how do I check for lean or rich fuel mixture?
12-18-2002, 01:14 PM
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how do I check for lean or rich fuel mixture?
Hey guys !! Ive been told by Danno how to tinker with the "Fuel Quality Switch" under the DME..but first I have to determinr the condition of my mixture, this because I'm getting sulfur smell (rotten egg) when I accelerate hard. So I'm wondering what my next step should be. THANKS GUYS.
12-18-2002, 01:26 PM
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Unplug the O2 sensor and read the voltage.
The curve is steep, but 0.5 V is perfect mixture. With your O2 plugged into the Motronic, you should see 0.5 volts under cruise and steady throttle, until you lay into it, then the O2 sensor is taken out of loop and the mixture goes on a pre-programmed fuel map for that engine. You may see as high as 0.9 volts then.
Some things to check:
Fuel Pressure...too much (maybe regulator has failed) will cause a very rich mixture which the computer can not correct. This happened to my 83 NA 944 causing it to blow black smoke.
Leaky injectors? Car would run really badly if this was the case.
Air flow meter or Mass air sensor, confirm it is working properly, etc.
For maximum power, most cars run in the 12:1 range, which is about 0.7 volts (I think)
Best simple way is with a wide band O2 sensor, because the standard sensors are very hard to read between lean and rich (steep curve). If you read the standard O2 sensor, you really need a circuit with some damping built in.
Another option is to get a gas analyzer, but the O2 method is accurate and is what your Motronic uses to set the mixture.
The curve is steep, but 0.5 V is perfect mixture. With your O2 plugged into the Motronic, you should see 0.5 volts under cruise and steady throttle, until you lay into it, then the O2 sensor is taken out of loop and the mixture goes on a pre-programmed fuel map for that engine. You may see as high as 0.9 volts then.
Some things to check:
Fuel Pressure...too much (maybe regulator has failed) will cause a very rich mixture which the computer can not correct. This happened to my 83 NA 944 causing it to blow black smoke.
Leaky injectors? Car would run really badly if this was the case.
Air flow meter or Mass air sensor, confirm it is working properly, etc.
For maximum power, most cars run in the 12:1 range, which is about 0.7 volts (I think)
Best simple way is with a wide band O2 sensor, because the standard sensors are very hard to read between lean and rich (steep curve). If you read the standard O2 sensor, you really need a circuit with some damping built in.
Another option is to get a gas analyzer, but the O2 method is accurate and is what your Motronic uses to set the mixture.
12-18-2002, 05:07 PM
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John, If I disconect the O2 sensor, where should I take my voltage reading? At the DME or at the wires from the sensor? I only have a cheap 15 dollar multimeter, would this one work? Thanks !!!
12-18-2002, 06:27 PM
#4
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You can read the O2-sensor either at the connector in the engine-bay or the DME connector. Would be easier in the engine-bay because you won't have to disassemble the DME connector to attach to the connector pin itself (the cable is coaxially shielded).
"For maximum power, most cars run in the 12:1 range, which is about 0.7 volts (I think)"
It's actually 0.84v.
"For maximum power, most cars run in the 12:1 range, which is about 0.7 volts (I think)"
It's actually 0.84v.
12-18-2002, 11:21 PM
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What about cars without a catalytic converter? I suppose there isn't a similar way to determine the fuel mixture? We are talking about reading the sparkplugs in this case, aren't we?
12-19-2002, 01:49 PM
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You can always read the O2 sensor, with or without the cats they are always before the Cats, close to the engine....just make sure you get the exhaust hot, or the sensor readings will be bogus.
You should read it off of the sensor signal wire, referenced to ground. Your meter will work, and you can even put it in the car.
I would recommend you read it "out of loop"...unplugged and read it "in loop", plugged in...in both cases, put the positive lead on the signal wire to the sensor and then ground the negative wire...use the 2 volt setting on your meter.
The issue you will have is in reading the meter...at cruise it will lay steady, but when you fluctuate the throttle, the numbers will jump...this is why you really should have a circuit with some inherent damping...the meter will respond quickly, but reading it is difficult at times.
Sounds like 0.84 V is the number for max power....with your sensor plugged in, you should see 0.5 V, this is done for emissions purposes, to get a clean burn at idle as well as to enhance fuel economy.
If you get the mixture close in open loop, then the computer will not have to make large corrections once you put it back in the loop.
You should read it off of the sensor signal wire, referenced to ground. Your meter will work, and you can even put it in the car.
I would recommend you read it "out of loop"...unplugged and read it "in loop", plugged in...in both cases, put the positive lead on the signal wire to the sensor and then ground the negative wire...use the 2 volt setting on your meter.
The issue you will have is in reading the meter...at cruise it will lay steady, but when you fluctuate the throttle, the numbers will jump...this is why you really should have a circuit with some inherent damping...the meter will respond quickly, but reading it is difficult at times.
Sounds like 0.84 V is the number for max power....with your sensor plugged in, you should see 0.5 V, this is done for emissions purposes, to get a clean burn at idle as well as to enhance fuel economy.
If you get the mixture close in open loop, then the computer will not have to make large corrections once you put it back in the loop.
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12-20-2002, 09:29 PM
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John , ..Danno...thanks a buch for the advice you gave me.. You learn something every day. I'll try the multimeter test. Should the 0.84 volts read at idle? sorry for the delay in my reply, I've been busy with work and christmas prep. <img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />
