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Re: I'd suspect AMM
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Fri, 28 Jan 2005 08:10:49 Share Post by Email
In Reply to: Re: I'd suspect AMM, Rmc, Fri, 28 Jan 2005 06:56:59
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Complex, so hang in with me.

The O2 sensor comes 'on line' after it warms up, usually about 2 minutes. Cooling fan cycling is all about the temperature of the coolant in the radiator; the O2 sensor operation is all about exhaust gas temperature, because that's what heats up the O2 sensor. So if the car is dead cold, about 2 minutes; if warm, that much faster.

The O2 sensor is a fine adjustment of mixture. At a reading of 0.5 volts, the mixture is perfectly stochiometric - the ideal air-fuel mixture. 0.2 volts is lean, 0.9 is rich. But the O2 sensor works around a very narrow band - 0.2 is lean, but only slightly lean. 0.9 is rich, but only slightly rich. And slightly rich, rich, or Oh My God Rich all read 0.9 volts - the sensor is pegged there.

The purpose of the AMM is to measure the, well air mass, because that's what fuel flow is scheduled by. It controls 100% of the fuel flow. The engine takes that air measurement and calculates how long to hold the fuel injectors open, and that's fuel flow. The ECU then measures the oxygen content of the exhaust (with the O2 sensor), and can make some fine adjustments on fuel flow.

OK, what about that swing? Modern cars have 3-way catalytic converters. Cool - what's that mean? It means that they handle both Hydrocarbons (HC) and Oxides of Nitrogen (NOx). Each requires a different operation. The engine is programmed to run slightly rich for a little bit, and load up the Cat with HC, then run slightly lean and load up the Cat with Nox. This cycling allows the Cat to work. Don't ask me how.

So if the AMM is adjusted to get the mixture almost exactly right, then the 'fine tuning' of the O2 sensor works. The ECU can swing the mixture slightly rich and lean about once a second, and the plugs look good, the car goes well, and the exhaust is cleaned by the cat.

If the AMM is off, too rich or too lean, the O2 sensor can only adjust the fuel mixture so much - the ECU is programmed that way. So the signal won't swing, because the sensor is pegged.

Now, if the O2 sensor says the mixture is lean, it tells the ECU to richen it up. But it can call for only so much extra fuel. If the AMM is way off, the additional fuel the O2 sensor calls for still keeps it pegged lean. The ECU may call this an O2 sensor fault because the O2 sensor is stuck lean. But that may be the sensor telling the truth - for example, you could have a vacuum leak causing the engine to run lean, and the O2 sensor just can't handle it. And that's air the AMM doesn't see.

The -005 AMM you have has an adjustment screw. The purpose of that screw is to adjust the 'bias' of the sensor. Ideally, you adjust the screw so that you get the mixture right, and the O2 sensor can swing properly. This screw is there to take out variations in the engine. Later versions of LH found ways to account for these, so the adjustment screw was deleted.

In general, the O2 sensor does not have enough authority over fuel flow to really mess up driveability. It takes the AMM to do that. The other possibility is a really blown Fuel Pressure Regulator. The FPR adjusts fuel pressure as a function of intake manifold pressure. Why? The fuel injector opens for a period of time; the amount of fuel squirted in depends on the pressure - what pressure? The pressure DIFFERENCE between the fuel pressure and the air pressure inside the cylinder. If the cylinder is at vacuum, it'll suck the fuel in; therefore, the fuel pressure is dropped so that the difference in pressure is the same. At boost, the cylinder is pressurized, so the FPR has to run at a higher pressure to force fuel into the cylinder. If the FPR is running at too high a pressue, it'll dump in way too much fuel all the time, and this can cause the engine to run like crap.

Now, if the engine is obviously running rich, as the spark plugs say (and spark plugs don't lie), the O2 sensor should be stuck rich - near 0.9 volts. But it's a very high impedance signal, so (1) you need to use a decent digital meter - avoid a cheap analog meter) and (2) you need a good ground. Don't use the battery ground. Best would be to measure it right at the ECU pins, and use the ECU ground. that's what the ECU sees.

If the car was running OK, and now suddenly isn't, I'd suspect AMM, not O2 sensor. To see if the FPR is totally whacked, find the thin hose that goes from the FPR to the throttle body. Pull it off, and suck on it. It'll taste a little like fuel. It should also hold vacuum. If you end up with fuel coming out of that hose, then the diaphram on the FPR is blown, and you're dumping in too much fuel. Make sure the hose is in good shape - no holes. Beyond that rough FPR test, the only thing you can do is stick a fuel pressure gauge on it to test it.

Do the AMM test above - that will be true at any engine operating temp. AMMs do fail, and I've seen your symptoms with a blown AMM. Try the various folks that advertise on this BB (See the bottom of the page) - a decent rebuilt AMM should run about $140 after core charge - much less than the dealer. And make sure you get the same part number - later model AMMs look the same (except for the lack of adjustment screw) and will fit, but won't work.

And if you do put in a new AMM, don't expect it to work perfectly right away. You still need to adjust it. Some folks will talk about 380 ohms (measured from signal to ground pins on the AMM) as being correct. Not true. That's a starting point for adjustment, where the engine should at least run. From there, you need to adjust the screw so that the O2 sensor swings back and forth, spending about 50% of the time above 0.5V, 50% below.


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