question about how idling works

[juntjoo]

what is it that determines how well the engine returns to idle after releasing gas pedal? I just recently replaced my second used ISC which like the other before and my original one didn't pass the pintle cycling in and out test, which led me to believe that whatever it is that is supposed to make it cycle in and out is at least partly due to something other than the ISC, so I installed it and at least at first the car seemed to run the best it has ever but by now it is idling like it did before, steady but a little weak with it coming close to stalling when I let off the gas pedal, ONLY while engine is cold or with the AC on.

so so far I believe I have the healthiest ISC but it with these symptoms it may also not be good but I'm wondering what else could be responsible for the weak idle response to releasing the gas pedal.

as far as my knowledge, and please contribute and/or correct me, you have the coolant temp sensor that tells the ECU to give more air/fuel when cold(maybe I should replace mine?) and the ISC and ECU themselves and the BISS screw that like the ISC affects the idle air flow correct? should I look into the temp sensor and BISS? does it appear my later ISC is also defective and doesn't respond fast enough? should I just fork up the cash a brand new one to be absolutely sure it is or not the cause. what is it that if defective would cause the engine to respond so weakly to idle with the AC on(under extra load)? looking for info on idle operation and opinions from you more experienced DSM'ers. thanks

 

[pauleyman]

ECU has multiple inputs to change ISC position. It has no feedback that it actually got to that position. The ISC has a limited range. This is why it is important to set the idle speed correctly via the biss with the check connector grounded. What that does is remove control from the ECU for the ISC. Only the biss would be controlling idle. This gives the ISC the proper range to operate under various conditions. I see three likely possibilities for you. Incorrectly adjusted idle, bad isc or bad isc drivers in the ECU (a simple visual check may point this out).

 

[Colt4G63 also]

Don't forget about the FIAV! It's an entirely different path for idle air to enter the engine, and works independently of the BISS, and the IAC valve. If that doesn't open properly you'll have your cold idle issues.

 

[pauleyman]

Only reason I didn't mention fiav is there is no way to adjust it. To the OP you should keep the coolant hoses to the throttle body. You must adjust the idle at operating temperature.

 

[juntjoo]

thanks for the replies. good info to work with...

 

[juntjoo]

Only reason I didn't mention fiav is there is no way to adjust it. To the OP you should keep the coolant hoses to the throttle body. You must adjust the idle at operating temperature.

what were you saying in that last two sentences?

 

[pauleyman]

Many folks remove the coolant hoses to the throttle body. I am not a fan of this. The FAIV works in conjunction with moving coolant through the throttle body. Get the motor up to operating temperature before adjusting anything.

 

[DSM1G90]

You need to reset the ISC before doing the BISS.

Read this from VFAQ:

1G BISS Adjustment

Tells it all here how to adjust idle on 1G 4G63 setups.

I reset my ISC then did my BISS after I replaced my ECU. My idle is dead set at 750 at normal op temp.

-DSM

 

[pauleyman]

You need to reset the ISC before doing the BISS.

Yes I've already stated that.

This is why it is important to set the idle speed correctly via the biss with the check connector grounded.

 

[95talongirl]

I'd also try buying a new IAC vs. using a used one, as well.

 

[juntjoo]

Many folks remove the coolant hoses to the throttle body. I am not a fan of this. The FAIV works in conjunction with moving coolant through the throttle body. Get the motor up to operating temperature before adjusting anything.

oh okay. thanks. yeah, I won't be removing any coolant hoses

 

[juntjoo]

so it idles fine w/out the AC on. But with AC on it squeals when cold (confirmed by turning off AC mid-squeal) and struggles to idle and drives like I'm towing a four wheeler, and will die when cold.

it doesn't cycle forward and back from the ignition "off" to "on" like it's supposed to but neither did two other ISC's I've tested AND again all have provided a good idle but only without the AC on.

since it appears the ISC functions fine w/o the AC, would it be safe to assume either:

1) ISC not receiving signal that AC is on or

2) AC is problematic

and if this would be a correct assumption how would I test both scenarios? thanks a lot guys

 

[DSM1G90]

You could simply adjust your idle speed up to 900 RPM with the BISS screw so you can run the AC at idle...not going to bother anything...

Sometimes, you gotta cheat a bit.

Really have to have that AC belt pretty tight since the compressor load is working the belt from the crank pretty hard..to stop the squeal.

 

[pauleyman]

You could simply adjust your idle speed up to 900 RPM with the BISS screw so you can run the AC at idle...not going to bother anything...

No you can't. The solution would only be temporary. The ecu searches for the correct idle. This is the function of the ISC. This is also the reason you ground the check connector to set idle. It removes the ISC function so that you can set the biss and give the ISC maximum range of adjustability for all sensor inputs. You have idle change based on power steering, ac, various electrcial loads put upon the alternator. All at idle.
Did you set the base idle correctly and are there no leaks at the throttle body or intake manifold? If that's true some datalogging may be in order to see what the ecu is telling the isc motor to do. Here's the catch. The ECU does not know if the isc actually did it. So if the ISC is bad nothing happens. ISC drivers are often damaged when the ECU caps leak. Did you check that? All you would be able to do is a simple visual check. If that passes you'll have to assume they are working. If you see any damage at all I'd assume there is some and it needs to be addressed.

 

[juntjoo]

No you can't. The solution would only be temporary. The ecu searches for the correct idle. This is the function of the ISC. This is also the reason you ground the check connector to set idle. It removes the ISC function so that you can set the biss and give the ISC maximum range of adjustability for all sensor inputs. You have idle change based on power steering, ac, various electrcial loads put upon the alternator. All at idle.
Did you set the base idle correctly and are there no leaks at the throttle body or intake manifold? If that's true some datalogging may be in order to see what the ecu is telling the isc motor to do. Here's the catch. The ECU does not know if the isc actually did it. So if the ISC is bad nothing happens. ISC drivers are often damaged when the ECU caps leak. Did you check that? All you would be able to do is a simple visual check. If that passes you'll have to assume they are working. If you see any damage at all I'd assume there is some and it needs to be addressed.

yeah, no leaks, adjusted BISS with check connector grounded and ECU has no visible issues.

so data logging won't help? what about the AC compressor? thanks Pauleyman

 

[pauleyman]

please re read my post.

 

[DSM1G90]

..and those ISC drivers are sensitive - doesn't take much to knock one out.