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Old 04 Dec 2009, 11:46 am
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Default why would a 94 civic ex 5 speed start pulsing from 1000 to 1500 rpm in neutral?

i drive a 94 civic ex 5speed and every time i crank it up it pulses from 1000 to 1500 rpm and keeps going. when i slow down and put it in neutral it also does it. the only thing i have done to the car is put a cold air intake on it. it has never done it before. i have replaced the idle control valve and the fuel regulator valve and still cant figure out where the pulsing is coming from. can anyone help me?
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Old 04 Dec 2009, 12:01 pm
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the idle control valve works by moving a plunger in and out of a passage that allows air to bypass the throttle plates. The more air that is allowed to bypass the throttle plates, the higher the idle; the less air that can bypass the throttle plates the lower the idle. the idle air control valve constantly varies the position of the plunger in the bypass port as the engine loads change to provide a constant idle.

if you have replaced the IAC valve, the chances are that the port is plugged with carbon (think of soot in a chimney). That is a very common problem on all cars. When the passage is blocked, air can't flow through the bypass passage so the IAC can't regulate it. The solution is to clean out the idle passage using the spray carburetor cleaner or throttle body cleaner (they are the same products, just different labels). That will dissolve the blockage and allow air to flow through the passage where the IAC can again be effective in regulating it.

if the passage is clear, then the only other thing that will cause the problem is a vacuum leak. that could be from a loose or rotted vacuum line or a leaking intake gasket. You are looking for something that is after air goes through the throttle body (that eliminates your cold air intake) so it would be any ported vacuum line connected to the intake or the intake mainfold gasket that would be suspect.

I think you're going to find the idle air passage plugged with carbon.

hope that helps
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Old 04 Dec 2009, 01:01 pm
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i had a very similar problem with my 95 accord,one problem could be the idle screw,try unplugging the sensor located on the back of the intake allowing the car to drop in idle and rise back up,then plug it back in,if it corrects the idle,you might need to loosen that screw.Hope it helps.
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Old 05 Dec 2009, 03:50 pm
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Default Re: why would a 94 civic ex 5 speed start pulsing from 1000 to 1500 rpm in neutral?

Low coolant level will also cause motor to surge. Lots of people forget to check their coolant and always think its something mechanical.
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Old 05 Dec 2009, 05:42 pm
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Default Re: why would a 94 civic ex 5 speed start pulsing from 1000 to 1500 rpm in neutral?

I have a 96 accord that started doing the same thing a few days back. I notices that the temperature goies all the way up an then comes back to normal abrubtly. Then the revving in neutral stops.
I'll have the oils and cooland changes this monday, ope its just this!
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