[Eldyfig]

I plan on changing out my ISC. I have been having some sporadic rough idle lately. Thought my one of my last two original coil packs might have been the problem. Well, now all four coil packs are new. Thought that was going to fix the problem. No.

I am not upset that the new coil packs didn't fix it. Two others were less than a year old. Now all four are new and that makes me feel confident in their performance.

This evening I was in the PCM Data menu again. At idle, I noticed my TPS degrees at idle going from -2.2 up to -1.8 and a little higher whenever the idle rpm jumped. This was PD01 in the data section of PCM. I went under the hood and watched the ISC. Sure enough, the ISC plunger was pushing on the throttle lever and clicking, making the idle jump.

To my question...I am looking for the part and price. On gmpartsdirect, part #17112866 is $85 shipped. What brand is it? I would assume acdelco. However, when I go to the ACDelco site http://www.acdelco.com/html/catalog/index_auto.htm, I can't find the part or the part #. I am trying to find the parts locally. Like with the coil packs, I found a store that had delcos, fortunately. They were the only one locally, and they had to drive them from their warehouse on the other side of town. So far, for the ISC I have not found acdelco. I have found Borg Warner, Niehoff, and GP Sorensen. I have tried a GP coil pack on the north*. Not good.

So what brand is the recommended? What comes from gmpartsdirect?

I have searched some posts on the ISC. Tell me if I got this right for the North*...

Take the old one out.
Put the new one in.
Start the car.
Push plunger all the way in.
When it retracts, unplug it.
Set the plunger .030 away from the throttle lever.
Plug it back in.
Do an idle learn procedure.

Is this correct for the North*?

 

[Eldyfig]

For any that are interested...I went to Rockauto.com and found that the ISC part #17112866 listed on gmpartsdirect is an acdelco part and the actual acdelco part # is 217425. Rockauto offers it for $83 shipped.

 

[WarrenJ]

It sounds as though your ISC is working just fine. The question is: why is the PCM commanding the ISC to change the idle speed?

A bad ISC would not account for a rough idle (unless your idle dropped below 475rpm), although it would account for the wrong idle speed.

More likely culprits include a dirty throttle body or EGR valve.

Regards,
Warren

 

[Eldyfig]

Of course I have checked the throttle body, easy stuff first is my philosophy.

The egr is fine. Egr pintle valve position when engine is off, engine at idle , or at WOT should be 44 in PD07 of the PCM Data menu. Egr valve should open during acceleration. This is how mine is functioning. At idle, when my rpms start flickering, my pintle valve position is 44.

A bad ISC would not account for a rough idle (unless your idle dropped below 475rpm)

This would be the case if the plunger was not extending properly.

although it would account for the wrong idle speed.

If the plunger was extended too far.

OK, but what can make the plunger extend and kick back slightly, causing the rpms to jump and fall? Could this be a sign of an ISC that can't hold its signal?

Thanks for the reply, Warren. Much appreciated.

 

[WarrenJ]

Of course I have checked the throttle body, easy stuff first is my philosophy.
OK, but what can make the plunger extend and kick back slightly, causing the rpms to jump and fall? Could this be a sign of an ISC that can't hold its signal?

I like your philosophy!

I won't deny that it's possible for the ISC to stick in such a way as to respond erratically to PCM commands, but that would not account for a rough idle. It would only account for an incorrect or varying idle speed. Some refer to it as "surging." I'm guessing you have to look elsewhere for the cause of the "roughness." Whatever is causing the rough idle may also be causing the PCM to incorrectly command the ISC to change position.

The ISC is basically a motor coupled to a reduction gear that operates the plunger. As such, it can't "lose" its memory. If the drive signal is removed the ISC will simply remain at its current position forever.

An intermittent vacuum leak might allow for a momentary change in idle speed; so would a noisy Throttle Position Sensor. But not that rough idle.

As always, I could be wrong. But I can't see worrying about the ISC until that rough idle problem is no more . . . .

Regards,
Warren

 

[Eldyfig]

Maybe I am wrong by stating that the idle is rough. I should say the idle is actually really smooth, but every once in a while the rpms will fall a hair and then kick up to 800 the setlle back out again. I noticed the TPS voltage changing and the ISC plunger moving when this happens.

I need to look into noisy TPS. If I had an intermittent vac leak causing this, why would the ISC plunger push on the throttle lever?

I went ahead and pulled the EGR valve and cleaned it. Even though the pintle valve position data looked good at 44 counts and 0% rescaled, I decided it couldn't hurt to use a cleaning. After I reinstalled it, it sat at 43 counts, still 0% rescaled. This is where it should be at idle. During accelaration, it moved fine and settled back correctly when the engine came to idle again.

 

[Eldyfig]

Did an ISC check through the OBD. Found out at PS03 (pcm override) the the pcm does have normal control of the ISC motor. Went to PO07 (pcm outputs), plunger retracts all the way in when commanded to do so. At this point, ignition control kept the idle at 475-500. If the rpms were above 700, a vacuum leak would be suggested.

So...no vacuum leak, verified again. Since my idle was under 700, next step is to check minimum idle values.

 

[Eldyfig]

ahh...screw my last post about no vac leak. It is intermittent and it reared its ugly head today. Finally, it continued even after I parked the car. Drivability was not effected at all.

Got my stethoscope on my ears. Found the whistle under the intake on the left front side of engine (passenger/ firewall side). Found it when I touched the spacer between the seals.

I will tackle this intake first before I worry about the ISC. Kind of like what you said, Warren. This would be the wisest decision. I do have a couple of more ISC tests through the PCM to complete when I have the time.

 

[Eldyfig]

Just got home from work and did a final test on the ICM. Measured the difference between the TPS minimum idle value and the TPS maximum extend value. My minimum is -3.0, my maximum is 7.5. My difference is 10.5. The correct range is 10-11. So far it checks out ok.

 

[Eldyfig]

No one locally has the intake gasket (part #MS93225). I will have to order it and do it next week. I don't want to reuse the old ones since they are probably original. For the time being, I will check the torque of the 4 bolts holding the intake down.

 

[Eldyfig]

Started torqueing them at 5 ft/lb and they were turning. When I got them to 5, I went on and torqued them to 7.5. I only took a short drive after I did this. Everything seems ok. I will know more this week when I do a little more driving. Maybe that will be all it needs.

 

[WarrenJ]

ahh...screw my last post about no vac leak.

Sounds like you may have it nailed.

Please don't forget to let us know the final outcome. And, thanks for tracking this for us. It may well help others.

Regards,
Warren

 

[Eldyfig]

Well, it has been a few days since I torqued down the intake to where it is suppose to be. No troubles at all so far. I guess I did nail it. No fluctuating rpm, no rough idle, no whistle.

I am still not positive about why the ISC plunger was pushing out on the throttle lever. My take on it is that the intake leak caused the rpm to jump up. The PCM, sensing this, maybe tried to lower the rpm? When the intake leak, being very sporadic, stopped, the rpm fell off more than the PCM was shooting for. The PCM sensed too low of an rpm and kicked out on the plunger, overcompensating some.

Does this sound logical or am I overdoing it. Engine is fine now. So be it. Never bought the ISC, never ordered an intake seal, so no money spent.

Thanks, Warren, for your input on this thread. It was much appreciated.

 

[WarrenJ]

Does this sound logical or am I overdoing it. Engine is fine now. So be it. Never bought the ISC, never ordered an intake seal, so no money spent.

I remarked earlier that I liked you philosophy regarding "easy stuff first."

I'd add a second axiom to your toolbox: "Never argue with success!" :cloud9:

Glad you solved it; it must have been maddening.

Regards,
Warren