Idle trouble...

[PlatinumDevil]

2000 GT, Slight delayed surge at idle, exists under regular load but extremely pronounced with A/C on... Car starts out idling around 900rpm and slowly the rpm's decrease until around 600rpm when they plummet to around 300-400rpm where the engine feels like it will stall and then it revs itself back up to 1100rpm and the cycle repeats.

I can also replicate this same scenario by depressing the brakes fully at idle or rolling up both windows simultaneously. Putting the car into gear sometimes has the same affect.

I've already cleaned the IAC but to no avail. Car has done this for probably the last 2 years, only driven in summer sits in a garage for ~7months a year. Runs extremely well otherwise...

In terms of mods, K&N, O/R prochamber, rebuilt and strengthened 4r70w custom dynomax catback, and Bama race tune (just recently installed so its not the tune)...

 

[trinity_gt]

You might want to check:

1) Alternator output. Check AC voltage (for rectifier issues) and DC (charging.)

2) Use a scan tool and check the O2 sensor output. The near-stall behavior may point to slow or lazy O2 sensors.

 

[wmburns]

Is the AC low on Freon? Does it make a difference if the AC is on or off?

What is the condition of the battery, battery terminals, and radiator core support grounds? Are all clean, tight, and corrosion free?

Have the alternator tested for output and AC ripple.

When was the last time the PCV valve was replaced? Are all of the vacuum lines in good repair?

Check the throttle linkage for excessive play. Do the butterflys move with the changing RPM's?

Do you have access to an ODB2 scanner? If so, monitor the IAC duty percentage. I'm interested in seeing the value and if it is changing in response to the changing idle.

When was the last time the MAF was cleaned? If monitored with an ODB2 scanner, what is the air flow doing?

Post the battery voltage while this is happening. I'm interest in seeing if the voltage is sagging along with RPM's.