Mechanics

SEDAN '96 AUTO ROUGH IDLE AFTER ~1 MINUTE AT STOPSIGN

1996 Chevrolet Lumina

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.

Details:
Lumina sedan
1996 Automatic Transmission
145,000 miles
3.4L DOHC

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?

Thanks
Avatar
Repaircarforever
May 31, 2007.



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