Pontiac erratic idle/lean condition

CDWILLIA815

MEMBER
1999 PONTIAC GRAND PRIX
6 CYL
FWD
AUTOMATIC
140,000 MILES

I have a 1999 Pontiac grand pirx gtp. The 3800 series2 ran great when I purchased it(used), but had a slight rotor slap in the supercharger. The snout is rebuilt, the supercharger itself has been replaced with a lower mileage unit, the vacuum lines are all new, the IAC motor is new, the PCV valve is new, new plugs, correct gap, new wires, cleaned hte coil brackets/mount, new throttle position sensor, new serpintine and blower belts. The car now has a very erratic idle, between 800 and 1200, fluctuating. The A/F ration fluctuates badly also. It misses at an idle, and once the fuel system is in closed loop the car runs very lean and continues to miss and fluctuate at idle. The A/F ration still jumps around between 6 and 11. It wont stay between 12 and 15 like it did before the parts replacement. I'm familiar with older vehicles, and antique motorcycles, but this newer OBD2 and MPI system is too new for my taste. I also have a "cheap" Actron obd2 scanner and an older Snap-On scanner analizer that is good to 2002 and older vehicles if this will help me trouble shoot. The only trouble code is MIL cicuit malfunction(check engine light), and multiple misfire. Thanks.

1 Reply

ERNEST CLARK

MECHANIC
1,730 POSTS

First, I need to know how you measured the air/fuel ratio. If it's by the o2 sensors, then they're supposed to jump around. The computer uses the data to calculate A/F over a given amount of time, not what it is for a few seconds.

With the combination of the high idle speed and lean condition, I'd say you either have a vacuum leak or faulty MAF. An ignition malfunction would cause the opposite. A failing pump/clogged filter would cause the lean condition but not the high idle. So too clogged injectors.