the uppipe. Before you go buy another, check all wires, pins, & connectors going to it. If they all look good then go get yourself a new sensor. This one should NOT be replaced with a cheaper aftermarket one. Go to the dealer or find a oem subaru one. It will cost about $170. Don't cheap out on this one or you will be fixing it again.

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#5 Old 06-23-2014, 08:50 AM
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Originally Posted by DBurwell View Post
That's what I've been thinking until today when it came up with a code. I've been looking for a factory bpv on craigslist and I found a guy who would do a straight across trade, but I think I can find one for cheap and make some money off the Greddy. Thank you for your input!
I agree with boo, and as for trading you... Haha, he's trying to rip you off. Go buy one for under $50 than sell yours for a quick $100

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#6 Old 06-23-2014, 08:51 AM
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Facebook and Nasioc are both great places to sell.
There are Wrx/STi sale pages on Facebook with thousands of members.

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#7 Old 06-23-2014, 11:41 AM
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To check the sensor and harness first, start the engine and let it warm to normal operating temps. With a multimeter check resistance between pin 1 and pin 2 on the sensor. Your reading should be less than 5 ohms. If it is more than 5 ohms then the sensor is bad. Same principal for checking the harness between the ecu and sensor connector. This reading should be less than 1 ohm. If it is more then you have a short or open in the harness that needs repaired. The reason I say to use a genuine subaru sensor here is because this one is a wideband up to a certain value and not all replacements are manufactured to read like this. If it were the other o2 sensor I would say sure, go with something else. As for everyone who say the stock front sensor is not a wideband, it is but, only up to a certain value. It does not have the range and capabilities of the ones used for tuning. Good luck op and get rid of that bov.

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#8 Old 06-23-2014, 12:30 PM
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A blow off valve will not affect your idle nor will it cause a CEL. And hope you're not backfiring.
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#9 Old 06-23-2014, 03:12 PM
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Thanks for your help guys. I'll check my o2 sensor to see how its working tomorrow and I'll let you know. Is there anything I can clean it with or is that a bad idea? I have some MAF cleaner, which I don't think would hurt it
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#10 Old 06-23-2014, 07:33 PM
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You could try cleaning it but I doubt you will have any success. The sensors workings are covered by a steel shield and unlikely to get clean. Maf cleaner would be fine to try. Compressed air would also be good. Also try cleaning your Maf sensor and checking for intake/exhaust/fuel leaks. O2 sensors rarely fail because of contamination. The plug would probably be the only part you could effectively clean. Even if you did get it clean and working you will eventually be right back in the position you are now and have to bite the bullet.

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#11 Old 06-25-2014, 09:20 PM
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Okay, so I have some more information. I didn't get to check my o2 sensors, but I did swap my bov with a stock bpv. That was my idle problem. I didn't notice it before, but the bov was staying open and causing an inconsistent idle. There has been a huge difference since I swapped them. I still have codes, but I'll just swap out my o2 sensor to fix that. I have another question not though. It seems like my engine is afterfiring a little more since I swapped the valves. I have a feeling that my exhaust setup is to blame. It has SSI headers, a catless downpipe, and a full 3 inch magnaflow exhaust, so there are a ton of unburned gasses and almost no compression for the engine to fight against. Do you guys think that is what might be causing the afterfire? It only happens when I take my foot off the throttle and I'm coasting.
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#12 Old 06-25-2014, 10:12 PM
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Yep, bov's do nothing when not in boost unless they are leaking, which many atmospheric ones are notorious for so, that explains your problem and why a stocker solved it.

As for the afterfire, The front o2 sensor directly effects the amount of fuel injected into the engine. The ecu reads the afr and adjust accordingly.Im not gonna say it will solve your afterfire problem but, it sure won't hurt if it is bad.

I saw in your original post you said no cel but you scanned for codes and saw the p0030. Maybe the previous owner put a washer or something on the o2 sensor to back it out of the pipe a little so that it don't pop a cel. This is a common cheap practice to remedy a catalysts inefficiency code but, no substitution for tuning when replacing exhaust components.

You should fill out your mod list on your profile or just go ahead and tell us all of them here. You never mentioned header and such before. These things help when diagnosing problems.

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#13 Old 06-26-2014, 05:06 AM
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Originally Posted by boo-key bear View Post
While you should still loose the bov, the cel is for a front o2 sensor. It is the one in the uppipe. Before you go buy another, check all wires, pins, & connectors going to it. If they all look good then go get yourself a new sensor. This one should NOT be replaced with a cheaper aftermarket one. Go to the dealer or find a oem subaru one. It will cost about $170. Don't cheap out on this one or you will be fixing it again.

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Funny...My uppie has an EGT sensor in it.(well, not anymore) My exhaust manifold has a front o2 sensor and my down-pipe has a rear o2 sensor. Why get a new one when you can just unplug it and put in a 2.2kohm 1/2 watt resistor? Is an 04 set up different?

All right, where is the distributor cap? I need to find the #1 plug wire to hook up my SUN machine.
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#14 Old 06-26-2014, 08:57 AM
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Your right. The one in the up is egt. I wouldn't put a resistor on the front one. The rear one is the one that causes the cel associated with a tbe. That's the one people use the resistor fix or anti-fouler fix on. Not the Front .

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#15 Old 06-26-2014, 09:34 AM
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It has SSI headers, a catless downpipe, and a full 3 inch magnaflow exhaust, so there are a ton of unburned gasses and almost no compression for the engine to fight against.=.

In turbo cars the turbo itself creates back pressure. Just thought i'd throw that out there
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