For the past 2 months or so I've been trying to track down why the idle on my GTO occasionally hunts. I'm at my wits end and am hoping someone may be able to provide some insight.

So far this is what i know and what I've done..

- The surging only happens once the engine compartment is nice and hot. It is not related to engine temp and not related to entering closed loop.The engine can get hot, and enter in closed loop without the surge. It's not until I raise the idle for a while or drive it for about 10 miles before it will surge. Pretty much waiting for the engine bay to get heat soaked I assume.

- At first it only happens when the A/C is turned on, once it's running longer it happens all the time regardless of A/C.

- When cold I can control the idle speed with the scanner. Once the surging starts, commanding the IAC open will no longer change the idle speed. (this is what caused me to replace the IAC)

- I've removed the throttle body and thoroughly cleaned the passages and even replaced the IAC valve with a new AC delco part. No change.

- The IAC counts follow the surge as does the Spark Advance. It changes from 17-7* and causes the idle to surge even with the IAC disconnected.

- I've tested for vacuum leaks using a smoke machine and propane. Found nothing. Even blocked off the PCV lines to see if that was somehow letting in air. Again, nothing. My fuel trims (both short and long for both banks) hover around 0 +- 3 at idle. Which is confirming to me that it's not a vacuum leak.

- MAF readings are normal. 9.xx g/s. Surge still happens with it disconnected.

- O2s are switching normally. ECT, IAT, TPS all read ok on the scan tool. I graphed the TPS with a DSO, it's reading perfectly.

- Fuel pressure is good and steady.

- The only value I see changing with the surge besides MAF, MAP and IAC values is the spark timing. At idle it hangs at 17*, when the surge happens it dips to 7* and then shoots back up to 20* when it rises again.

- *** If I disconnect the IAT sensor while the engine is surging, the surging will stop. The IAT sensor is working perfectly however. At first I thought it was entering into open loop with the sensor disconnected but the scanner still indicates it's in closed loop. My theory is that when the IAT is giving a false reading (-38 when disconnected) it goes into a different operating strategy that ignores whatever it is that is causing the surge. I can't find any information as to how it calculates timing when the IAT is disconnected.

I have access to a tech 2, and various snap on scan tools but nothing to log with and upload here unfortunately.

04 GTO, 136k miles, Stock with the exception of CAI and cat back.

 

Have you checked into your coolant temp sensor? A malfunctioning one can cause odd idles because the computer thinks the car is cold starting. That could be your hanging idle issue. It could be complicated more with the ac because of the increased engine load. That's my guess from skimming your post.

 

I would say that You have a wiring connection problem . I've seen this a cpl of times before where it would run perfect for about 25 to 35 minutes then would die sometimes or just run like crap ,,,,,,,if You shut it off or it died and You let it cool down for 5 or 10 minutes it would run good for about another 5 or 10 minutes . I would pull apart all the engines electrical connectors and use ann eyeglasses flat screw driver and push the screw driver in behind the female connectors bending them in towards where the male prong occupies when together and if there is any corrosion scrape it clean first then tighten it up . I've had connectors that did'nt want to stay tight where I cut a very small piece of metal or plastic and wedged it behind the female part to permanently tighten it up so it would positively make good contact for many years to come . You might also want to check and tighten all the grounding points . A bad ground can and will badly corrode the cooling system if it's an engine ground .

 

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