VX II Auto Cold engine idle erratic/surging

[kais3r]

Hi, I've been trying to sort this issue for a while but am not sure what to try next.

The engine will rev higher than normal when cold, usually in the range of 1200-1700, typically at 1500 approx, this goes away once the engine comes to temperature or has been turned on and off a few times before driving (takes a fair few tries and a couple mins to settle), during the time the problem exists the revs will fluctuate at times jumping by a couple hundred (usually upwards) also pushing the throttle does at times make the revs sit higher than they were before, though not as high as I pushed them, eg. 1500, rev to 2500, drops back to 1700.
Throttle cable and associated parts are all ok, this seems to be isolated to the IAC motor.
Engine will also keep the minimum revs at the elevated level during driving.

There are no vac leaks and I have tried another 2 used IAC motors with the same effect.
I have had the IAC disconnected from power during correct cold idling and it will not fluctuate and will idle fine, except understandably low revs when engine gets warm as it requires more air flow when hot.

This proved to me where the issue was, however having tried a few IAC motors I am suspecting this may be a faulty PCM.

Are there any other worthwhile things I could try first?

If it is the PCM, am I able to replace it without linking it to the BCM, do I need to or can I use the chip from the old in the new, and are there any systems that won't function like the stock security system if the PCM were replaced and linked up with the BCM?

Main reason I suspect the PCM is that the engine revs jump distinctly during idle at times like the PCM is managing it and enforcing higher revs where it will remain unless restarted or sometimes revving the engine.

Happy to do some more diagnosis if anyone thinks It worthwhile and has a suggestion...
I have software and cable to monitor sensors and IAC etc.

Thanks in advance.

 

[Nut Kracker]

When installing a new IAC, certain re set procedures must be used
to ensure proper operation......

 

[kais3r]

Hi, thanks for the tips, I have tried those things, the problem is the revs keep fluctuating and/or rev too high even during the learning phase for idle control.
Even after I do the reset correctly, the problem still exists afterwards, the only thing that seems to eventually resolve it is a warm engine.
an example of what its doing:
start, revs to 1500, eases down to 1000 in 2-3sec, surges up to 1300 and other similar revs for a few seconds and drops back down for a second, and keeps repeating as if it can't decide where it wants to sit.
any ideas?

 

[Pollushon]

I've always laughed at these crazy arsed write ups on IAC installation procedure. Make sure the dick isn't too far out (I usually move it right in), screw it up and the ECU does the rest.

Sounds like the IAC is fine just something is giving it the wrong info cold. Had a similar issue on mine that was resolved by a new temp sensor (for the ECU). Seems when they're getting on in age they can't tell low temp deltas very well and become more of an off or on sensor.

 

[Nut Kracker]

Other components may need checking....
The IAT (intake air temperature) sensor.
CTS (coolant temperature sensor).
o2 (oxygen sensors).
Could also be a vacuum leak somewhere.
Try checking for Error Codes

Also have a look at This Forum Particularly Post #2

 

[Nut Kracker]

I've always laughed at these crazy arsed write ups on IAC installation procedure.

This "crazy arsed write up" comes directly from the Holden Factory Service Manual.

 

[Pollushon]

Other components may need checking....
The IAT (intake air temperature) sensor.
CTS (coolant temperature sensor).
o2 (oxygen sensors).
Could also be a vacuum leak somewhere.
Try checking for Error Codes

Also have a look at This Forum Particularly Post #2

IAT is a good suggestion, although a faulty IAT will usually impact cold or warm much like a vacuum leak and doesn't really care about the engine temp. O2's run open loop until the engine reaches op temp so it couldn't be them.

Got to agree, checking for error codes is probably the best start.

This "crazy arsed write up" comes directly from the Holden Factory Service Manual.

Not having a go at you mate. I have that exact manual and I've never once opened it to that page. Just saying it's a lot of words to say, shove pintle in, install, start car, profit.

 

[kais3r]

Ok thanks for the info guys,
I have checked error codes and they are all clear, I then tried cold idling with each sensor disconnected separately (IAT, TPS, Coolant) none of which changed the fluctuations in idle revs or the high RPM.

I did test something new though and found that if I turn the ignition on and off without starting numerous times and then starting the engine, it was suddenly all good, this I'm guessing leads back to either the IAC or PCM as the issue as they are the only two things which either move or reset during ignition on off cycling.. Any ideas?

The IAC and housing are clean and rubber O-Ring is fine and have tried 3 IAC valves, so I'm guessing maybe the PCM is faulty and needs a few power cycles to work right....

Revs are at about 1k and stable with cold engine when it works after cycling ignition.

 

[Nut Kracker]

Got to agree, checking for error codes is probably the best start.

I think the OP will have a heap of codes now, after disconnecting the IAT, TPS and CTS.

I then tried cold idling with each sensor disconnected separately (IAT, TPS, Coolant) none of which changed the fluctuations in idle revs or the high RPM.

Not having a go at you mate. I have that exact manual and I've never once opened it to that page. Just saying it's a lot of words to say, shove pintle in, install, start car, profit.

That's OK mate. It is a lot of words to say for a simple procedure, but it keeps someone in a job.
It's just easier to post that, than to have to type it out.
Besides, it looks more official and people tend to believe it more, if you say it's from the FSM
and not just out of someone's head lol.

 

[kais3r]

I think the OP will have a heap of codes now, after disconnecting the IAT, TPS and CTS.


.

Yeah till I clear them, as I said there were none prior to disconnecting the sensors.

Looks like I may need to try a new PCM, can I do this without needing to link it with the BCM?
Just not sure If it's the chip on the PCM or the PCM itself that holds the link for security system etc, so I could reuse the chip and hope a new PCM fixes it.