Low, fluctuating idle when warm…large RPM drops

[Rob Brown]

Hi there! 1998 Saturn SL2, automatic trans. 250K miles, though the engine was worked on prior to me purchasing it a month ago…has great compression in all cylinders (190-200psi).
The symptoms:
Once the car has warmed up, I will often experience a low, fluctuating, rhythmic idle when stopped at a light in gear. 700, 500, 700, 500, etc. in a rhythmic pattern, 1-2 seconds between fluctuations. Sometimes these fluctuations figure themselves out, other times they continue.
Additionally, when the engine is warm, shifting from park into drive will drop the RPM’s to 400-500, the car will shudder, dash lights dim, and then it jumps up to a normal idle.

I’ve looked at most everything and am getting pretty discouraged. Posts on other forums that sound like mine seem always to fade in to oblivion with no satisfactory solution produced.

Things I’ve done:
1. Replaced plugs and wires (one wire was too resistant)
2. Replaced a failed alternator, tensioner pulley, and belt.
3. Swapped idle air control valve for a known good from another car. No change.
4. Checked voltage output of Throttle Poster Sensor with engine off. Smooth transitions through voltage range.
5. New air filter.
6. Swapped TCC solenoid with 2nd gear solenoid. No change.
7. Checked for intake leaks with carb cleaner and smoke. No apparent leaks.
8. Checked exhaust for leaks with smoke and soapy water. No leaks found.

Any insight would be appreciated, especially if you’ve experienced similar symptoms before.
Many thanks!
Rob

Topic

[college man]

Try cleaning the throttle body and all the passages going to the IAC.

[Rob Brown]

Thank you for the suggestion. I should have noted that I did that as a first step.

[Craig]

While I understand that your problem is likely fixed or the car may have been shuffled out of your inventory, I am curious if you noticed peculiar sights/sounds while idling the engine with the hood open, such as clicks, hisses, or whirring.

I have a sneaking suspicion (or wild guess) that the regular RPM drops were intelligently controlled and may have been related to the A/C pump clutch engaging (additional load), an error occurring, and it disengaging (return to normal).

[Mason]

I am having this exact problem. Except my car is a 95 SL1 and a manual. I’ve done all of the same stuff.
Cleaned the throttle body, replaced pretty much all the sensors, checked for leaks… It happens when the a/c is off or on.

Sometimes when it levels out i can turn on the lights, or the blinker, or hit the breaks, or anything that uses more electricity and it starts idling rhythmic again.

Also i replaced the alternator to the 105amp and it didn’t help. As well as the PCM and ICM

[video]https://www.youtube.com/watch?v=mtyz5Y8flkY[/video]

[David Bean]

Hi Kimohands. Sorry for reviving an old thread but was wondering if you managed to sort this problem out?
I’m in the UK and have exactly the same problem. No fault codes, no spanner light and my car drives fine though reversing up a hill is a nightmare.
Did you find that the fluctuating also depended on engine temperature? With mine the difference between high and low fluctuations decreases as my engine warms up and the idle speed increases to about 1K rpm. Then when the rad fan comes on the idle reverts back to 950 high and 650 low roughly two seconds for each. As the engine gets hotter it starts all over again.
Thanks
Dave

[Billy]

You are more likely to get helpful responses if you start a new thread and include your car’s make/model/year/engine/transmission/mileage.

[Author]

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