Misfire at idle

GPGT97

MEMBER
1997 PONTIAC GRAND PRIX
6 CYL
FWD
AUTOMATIC
550 MILES

Enter your question.1997 Grand Prix GT has multi. Cyl. Misfire and Bank 1 too lean. Car has 500 miles on rebuilt engine, and runs great over 1500 RPM. Replaced MAF, cleaned and checked injectors, FP is 48 PSI with new plugs, wires and coils. Any vac. Leaks have been sealed. This 3.8 NA has a comp cam with.531 gross lift, which makes the idle even more erratic. Engine had same symptoms before rebuild. The exhaust is extremely rich at idle.I have two Snap-On Scanners and the only codes they show are PO300 and PO171.
Could the O2 sensor be causing this?
Crank and cam position sensors were replaced about two years ago.

3 Replies

MERLIN2021

MECHANIC
17,250 POSTS

02 Sensor will cause it as will vacuum leaks, 02 will take over fuel management when it's warmed up, cold operation the MAF does the enrichment. Check also the ECT(engine coolant temp) sensor.

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Sunday, January 23rd, 2011 AT 6:29 PM

GPGT97

MEMBER
12 POSTS

Thank for your advise and a remarkably fast answer.
I was going to purchase an O2, but smelled raw gas from the lower I.M.
Cylinders 1, 3, 5 had fuel bubbling under each injector on the intake.
I, m assuming the fuel rail must be cracked over the injectors because the o-rings and bodies were dry. It did throw a P1200 code before a 52 mile trip. All dry. Ran great on I-20. Idles bad.