Lean on bank 1 - Subaru WRX Forum
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post #1 of 3 Old 04-22-2007, 08:12 PM Thread Starter
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Lean on bank 1

I have an 02 WRX with catback and a bov.... Latley i was driving and about 4000 rpms the car would hesitate until about 5000rpms in every gear.... i just bought the car two weeks ago and i never had the problem the first couple days.... then one day it threw a CEL code... the code read lean on bank one.... one of my buddies said to put new plugs in it but i didnt see how that could cause it to run lean(maybe im wrong).... another buddy said it could possibly be a fuel pressure reg.... then i was told it could be a fuel pump..... i just replace the fuel filter hoping it was that... it still does it... so i need help.... where do i start?
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post #2 of 3 Old 04-23-2007, 11:17 AM
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That sounds more like a fuel system problem than a spark problem. If it were the fuel pressure regulator, than you would get an overall lean code, as it would lean out all the cylinders. Same thing with a clogged fuel filter. Really, only a couple of things can cause a single cylinder to run lean...fyi, if none of this makes any sense to you, you probably want to get an experienced mechanic to look at the car. This type of diagnosis can become very complex, you can even damage the electrical system if you probe in the wrong place, or provide ground or power to the wrong plug. Just be careful and you should be fine.

First thing i would check is the electrical signal to the fuel injector in #1 cylinder. Using a paperclip, backprobe the electrical connector to the #1 injector, start the car, and check for voltage pulses to the injector plug. You will need a test light to do this, a voltmeter will not give you a pulse signal. Once you have established you are getting power to the injector, then check the ground. You should have a good solid ground coming through the connector. Using a voltmeter, put the red probe on the positive terminal of the battery, and the black probe on the negative. Record that number (should be about 12.7-12.8 volts). Leave the red probe on the positive side of the battery, and begin checking grounds by touching all the grounding locations in the motor with the black probe. The number you come up with by doing this should be within 0.5 volts of the number you wrote down, if not, it's a negligible ground. You can use this process to check all the grounds, and even check the ground to the injector pigtail (although the car must be on to get ground to the injector).

If the elctrical system looks good, then you need to check the injector itself. There really isn't a good way to do this, pretty much all you can do is an ohm test on the injector to make sure it has the proper resistance. The resistance depends on the year of the car, and the specific injector you have. You can probably find it on-line somewhere, if not, let me know and i'll try to look it up. More than likely, this is going to be your problem. I have seen a lot of subies, especially ones on colder climates have injector problems. It could also be the result of the previous owner running poor fuel through the car and never having an injector clean done on the vehicle. The injector pintle could be sticking, you could try having the injectors and intake tract professionally cleaned and there is a 50-50 chance that could fix the problem.

If the fuel system is good, and no problems are found, then the next place i would look would be the control system. Your profile doesn't have a mod list in it, so i don't know if the car has currently or ever had any engine management on it, but the ECM could be sending a bad signal to the injector and causing an issue. The only way to check the injector pulse width in a stock ECM is with a scope or scanner, if you have access to one, then use that. Otherwise, must go to the dealer to have ECM parameters checked. If you have aftermarket engine management, you may be able to use that to check injector pulse.

Cylinder #1 tends to get much hotter than the other cylinders in the motor. This can cause a lean condition as well. It's caused by the design of the exhaust manifolds on the car, as the engine ages, and carbon builds up in the combustion chamber, the temps rise. As silly as it sounds, adding a header to the car has been known to fix this problem from time to time as well, because it lowers all the cylinder temperatures and they have less tendancy to lean out.

If none of this fixes your problem, then i'm sad to say, you have internal engine failure. Either a bad valve, piston, or ring allowing the fuel mixture to leak out and causing improper combustion. Perform a leakdown check and compression test on cylinder #1 and see if there are any problems there.


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post #3 of 3 Old 04-23-2007, 07:11 PM Thread Starter
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Thank you for all your info!!!! i have the problem fixed.... It was actually a bad air/fuel sensor..... One of my good buddies is a subie mechanic... thanks
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