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.

[David T]

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.

No problem. I will refer you again to the Workshop Manual D:05:02 fig 38 for the thermistor for the idle circuit and D:04:11 fig 31 for the thermal switch in the Lambda system. Easy to confuse the two switches. I haven't checked but it may be possible to mix them up and hook each other's wire to the wrong one too. While they look similar and are supposed to operate at the same temperature, they have different part #'s. I don't know the consequences of mixing them up.

[NckT]

David, I was being a bit facetious. The thermister is basically varying resistor in relation to temperature (ie NOT is switch) and is a 2 pin connector. The lambda switch is digital ie on or off and is a 1 pin connector on the switch ie a blade connector. You cannot physically get them swapped over by accident.

In addition, Rob Grady is correct in both responses and the air to fuel ratio (mixture) will become richer with the frequency valve disconnected, not leaner.

At the end of the day the op had resolved his issue, which is great news.

[Ron]

No problem. I will refer you again to the Workshop Manual D:05:02 fig 38 for the thermistor for the idle circuit and D:04:11 fig 31 for the thermal switch in the Lambda system. Easy to confuse the two switches. I haven't checked but it may be possible to mix them up and hook each other's wire to the wrong one too. While they look similar and are supposed to operate at the same temperature, they have different part #'s. I don't know the consequences of mixing them up.
Fig 38 shows the right location for the thermistor but pictures the lambda thermal switch.
Hard to hook up wrong -- The plugs are different, single large spade vs two smaller spades.

[PJ Grady Inc.]

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!

Wow that is one weak throttle spring! Idle spring/rod adjustment issues don't usually leave the idle that high. FWIT we have a 10% stronger spring available at about $10- less than the NOS cost.
It is a little pricey either way but it's not worth risking damage or an accident on a $30- part.
Rob

[PJ Grady Inc.]

David, I was being a bit facetious. The thermister is basically varying resistor in relation to temperature (ie NOT is switch) and is a 2 pin connector. The lambda switch is digital ie on or off and is a 1 pin connector on the switch ie a blade connector. You cannot physically get them swapped over by accident.

In addition, Rob Grady is correct in both responses and the air to fuel ratio (mixture) will become richer with the frequency valve disconnected, not leaner.

At the end of the day the op had resolved his issue, which is great news.

Thanks Nick but I was just trying to let David be right for once even though he was wrong lol!
Rob

[David T]

Thanks Nick but I was just trying to let David be right for once even though he was wrong lol!
Rob

I may not always be right, but I am never wrong! (As long as we are being factious!). As for the A/F ratio, if the motor was running well and was properly adjusted, losing the Lambda system and the frequency valve, the mixture will be leaner. The Lambda system adds fuel to the leaner, base mixture to get to the right A/F ratio. That's why, when the system doesn't work properly (or not at all) the first impulse is to adjust the mixture screw to enrichen the A/F ratio. Another reason commonly used to mess with the mixture screw is to overcome vacuum leaks which lean the mixture.

[Ron]

I have to agree with David WRT disconnecting the FV making it leaner -- Current through the FV decreases the pressure in the FD lower chamber......

[gullwinger]

Freq valve is controlled by the WUR and the WUR (warm up regulator) is controlled by a thermistor under the intake. There is a vacuum line under the WUR on the right side see if it has air coming out of it at idle. If so clean the screens on the WUR. The freq valve adds more fuel to the distributor when the car is cold.


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[Bitsyncmaster]

I have to agree with David WRT disconnecting the FV making it leaner -- Current through the FV decreases the pressure in the FD lower chamber......

When I get time, I will have to hook my wideband display up and do that test (unplug the FV).

[Ron]

When I get time, I will have to hook my wideband display up and do that test (unplug the FV).
Cool! It would be interesting to know how it could go rich.
My understanding from the manual is that the ECU normally holds the duty cycle at 70% (open 70% of the time) and, as in a manual example, raising it to 80% enriches the mixture. If you disconnect the FV, it would be open 0% (always closed). If the valve is closed, no fuel/pressure would be bled from the lower chamber. The resulting higher pressure would reduce the normal deflection away from the injector ports in the upper chamber, causing less flow through the injectors. The O2 sensor would see it go lean but it wouldn't be able to communicate with the disconnected FV.
...The temp would be virtually the same, so the WUR wouldn't change the Control Pressure and the thermistor wouldn't signal the ECU to change the Idle Speed Regulator. WOT and acceleration enrichment systems are out -- I can't think of anything else able to change it...

[Bitsyncmaster]

It's been a long time since I connected Logworks to my wideband. I finally found a download for my new laptop and it actually worked.

My car is displaying about 14.2 AFR just sitting at idle. I did not really warm up the engine much and the AFR is swinging from about 13 to 15.

I unplugged the FV and engine almost died but it did stay running displaying 20.5 AFR. So with my test is it did go lean when the FV was unplugged.

[NckT]

Well I got that the wrong way round, my mistake !

It's when I fix the lambda circuit/ computers and reinstate the frequency valve's operation it goes rich and it had to be leaned out accordingly.

Thanks for taking the time out to use your wideband o2 sensor :)