'93 Sable (3.8): Rough idle/stalling/surging/stumbling
I have a 1993 Mercury Sable GS with the 3.8 engine.
I have replaced the MAF sensor, O2 sensors, PCV valve, spark plugs/cap
and rotor/wires, air filter, fuel filter, have had the throttle body
cleaned twice, and have run Chevron Fuel Cleaning System through the
Also, not necessarily relevant, the alternator, starter and thermostat
have been replaced and the car starts perfectly (also, all charging
(ALL parts above are MOTORCRAFT parts).
Of note, after replacing the O2 sensors, the car is more powerful and
apart from those idle-related symptoms, performs VERY well (I can
pass on the freeway effortlessly/it accelerates like a
My mechanic is exceptional and very honest but this has him stumped
and he has suggested waiting until the check engine light comes on.
My concern is the last time we had the check engine light, it was the
O2 sensors but there was no code (just the light...they checked for
codes even with the car running and the light on!). They figured out
the O2 sensors by taking them out and testing them directly, I
I'm just curious if anyone here has any other ideas. Symptoms are:
1. Intermittent rough idle (primarily cold but also warm) including
dipping (from 650RPM to maybe 450RPM), shaking, sputtering out the
2. Rare stalling (has happened 2-3 times in the past 4 months).
Stalling happens when I put in gear (reverse) a minute after starting
cold in the morning
3. Surging: this happens after starting the car (primarily cold but
also warm) for around 15-30 seconds at 1500RPM AND while out driving
hot or cold. While sitting at a light, the car will nudge forward
lightly or if in neutral, the tach will go up about 100RPM and back
down in a second. Also, while warming up last week, the car surged to
1400-1500RPM for 2 minutes and 50 seconds (I waited it out).
All this surging is new, as I've had the car for 2.5 years and it
never did this until 2005.
4. Just in the last week with the car warmed up, while sitting at
lights, the car would stumble (like 100RPM) about once every 10
seconds (stumbling is as quick as the surging...like a second).
Also, symptom #1 has been happening for about 1.5 years and #2-4 have
started, in that order, just this year.
If anyone has any other ideas, that would be much appreciated.