Rough idle and stalling after warmed up

Tiny
ACRINSD
  • MEMBER
  • 1999 SATURN SL1
  • 1.9L
  • 4 CYL
  • 2WD
  • MANUAL
  • 245,000 MILES
My car runs fine at start up, but once it's warmed up, it idles rough, bouncing between 400-600 rpm, and has started stalling when idling. It runs normally when driving at speed, the issue is only at idle. There are no codes and the check engine light is not on.

My plan is to replace plugs and wires, clean the throttle body and replace the the IAC. Any suggestions on what else should be checked? I've been searching the forums and the IAC, along with carbon build up in the throttle body, seem to be common issues. Any suggestions are appreciated.
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Thursday, May 16th, 2019 AT 8:54 AM

3 Replies

Tiny
JACOBANDNICKOLAS
  • EXPERT
Welcome to 2CarPros.

Actually, you are right on with your ideas. Most commonly a bad idle is the result of a failing idle air control valve (IAC). The IAC is responsible for maintaining the engine idle speed. Also, an excessively dirty throttle body can be the cause as well. One thing you didn't mention is an engine vacuum leak, which can also cause a rough or uneven idle.

Here are a few links you may find helpful. Take a look through them. The first explains how to check for an engine vacuum leak and the second, how to service the throttle body actuator.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

https://www.2carpros.com/articles/throttle-actuator-service

_______________________

Next, here is a list of things that could be related to the condition. Note that the first few require a live data scan tool to be checked.

ROUGH, UNSTABLE OR INCORRECT IDLE/STALLING - Definition: The engine runs unevenly at idle. If bad enough, the vehicle may shake. Also, the idle may vary in rpm (called hunting). Either condition may be bad enough to cause stalling. Engine idles at incorrect speed.

Check:
Misfire counts using Scan tool (if any exist)
Throttle linkage for sticking or binding
Throttle position sensor should read zero percent at idle on Scan tool.
IAC System.
Generator charging output voltage. Repair if less than 13 volts or more than 16 volts with all accessories off.
Injector Balance.
EVAP canister purge system
Minimum air rate
Catalytic converter blockage
Fuel quality.
Powertrain mounts.
For DTC P1599 - see chart.
EGR pintle valve held open
Check Terminals and wires at EI module
Check Terminals and wires to crank position sensor
PCM power and grounds
Fuel pressure
Spark plugs and wires
Cylinder compression.

Monitor long term fuel trim values on Scan tool if system is running lean (block learn near 147).

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

If you determine the IAC is bad, here are the directions for replacement. The attached pictures correlate with the directions.

IDLE AIR CONTROL (IAC) VALVE
Idle Air Control Service

pic 1

Idle Air Control Valve.

pic 2

REMOVAL
1. Turn ignition Off.
2. Remove air intake tube and resonator.
3. Remove electrical connector from the IAC valve.
4. Remove screws and IAC valve.

NOTE:
The IAC valve is an electrical component and must not be soaked in any liquid solvent; otherwise damage could result.

5. Remove O-ring from IAC valve and discard.

INSTALLATION

1. Clean the IAC valve seating surfaces on the throttle body to assure proper seal of the new O-ring and contact of the IAC valve flange.

NOTE:
Before installing a new IAC valve, retract the pintle using the Scan tool. Refer to Scan tool operating instructions.
Normal wear of the IAC pintle, characterized by a ring around the cone, is not an indication of a bad IAC valve.
If replacement is necessary, be sure to replace with an identical part. IAC valve pintle shape and diameter are designed for specific applications.

2. Lubricate new Oring with clean engine oil and install on IAC valve.
3. Insert IAC valve in throttle body.
4. Install IAC valve attaching screws using thread coating, Saturn P/N 21485277 (Loctite 242 Threadlocker or equivalent).

Torque: 3 NM (27 in. lbs)

5. Connect electrical connector to IAC valve. Push in connector until a click is heard and pull back to confirm a positive engagement.

________________________________

Let me know if this helps or if you have other questions.

Take care,
Joe
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Thursday, May 16th, 2019 AT 6:48 PM
Tiny
ACRINSD
  • MEMBER
Thanks Joe! That was very helpful. I'll start with a standard tune-up, then replacement of the IAC and cleaning of the throttle body. If the issue continues, I will purchase a scan tool capable of live feed and go from there.
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Thursday, May 16th, 2019 AT 8:40 PM
Tiny
JACOBANDNICKOLAS
  • EXPERT
Welcome back:

You're very welcome. Let me know how things work out for you. Also, please feel free to let me know if you have questions or need help.

Take care,

Joe
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Friday, May 17th, 2019 AT 7:22 PM

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