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Re: Running Lean
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Thu, 7 Mar 2002 09:40:04 Share Post by Email
In Reply to: Running Lean, Saabiesaab, Wed, 6 Mar 2002 23:58:09
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This may sound silly, but just because the computer says the engine is running lean doesn't mean that it IS running lean.

The computer uses the oxygen sensor to measure mixture. If the sensor says the mixture is lean, you'll get a lean fault code. So you're down to two possibilities:

1. The O2 sensor is bad
2. The car really is running lean.

To check the first one, pull out a couple of spark plugs and look at them. Fault codes aside, the spark plugs live in the engine, and they know the truth. Do the tips look white? That's a sign of lean running. Or do they look black, with a fluffy carbon buildup? That's the sign of running rich.
If the O2 sensor is telling the engine control that the engine is running lean, the control will add more fuel. If the O2 sensor is lying (bad sensor), the mixture will be over rich, and you'll see black fluff. If the engine is really running lean (whitish plugs), then the O2 sensor is probably telling the truth.

If the engine really is running lean, it works out to three possibilities - too much air, not enough fuel, or bad information about how much air is coming in.

Too much air - look for vacuum leaks. Unmeasured Air is getting in. Look for hoses that have fallen off, or have cracks or holes. Check the hooter valve for a blown diaphram. My favorite leak is the charcoal canister. It likes to blow its hoses off, but since it's buried in the fender, you never see it. Remove the right front (passenger side, US) turn signal assembly and look in. You'll see the top of the canister, with some hoses running to it. Make sure the hoses are on OK.

Too little fuel - bad fuel pressure regulator. This one is tougher without the right tools. The FPR has a thin hose running to the intake manifold, and it uses this pressure to regulate fuel. Pull the hose off the intake manifold and suck lightly on it. If you can pull air (or fuel - yuuk!) through it, then the FPR diaprham is bad. However, the best test is to hook up a fuel pressure gauge.

Bad information. The control schedules fuel based on how much air is coming in. It uses the Air Mass Meter (AMM) to measure the air. If the AMM is bad, the control will be told the wrong amount of air is coming in, and not supply the right amount of fuel. This one is another toughie. Your best bet is to swap the AMM out for a known good one. You can by good rebuilt units via mailorder for about $150. Your best bet is to find a buddy with a Saab and see if you can swap the parts for a little bit - replacing the AMM is about 2 minutes and no tools other than your fingers. NOTE: THERE ARE TWO DIFFERENT PART NUMBER AMMS, AND THEY ARE NOT INTERCHANGEABLE! Make sure you have the same part number.

At the very least, you can pull the spark plugs and also look around for vacuum leaks. Replacing an O2 sensor is very much a DIY job.

Good luck!

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