Idle sporadically jumps to 2000 RPMs

[villevol666]

Hi All,

I am having an issue where my idle will randomly jump to 2000 RPMs. It will do so for a few seconds, or for 30 minutes. The problem subsides if I unplug the frequency valve. I haven't noticed yet if it is buzzing while it is happening, but I believe it is not.

Besides suggestions on what the problem might be, I'm hoping someone can answer me the following:

What exactly is the frequency valve doing? Not just tuning the fuel mixture, but step by step, how does it decide when to turn on and off?

And when it does turn on, what other sub processes are running because of the request to turn on?

Is the point of the valve to lean or enrich the fuel mixture? Both?

And lastly, I'm getting the car smogged this weekend, no way it will pass with the RPMs shooting to 2000, any chance it can pass without the valve?

Edit: Bulkhead connectors have been thoroughly cleaned.

Thanks,

Brandon

[kbrown84]

i would check the throttle micro-switch, to make sure the idle motor circuit is working properly.

[PJ Grady Inc.]

Hi All,

I am having an issue where my idle will randomly jump to 2000 RPMs. It will do so for a few seconds, or for 30 minutes. The problem subsides if I unplug the frequency valve. I haven't noticed yet if it is buzzing while it is happening, but I believe it is not.

Besides suggestions on what the problem might be, I'm hoping someone can answer me the following:

What exactly is the frequency valve doing? Not just tuning the fuel mixture, but step by step, how does it decide when to turn on and off?

And when it does turn on, what other sub processes are running because of the request to turn on?

Is the point of the valve to lean or enrich the fuel mixture? Both?

And lastly, I'm getting the car smogged this weekend, no way it will pass with the RPMs shooting to 2000, any chance it can pass without the valve?

Edit: Bulkhead connectors have been thoroughly cleaned.

Thanks,

Brandon

It is probably a poor connection to the idle thermister switch in the VOD. It is hard to access but doable if you have thin hands. The workshop manual has good theory of operation section on the CIS, Idle control and Lambda circuits that you should read before trying to diagnoss engine problems. It will run richer without the F.V. connected so no it will not pass smog.
Rob Grady

[villevol666]

I just found this posted in another thread:

"If you disconnect the entire connector on the passenger side of the idle ECU behind the driver's seat, it will keep the idle right at 2,000 RPM. When you said yours sticks near 2,000 RPM, it made me think of this. Could be part of your problem or it might not be. You might add this sort of test to your troubleshooting to figure it out."

I'll have to test this later, but this is the exact problem I am having.

[PJ Grady Inc.]

I just found this posted in another thread:

"If you disconnect the entire connector on the passenger side of the idle ECU behind the driver's seat, it will keep the idle right at 2,000 RPM. When you said yours sticks near 2,000 RPM, it made me think of this. Could be part of your problem or it might not be. You might add this sort of test to your troubleshooting to figure it out."

I'll have to test this later, but this is the exact problem I am having.

That is what I'm talking about although if the engine is set up properly it will generally rev to 2500 RPM. An incorrect CO adjusment etc could alter that speed however. You need to check for continuity with an ohm meter between the Blk/Ylw and Blk/Slate wires. If it is an "open" or no continuity then it is probably a loose thermister connection in the VOD.
Rob

[David T]

It is probably a poor connection to the idle thermister switch in the VOD. It is hard to access but doable if you have thin hands. The workshop manual has good theory of operation section on the CIS, Idle control and Lambda circuits that you should read before trying to diagnoss engine problems. It will run richer without the F.V. connected so no it will not pass smog.
Rob Grady

Now who is referring to the Workshop Manual. Just kidding but it IS a wealth of information. I agree it sounds like a bad/intermittent connection in the Lambda system. BTW, the motor will run LEANER without the FV unless the mixture screw was adjusted richer to compensate for the FV NOT working.

[PJ Grady Inc.]

Now who is referring to the Workshop Manual. Just kidding but it IS a wealth of information. I agree it sounds like a bad/intermittent connection in the Lambda system. BTW, the motor will run LEANER without the FV unless the mixture screw was adjusted richer to compensate for the FV NOT working.

Ok David but the idle thermister is part of the Idle Control Circuit not the Lambda Circuit. Quid pro quo.
Rob

[villevol666]

Thanks everyone. After reading the manual, I went through the entire system. The ECU and all of the wiring is fine. Turns out the problem was the spring around the throttle control. It has seen better days! For now, I have just extended the screws a little bit to compensate. Will replace as soon as possible.

Thanks again everyone!

[David T]

Ok David but the idle thermister is part of the Idle Control Circuit not the Lambda Circuit. Quid pro quo.
Rob

There is also a thermistor in the Lambda circuit but you're right, the thermistor in the idle circuit would can cause the idle to increase. Any we over-thunk it. The throttle was not returning all the way to idle. It's almost always the simple stuff!

[NckT]

There is also a thermistor in the Lambda circuit but you're right, the thermistor in the idle circuit would can cause the idle to increase. Any we over-thunk it. The throttle was not returning all the way to idle. It's almost always the simple stuff!

Cate to enlighten where the thermister is in the Lambda circuit? The only temperature dependant element I know is the temp switch, which is nothing to do with idle control and this does not effect idle, just the duty cycle for the frequency valve.

You've got to be careful with your past as some may interpret what you've put as a means of covering up the fact you made a mistake in a previous post.