986S throttle hunting/fluctuation at low throttle/high load situations

[grover]

2004 986S, tip, 84k miles. I've been having issues with throttle hunting/fluctuating at steady-state 25-45mph while going up light grades, and other places the engine is seeing high loads in low-rpm overdrive type situations where I've never had an issue before. No issues at idle or when driving aggressively- mostly in gears selected by the tip computer in auto mode in the 1500-3000rpm, and manually selecting one gear lower worked around it (no issues). No apparent drop in power, the throttle just seems to surge/fluctuate. Saw slight fluctuations in MAF obdII readings, but not sure if that's the cause or a symptom. No CEL codes. With no issues at idle, I'm thinking it's probably not a vacuum leak.

I'm in the middle of a road trip (35mph speed limit on the Shenendoah parkway, grrr...) with limited tools available and a 6 hour drive home this weekend, but if it's something serious, I'll buy tools and get 'er fixed. First noticed the issue driving for lunch about 30 minutes after 8 back-to-back autocross runs, but it did not seem to impact performance at all the next day's autocross. Was hoping it was a fuel issue as I had gassed up at a really cheap station and put in 2 gallons twice to get to race levels (never sure how much 87 octane is already in the pipes before the 93 starts flowing...), but the issues persisted even after a full tank of 93 octane at a reputable station (Hess).

Edited July 25, 2012 by grover

[Loren]

Most problems with hunting at idles are a dirty throttle body.

Start by removing and cleaning the throttle body.

[grover]

Idle is fine, though; the only issue seems to be in low-speed "overdrive" type conditions, whch has me a bit baffled.

[grover]

I cleaned my throttle body last night. Looked significantly different than Pedro's which I used as a guide- no idle control valve or any extra passages in the throttle body at all, just the butterfly valve and servo. Buildup looked superficial, but I went at it anyhow with foaming degreaser + elbow grease (elbow degrease?) + electronics cleaner and got nearly all of it off. One vacuum line came loose as I worked (from the AOS)- not sure if it was loose all along and causing my problems, but it's on good now. I drove around a bit at just above stall-speeds in 5th gear trying to replicate it; if it's still there, I couldn't tell. Time will tell, I suppose! Thanks again for the insight, Loren! :)

Before:

After:

Edited August 4, 2012 by grover

[grover]

The throttle hunting/fluctuation was not fixed by cleaning the throttle body. What else may have caused this, what should I try next?

[systech]

The throttle hunting/fluctuation was not fixed by cleaning the throttle body. What else may have caused this, what should I try next?

The MAF is often a contributor to poor running. Try disconnecting the wiring to it...if it runs better (using the default map in the ECU) then that's a good signed that the MAF is US or may need a clean with electronics cleaner.