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Old 01-03-2011, 09:27 AM   #1 (permalink)
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Join Date: Nov 2010
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Default fluctuating idle on cold start

93 NA stock engine expect for a randall style intake. I park outside and it's pretty cold out. Car starts up fine, but before the engine is fully warmed up, my idle would fluctuate, is this normal? I did a google search, but could only find idle fluctuating problems for turbo miatas or it would be something else not really related to what my car is doing.
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Old 01-03-2011, 09:53 AM   #2 (permalink)
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Join Date: Sep 2009
Location: Toronto
Posts: 650
Default Re: fluctuating idle on cold start

Loose vacuum lines?

When cold, the engine runs in closed-loop, meaning it runs off a pre-set tune from the ECU.

When warmed up, it's in open-loop, meaning it tries to remain at stoich while taking readings from the oxygen sensor (which is finally warm), as well as a few other sensors. If these sensors notice something off, it'll try to compensate, possibly a loose vacuum line, making the car run lean, so the ECU has to compensate and try to run rich. It runs too rich and it again tries to compensate, this time overcompensating because of the loose vacuum line and runs too lean, causing your idle to fluctuate.
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Old 01-03-2011, 11:24 AM   #3 (permalink)
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Default Re: fluctuating idle on cold start

I'll check the vacuum lines later. Wouldn't a loose vacuum line cause it to fluctuate all the time? Mine only does it when the engine is cold.
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