Pagoda SL Group
W113 Pagoda SL Group => Drive train, fuel, suspension, steering & brakes => Topic started by: wwheeler on October 20, 2013, 20:26:47
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I just tested my engine today at idle with a new Gunson tester. All seemed to go OK but I have a question regarding the calibration value. I set it at 2% in the open air per the instructions. So when you take the reading with the probe in the tailpipe, do you take the reading on the display as the CO or do you subtract the 2.0% calibration from the reading?
Reason why I ask is because the lowest reading I can get with the engine still running well is around 5.8%. When I go leaner than that, it runs rougher and the split linkage test shows it to be just turning lean. When I first started the test, the idle was 8% and I used four clicks lean to get it below 5.8%. With only three clicks lean the reading is right at 6% and engine runs OK.
What do you think?
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I took the actual reading on the Gunson, no maths!
Seems ok.
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Same. 5.8 is not a bad number.
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I set it at 2% in the open air per the instructions.
Are you sure that's what it says?
The gauge should be reading zero before you begin to sample the exhaust gasses and you should be seeing around 4% +/- 0,5%.
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OK and thanks. Because Gunson is british made, the instructions are UK english. I have noticed sometimes my translation with technical terms into US english is flawed. So maybe my problem is in the set up and you guys across the pond can help with my translation of the instructions. 5.8% isn't bad except that all indications are that it is becoming lean which should be about 3% or so.
When I hooked the unit up to a 12V battery, I set the unit near the exhaust outlet with the car running and the probe NOT in the tailpipe. You then set at 2%, let sit for a few minutes until the reading stabilzes. At that point you stick the probe in the tailpipe 3/4" the way in and take a reading after a minute or so. That is what I think the instructions said.
It almost sounds like you let the 2% reading stabilze in the open air WITHOUT the car running. Is that correct?
Thanks again!
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I'll see if I can scan the instructions tonight. Again, my translation may be flawed.
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I set it at 2% in the open air per the instructions
: Are you sure that's what it says?
This is right, per the instructions that accompany the device.
Some previous texts from this Forum, see:
http://www.sl113.org/forums/index.php?topic=14235.0 (http://www.sl113.org/forums/index.php?topic=14235.0)
http://www.sl113.org/forums/index.php?topic=12168.0 (http://www.sl113.org/forums/index.php?topic=12168.0)
Some notes to measuring with Gunson: "After inserting probe in exhaust of your running car, wait for a period of 15 seconds for the meter to respond and a further 1 minute to stabilize. Typically, the indicated CO will rise above 2% for a few seconds, then start descending. It will take about 20 to 30 seconds to 'bottom out', before it climbs again. This bottomed out reading is your CO level. It starts to climb again as the cell in the instrument gets contaminated [saturated]".
/Hans in Sweden
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The ambient air around us contains 2% CO - this is thus the [correct] calibration setting for the Gunson reader.
This isn't quite right - I hope that the clean air we are breathing does not contain 2% CO.
The Gunson meter works by measuring changes in thermal conductivity, as the thermal conductivity of exhaust gas will change with CO levels. The 'clean air' setting process is basically you calibrating the instrument, i.e. telling the instrument that when it measures thermal conductivity of 'x' then that is equal to 2% CO in the exhaust gas. It then uses that as a reference when it is measuring your exhaust gas.
The Gunson documentation states that the thermal conductivity measured by the instrument in clean air is equivalent to what exhaust gas with 2% CO would be. What you are doing here is using clean air as a proxy for 2% CO exhaust - if Gunson are to be believed then the thermal conductivity measured is the same.
Instruments like this are prone to error (Gunson quotes an accuracy of +/- 0.5%) and other factors influence reading too, so one shouldn't take the absolute readout obtained as being spot on. They are useful, however, for watching the change as you alter fuel/air mix etc, so they do have their uses.
David
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Wallace,
you also might want to calibrate the zero point as described in the PDF that Jack posted here:
http://www.sl113.org/forums/index.php?topic=950.msg134154#msg134154
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OK! Thanks for the replies. Somehow in my Texan translation I thought that I was supposed to calibrate the device NEAR the exhaust with the car running. So now I understand that it is in air NOT around the exhaust and NOT with the car running.
I think now the readings will be on the money. As with many things on an old car, the factory settings are a good starting point and you fine tune to your car.
Thanks for the help and the extra documents!
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This isn't quite right - I hope that the clean air we are breathing does not contain 2% CO.
You are right of course David I have removed that text string /Hans