Hello Mr Aguire,sorry to hear that the IAC and cleaning didn't fix your problem. I did some searching for similar problems on your model car and found many reasons for the idle problem. You didn't say if it happens only cold or at all times. If cold I found that things like a vacuum leak,to high oil pressure causing pumped up lifters have been found. I will send one of the help requests and what was found.It turned out to be a rare problem that took some time to find. To check your problem out further would require some in depth testing with a scan tool, to check the sensors and fuel trims when this problem occurs. Sorry what I suggested didn't help,it does cure the problem 90% of the time, sending this to try to help you get some direction to the car fixed,no need to hit accept again, Jerry.
1996 Pontiac Sunfire
Engine: 2.4 L / 146 CID / 4 cyl / Gas / DOHC
Fuel: Fuel Injection
Trans: 4-speed Automatic Transaxle (Electronic)
Mileage: 141,000 mi
Emissions: OBD-II Compliant
Misc Data: NO Codes
Symptoms: Poor Idle Quality, Hesitation, Surge,
Cuts out, Stall, Misfire, Poor Fuel
Occurs: Cold, Hot, Idle, After a Hot Soak,
More Info: Test results & repair history included below
_____________________ ORIGINAL MESSAGE _____________________
Argh! This vehicle has been driving me nuts for the last
couple of days. The problem is it only acts up very
intermitantly with NO CODES... What happens is after the
engine warms up to operating temperature it will begin to
stall/cut out down to 300rpm then recover to 900rpm then stall
down to 300rpm and repeat indefinatly. It will do it after a
continuous run from cold to warm, or from a warm restart after
30min-1hour after a shutdown. Cannot perdict when it will do
it. You can rev the engine up to 2000rpm and you can still
feel a slight mis and the engine will buck periodically.
With the engine acting normally I drove it on the highway and
doing 100Kph(55mph) and mashine the accelerator there is no
pickup, just a lot of noise, you have to manually shift the
trany (auto) and you hear what sounds like an exhaust leak
when the engine is reved above 4500rpm under load only. There
is some acceleration but nothing to write home about. There is
better part throttle acceleration at these speeds than at WOT.
I disconnected the iac with the engine idling normally at
900rpm and drove it throught he city with multiple stops and
starts with a 45minute shutdown and restart with no problem.
The engine did stall after it sat and cooled down and tried to
fast idle(3 hour cooling with outside temp at 6^C (44^F).
There is oil leaking from the cam cover and from around the
oil presure sensor boss.
At the time I hooked the scan tool up to it, it behaved
perfectly for over an hour of trying to get it to misbehave.
Even when the idle is surgine the alternator is putting out
13.8V+ at 300rpm, Here is what I have for the only real scan
Idling At 900rpm.
ECT 115^F(Verified w/ Thermometer)
TP Sensor .35V (Operated within Spec)
Baro Pressure 11 in-Hg
LT Fuel Trim -17.2%
ST Fuel Trim 0 - -7% (cycled from 0 to -7%then Reset to zero
Map Voltage 1.37V (operated within spec)
B1S1 O2 Operated Between 300-600mV
B1S2 O2 Operated Between 400-500mV
I have looked at tons of iatn posts from Oil pressure to
timing belt skipping to reflashing the comp. I'm not really
into throwing parts at vehicle untill i'm entirely desperate,
I'm not there...yet! I know iatn's experiance will let me make
the correct choices and repairs.
Thanks in advance, I'll update when I obtain some more
___________________ TEST AND REPAIR DATA ___________________
Checked for codes and none were found.
RELATED REPAIR HISTORY
Replaced IAC with known good unit with no change in
drivability. Replaced Ignition Boots, Plugs Were Replaced
Three Years and 40K ago.
______________ COMMENTS ADDED ON Dec 29, 2007 ______________
Thanks For Your Patience. This vehicle came in for unrelated
mintenance and we did the back pressure test. It was perfect.
At idle the gauge was below 0.25psi, at 2000rpm the gauge was
around 0.25psi, and at 4000rpm the gauge was between 0.25psi &
0.50psi. This was done at operating temperature, before and
after the cat, and with the engine running perfectly...for an
11year old quad4!
The noise that is being heard is from the balance shaft chain.
The chain tensioner was broken causing it to vibrate and make
Thanks again for all who replied Matthew
____________________________ FIX ___________________________
Thanks to all who replied, I'm with the majority and believe
that there is also an exhaust restriction, but after the
vehicle was repaired the customer chose to take the vehicle
and has not returned yet for the back pressure test.
The problem turned out to be a faulty diode within the
alternator. WHen I looked throught he other IATN posts, others
have had leaking AC current drivability problems. I hooked my
DVOM between the positive battery terminal and alternator case
and recorded these reaedings before replacing. At idle before
the car would surge and idle rough there was 0.050V of AC
current. when the idle began getting rough and cylinders would
start to drop out the AC volts went up to 0.100V, when the
vehicle started to surge the AC Volts whent up to 0.200V, and
when the vehicle would stall the AC volts would rise up to
0.355-0.400AC-V before the engine stalled out. The quality of
the idle was in direct proportion to the amount of AV Current
being produced. If I get the vehicle back to do a back
pressure test I'll update this post with the readings.