Lumina Sedan '96 auto rough idle after ~1 minute at stopsign


Car develops rough idle/stalls when stopped/parked after ~1 minute warmup; runs okay when driving particularly at highway speeds - fuel consumption may be slightly higher

This symptom appears to be different than most of the other stalling posts for the Lumina except for maybe one without a response.

Lumina sedan
1996 Automatic Transmission
145,000 miles

Recent History:
According to spouse this problem started slowly possibly a few days (not sure) after filling up at a "reputable" name brand gas station with standard 87 Octane gas

What I know:
-Currently code P0171 (bank 1 lean) sets after 2 drive cycles, reproduced multiple times (after clearing)
-Exhaust at idle tends to smell rich (even with above code indicating lean)
-Freeze Frame data from code reader indicates Fuel Sys 1 is CL, Fuel Sys 2 is CL-Fault, but I can't find anything suggesting two different fuel systems on this vehicle - however my understanding of OBD II is that CL-Fault is not a default state in the absense of a second fuel system (but not sure)
-Used a brand name additive to displace water in fuel - no significant improvement
-Replaced air filter and gas cap - no change
-Checked intake manifold pressure steady at 18-20 inHg at startup (begins to waver as warms up at idle eventually swinging between 5 and 20+ before stalling)
- plugged vacuum lines to fuel regulator, vacuum assist to power brakes, and vacuum assist to transmission shift assist vale - same problem still exists
-Checked IAC valve, observed even resistance at ~60 ohms on both circuits, cleaned pintle and opening in throttle - no apparent improvement
-Checked Intake Air Temperature - calibration is good to about +/- 3 degrees F
-MAF is getting 12V, can't check output but freeze frame on code reader shows a variety of values possibly attibutaable to different engine speeds
-TPS appears to be working based on volt meter measurements (always 0% in freeze frame as a pre-requisite for obtainin P0171 code)
-possible oil leak from rear valve cover, but cleaning backside of engine did not seem to fix problem
-Disconnecting Intake Air Sensor keeps car in open loop and helps prevent stalling somewhat but not far from completely solving problem
-Problem is getting somewhat worse with time
-I hear a possible hissing noise occasionally near the fan belts that coincides (goes off and on) with the cycling of the idle speed from slow to fast but I can't identify a location of a leak. Noise appears to come from low on the vehicle near the belts. It may just be belt noise associated with changing engine speed but I am not sure. My vacuum tube chart doesn't show any vacuum lines in that location
-New PCV valve didn't fix problem, Even with plug in PCV inlet to intake manifold problem still exists

Lots of details. Any ideas?


Do you
have the same problem?
Thursday, May 31st, 2007 AT 3:08 AM

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