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Join Date: Mar 2005
Location: Indidnapolis, IN
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Setting ECU idle Mixture

I need to set the ECU idle mixture since I did a bunch of work (replacing seals, gasjkets, hoses etc... ) I also have a new MPS.

The car idles like crap! Hunts surges, etc.... I would like to check the idle mixture setting because I think it is pretty rich.

Based on what I've read, the procedure is to get the car thoroughly warmed up and then set the mixture with a %CO monitor and adjust the idle screw (iterative process I'm sure!).

I would like to do this myself and was looking at the gunson tester in order to set the mixture. I know they are finicky but I think it would work.

However, I have been itching to buy a wideband O2 sensor for tuning my other car. So my question is can I use the O2 sensor (with the tailpipe fixture for the O2)?

There is a chart that seems to relate %CO to an A/F ratio (14.1 seems to ring a bell for 3%CO). Will this setup get me where I want to go??? DO I need to disconnect the smog pump or does the 3% CO numbner take the extra air into account.

Thanks in advance for your help.
Old 03-30-2005, 03:53 PM
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The 3% number does not take EGR, supplementary air (from the smog pump), or catalytic converter action into account. You are supposed to bypass all of those emissions-control devices. I don't know what AFR it compares to, but I would think that setting your idle up for stoich (14.7:1) or a bit on the rich side (14:1) would be more than good enough.

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Old 03-30-2005, 05:08 PM
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Do you have the Haynes manual? If not, here's some info from the manual on how to adjust the idle speed and CO plus a couple links to information on the proper range for air/fuel ratios.

Haynes

http://www.aircooled.net/new-bin/viewproductdetail.php?keyword2=LGU0016&cartid=

http://www.widebandcommander.com/faq.htm#2
Old 03-30-2005, 06:50 PM
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I recommend you go to a mechanic with a shop-quality emissions analyzer and do your idle mixture adjustment setting there, with the engine FULLY warmed up. Bring along your Gunston or other meter to cross-calibrate if you want.
Old 03-31-2005, 07:34 AM
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Actually, the best thing to do would be to put your car on a dyno so that you can check the air/fuel mixture throughout the entire rev range when there's a load on the motor. Here's a graph (wheel hp) of a stock 2.0L.

Old 03-31-2005, 08:56 AM
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I'm doing the same thing on mine right now. I have a Gunson tester and it works great, I set my idle at 3% and that is perfect. Non-load at 3k dropped it down to the 1.5 percentile area. I was told by one of the guys on the 914club site that under load I should be in the 4% area. This is fine for me.
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Old 04-06-2005, 04:56 AM
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