Open vs. closed Loop, VX - Fuelly Forums

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Old 07-06-2011, 05:16 AM   #1
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Open vs. closed Loop, VX

I hope to shed some light on this topic with a little help from you all. As is common with many obd1 hondas, the idle air control valve (iac) on my VX malfunctions and the car idles high, and occasionally the idle will fluctuate at a stop light when hot. As a hyper miler, I was getting irked by the high idle and surging, so I turned the idle adjust screw way down. Once in a blue moon, when hot, the car will actually settle down into low idle, but usually the iac keeps the motor running around 1000 rpms. I made the decision to disconnect the iac, by simply unplugging the 2 wire connector on the bottom. The new idle is low, I mean way low, around 400 rpms. I had it around 100 but the frequent stalling convinced me to come up a bit. Now however, the check engine light is on. So here are the three questions:
1. Is the car in open loop with c.e. light on and does that mean poor efficiency?
2. What are the negatives effects of low idle?
3. Is there anyway to trick the ecu into thinking that iac is still plugged in, via a resistor perhaps, or just jumping the terminals, so the check engine light goes off?

I will be taking measurements soon of the current, voltage, and resistance across the terminals of the iac to determine what if anything I could use as a jumper to bypass the iac.
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Old 07-06-2011, 05:45 AM   #2
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Re: Open vs. closed Loop, VX

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Originally Posted by Farlander View Post
3. Is there anyway to trick the ecu into thinking that iac is still plugged in, via a resistor perhaps, or just jumping the terminals, so the check engine light goes off?
Is it possible to remove the IAC and keep it plugged in, so that it operates normally but has no effect? If not, you could get another IAC from a junkyard and leave it plugged in.

However, I suspect that (or a resistor or jumping the terminals) would throw a code too when the ECU tries to use the IAC but sees that it has no useful effect. The best course of action would be a proper repair.
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Old 07-06-2011, 06:00 AM   #3
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Re: Open vs. closed Loop, VX

I used a radio shack resistor on my 87 Acura Integra MPFI (I also unplugged my IACV to reduce idle speed after cleaning it didn't cure the problem).

The resistor worked well/kept the check engine light off and I was able to get good mpg (36 mpg on long highway trips).
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Old 07-06-2011, 06:51 PM   #4
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Re: Open vs. closed Loop, VX

I measured the resistance across the iac terminals, exactly 12.5 ohms. I've read the iac uses a +12 or -12 reference signal to actuate, which explains the surging, (up down up down +-+-)

I decided to actually clean the iac and see whether there would be any improvement. Huge success. I took off the iac (only 2 bolts on '94 vx) and you do not have to disconnect the two vacuum hoses to be able to tilt it far enough back to where you can fill up the 2 holes with sea foam. I then let it soak, and after 10 minutes used brake clean to power blast it all out until the fluid ran pretty clean. I also sprayed a little heavy duty silicon in the screened hole to help lubricate the actuator. Car started fine with no problems even though there was still a puddle of sea foam and brake fluid in the bottom of the iac. I will report on the results of longer driving tomorrow. Right now the car idles at around 600, and falls to about 300 with lights on, but there is no feeling of stalling.
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Old 07-06-2011, 06:54 PM   #5
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Re: Open vs. closed Loop, VX

I still want to know about open loop.
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Old 07-06-2011, 09:58 PM   #6
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Re: Open vs. closed Loop, VX

CEL with keep you in open loop, as far as I know.

Also, are you sure that the vehicle is idling at 300? You know about the low mark on the RPM dash gauge that represents roughly 500-550 RPM? Could such a low RPM would be bad for the engine somehow?

The IACV and EGR/passages cleaned fixed this same surging soon after acquiring this vehicle two years ago. Since then, prior to installation of the replacement 02 sensor(5-wire), running power foam through the top end gave another chance to spray towards the IACV in case it was starting to gum up again. No idle problems since, in fact it seems to idle slightly too low full temp with lights on etc. Perhaps by as little as 50 rpm would help the bottom end of the idle smooth out.
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Old 07-07-2011, 08:28 AM   #7
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Re: Open vs. closed Loop, VX

This morning on the drive to work the car drove perfectly, idles fine.
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