SOHC: 2000rpm Idle and Stuttering

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I have an 89 240sx that was sitting for about 2 years. I put a manual transmission and ECU in it, replaced the fuel filter, air filter, PCV valve, plugs, and wires.

It was running great (for abour 3 months). Now it wants to idle at 2000rpms. And when driving at a steady RPM, it seems like it cuts out or surges.

Is there a "more often than not" cause or solution? The car has 95k miles on it.

Thanks for reading.

[NISTECH]

check your tps setting at op temp. The closed throttle switch might be off instead of on at idle.

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Thanks for the reply. I'll have to check the manual to locate and test those.

I've notice that the ide works it's way up to about 2100rpms once the needle on the coolant gauge is about 1/4 of the way.

I've cleaned the AIV, and put some carb cleaner through the 1/2" hose that leads to the air valve and IAA.

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When it's cold, it starts up to about 1800rpms, then goes down to 1100rpms and then as soon as it warms up (coolant needle around half), it goes right to 2200rpms and stays there.

I checked the ecu and it gave a code of 55 no malfunction.

[NISTECH]

What you are saying here is it doesnt do it till the fuel control system goes into closed loop. the only sensor the ECM doesnt look at during warm up is the O2 sensor. It may be the sensor is phase shifted lean and its adding too much fuel. You can usually see a phase shifted O2 with a simple volt meter if you watch it its fluctuation will usually not climb above around .5 to .6 volts and drop to 0 or in some cases below 0. Try giving that a look.

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The A/F on my multi-checker is about 25 when at idle and then it goes as low as 10 when driving.

[NISTECH]

I have no Idea what this multi checker is your reffering to.

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Here's a link: Apexi Multichecker

[NISTECH]

ok that is a really... hmmmm.... "Neat" tool but the measurements it takes are usless to me since it does not take measurements that coincide with service manual info. I have no experiance using a device like that. From my point of veiw I could use another word to describe that flashy device.

You need to use a volt meter or a graph on the O2 wire it self to get an ACCURATE interpitation of what it is doing.

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So just tap into the O2 to get the voltage?

[NISTECH]

yes while it is still connected

[NISTECH]

you want to look for its max high voltage and its min low voltage.

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OK. I'll check it out tomorrow. Someone had suggested taking off the air valve at back of the intake manifold and cleaning it. -Saying that it might be stuck open, too.

[NISTECH]

that would not cause it to idle rough but idle high instead. Its air supply still comes from the intake ducting which is measured by the airflow meter if it were stuck open the airflow meter would see the additional air being drawn in and compensate by adding more fuel.

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If I pull the vacuum hose off of the fuel regulator, there is no change in idle. I hooked a vacuum gauge up to the hose and it had no vacuum. I don't know if that is normal or not.

When I unplugged the temp switch, the idle went from 2200rpms to about 1800rpms.

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The temp sensor, throttle plate sensor, IAA valve, throttle sensor, and air valve seem to have the right resistance. The O2 sensor, when I put the voltmeter on it seemed to stay pretty low as in below .5. So...I picked up an O2 sensor. Hopefully, NISTECH is right.

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Well, I put a new Bosch O2 sensor on and a new cap and rotor. It seems to run smoother, but still idles around 2000rpms.

[NISTECH]

moniter the new O2's voltage was there any change? If not you need to recheck for vaccum leaks. there would have to be one somewhere if that voltage is still low. low voltage indicates a lean condition.

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Thanks for your help. Where is the best place to put a vacuum gauge? When I unplugged the vacuum line from the fuel pressure regulator at the end of the rail, there was no vacuum.

[NISTECH]

check the other end of the hose see where it comes from. You should have vaccum on that hose at idle. if it doesnt have vaccum it is richening the mixture up a bit. I just realized you said something about that in an earlier post. sorry I didnt catch it. you need to find the reason that line does not have vaccum as that is likely your problem, not the fact the pressure in the fuel system is up,thats just a symptom. the vaccum loss is your problem.

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I replaced a lot of vacuum lines. I noticed that if I unplug (power wire) the AAC (auxilary air control) valve it idles and seems to run fine. Does it need to be plugged in? Is it just to stabilze idles for A/C or something? I also noticed that unplugging (power) the Air Control valve did nothing.

[NISTECH]

when you unplugged it the idle dropped down to normal?

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Yes. It idles smooth and fine with it unplugged. I did notice that when the engine is cold it doesn't idle high to try to warm it up. At that time it's not stable but once it's warm it is.

[NISTECH]

then possibly the aac valve is your problem as mentioned by someone else.

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Does it hurt to leave it unplugged? The only difference that I notice is when the engine is cold.

[NISTECH]

you wont see much of a difference. just no high idle cold as you mentioned and a dip in rpm when you turn on an accesory

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Thanks for your help!