Surging at Idle

[Scarlet2003]

Well, I have changed all plugs and all spark plug wires and the surge is still there. Also tred 89 octane vs. 87 on the vapor lock theory. Still does it. Also tried removing the K&N air filter and replaced it w/ a Purolator paper filter. So far it seems to be ok.... Keeping my fingers crossed....

[CREWZIN]

Did you try cleaning or changing the IAC (Idle Air Controller) in back of and attached to the throttle body?

[Scarlet2003]

I did have the throttle body cleaned a few days ago - would that be part of that service or would I have to ask the Tech to do that?

It is 90 here today and the car is still surging.

[CREWZIN]

It's hard to say from here if they touched the IAC or not. Don't know what else to suggest except to spend some money and have it diagnosed with the latest equipment form a skilled tech. Take whoever you choose for a ride and let them see what's happening.

[slartyblarfast]

Usually surging is a sign of a vacum leak. Also could be a/c compressor on and off. How bad is the surge?

[Scarlet2003]

Slarty - it feels like it about to stall in gear at idle w/ the AC on and is very disconceritng to my wife (her car not mine) especially w/ 2 little kids in the car. The dealer is an idiot. Been there twice and all I get is, "Duh, not throwin' a code". It did not do it all winter (or what passes for winter here in NC).

[slartyblarfast]

Ok.... It only does it with the a/c on? Does it feel like it's chugging and then al of a sudden it idels up fast and then smoothes out? If that's the case then I'm pretty sure that your cooling fan is inoperative. The a/c compressor comes on and the head pressure builds too high, the engine lugs down and the compressor turns off. I'd bet that the a/c also doesn't work real good at idle? It's also probably intermittent?
A dollar to a doughnut that's what the problem is.

[snovak]

Sorry for bumping a thread that's a few years old. Was wondering if there was ever a solution to this issue. My wife's '04 PT GT (~75K mi) has the same symptoms. I've gone through most of the threads on the subject. Yesterday, I...

• pulled off the throttle body, cleaned it.
• Pulled out the IAC, cleaned it (didn't look real bad, a little soot, nothing serious).
• MAF looked clean.
• Radiator fan works, is there a separate condenser fan?

My wife was driving it for a while with a check engine light, PO551, Power Steering Pressure Switch. I replaced the sensor and cleared the codes and they're not coming back. So, that little issue is fixed, but I'm pretty sure it's unrelated to the idle rpm surging. And similar to the posts above, the surging only occurs with the A/C on. Off idle, she runs fine.

[CREWZIN]

Somethng that hasn't been talked about and even I forgot to add is that the PCM adjusts the idle when the compressor is engaged. The idle is is raised by the PCM to compensate for the power needed by the compressor. If it's not ding that, the RPM's will drop considerably and seem like it's going to stall.

I don't know the in's and out of how that works but it's in the programming of the computer. Slarty mentioned something about it when he said it might be the fan but on mine,and most PT's) the on and off of the compressor is hardly noticed because the computer compensates for any power needed by the A/C.

[chasinpr]

This sounds like what my cruiser has started doing recently. What is the PCM? How do I confirm this theory?