Multiple cyl. Misfire at idle below 1400 RPMs

GPGT97

MEMBER
1997 PONTIAC GRAND PRIX
6 CYL
FWD
AUTOMATIC
550 MILES

Recently replaced upstream O2 and TPS to fix PO300 and PO171. After replacing O2, car idled more erratic, then after new TPS, idle was smoother until engine was off and cooled for an
hour. Restarted engine and idle and misfire were
almost as before sensor replacement. The ETHOS scanner will show the number of misfires for each cylinder, above 1400 RPMs, the misfire numbers stop accumulating, back to idle = more misfires. Eventually threw a po171 and po172 with the po300. FP @ idle 48 psi.

4 Replies

RASMATAZ

MECHANIC
75,992 POSTS

Could be one of the following:

1. Vacuum leaks
2. EGR and PCV valve.
3. Low fuel pressure.
4. Dirty fuel injectors.
5. Mass Air Flow Sensor.
6. Oxygen Sensor.

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Saturday, January 29th, 2011 AT 6:47 AM

GPGT97

MEMBER
12 POSTS

All vacuum leaks have been sealed, upper I.M. Is new with new PCV and O-rings. FP is 50 with KOEO, 48 at idle and 43 before rev limit.
All injectors were removed and cleaned. MAF replaced last week.
O2 and TPS replaced yesterday, crank and cam sensors about 2 years old. IAC motor is good, unsure about EGR. Tried new IGN. Module and coils, new wires & fresh plugs. Still misfires under 1400 rpm.