Eratic idle

Tiny
WILLIAMDA
  • MEMBER
  • 1996 TOYOTA COROLLA
  • 1.6L
  • 4 CYL
  • 2WD
  • MANUAL
  • 200,000 MILES
Car runs great cold and idles smoothly until it warms up and then develops a slight hesitation on acceleration and idle starts to surge from low to high and so on. When cold again idle is steady and smooth. Have replaced EGR valve and cat, Cleaned intake, replaced fuel pump, fuel filter, injectors, fuel regulator, new plugs, distributor coil cap rotor, new ignition wires, new ECT sensor, MAP sensor, thermostat. Cleaned throttle body checked all hoses for leaks and replaced air filter. Checked timing belt, new O2 sensors upstream and down stream, checked all fuses and relays, flushed coolant system and replaced radiator. I did a test on the TPS sensor and it seemed to check out but I really don't know how to tell its set right. I'm an old school mechanic and didn't have all these things on the old cars.
Wednesday, September 15th, 2021 AT 3:29 AM

5 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,141 POSTS
Hi,

I agree with you. LOL And the newer they get, the more complicated.

As far as your problem, if the idle is going up and down, it is likely related to the idle air control valve, (IAC). The IAC is responsible for maintaining the engine idle speed.

Basically, on this vehicle the throttle plate fully closes when your foot is off the throttle. At that point, the engine would stall. However, the IAC allows a metered amount of air to bypass the throttle plate.

If you look below at pic 1, I highlighted the part I'm referring to. Pics 2 and 3 are the diagnostics (very simple). The hardest part is removing the throttle body to access the IAC.

Now, what can happen is this. Carbon can build in the IAC causing a restriction of air. That can cause what you described. If you find there is dirt/carbon that can be causing the issue, try servicing the unit first. Here is a link that shows how it's done:

https://www.2carpros.com/articles/idle-air-control-valve-service

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

Take care,

Joe

See pics below.
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Wednesday, September 15th, 2021 AT 8:36 PM
Tiny
WILLIAMDA
  • MEMBER
  • 4 POSTS
Thank you for this advice. I will take it apart and clean it best I can and if not replace the valve.
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Thursday, September 16th, 2021 AT 4:05 AM
Tiny
WILLIAMDA
  • MEMBER
  • 4 POSTS
I also would like to know how to properly set the TPS with a multimeter so it is at idle position. Can't find any videos that show clearly how to perform this task. Most rush you through it and don't even tell you what to set the meter on so its very confusing to an old mechanic who can use a dwell meter and distributer machine.
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Thursday, September 16th, 2021 AT 4:10 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,141 POSTS
Tach and Dwell meter. I miss them. LOL

As far as adjustment, take a look at the attached pics. They are the directions for adjustment.

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

Take care,

Joe

See pics below.
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Thursday, September 16th, 2021 AT 7:29 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,141 POSTS
Tach and Dwell meter. I miss them. LOL

As far as adjustment, take a look at the attached pics. They are the directions for adjustment.

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

Take care,

Joe

See pics below
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Thursday, September 16th, 2021 AT 7:29 PM

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