eratic idle

1996 TOYOTA COROLLA
200,000 MILES • 1.6L • 4 CYL • 2WD • MANUAL
Avatar
WILLIAMDA
  • MEMBER
  • 4 POSTS
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.
Sep 14, 2021 at 8:29 PM
Advertisement
Avatar
JACOBANDNICKOLAS
  • Experienced Mechanic
  • 110,240 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.
Sep 15, 2021 at 1:36 PM
Avatar
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.
Sep 15, 2021 at 9:05 PM
Avatar
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 .
Sep 15, 2021 at 9:10 PM
Avatar
JACOBANDNICKOLAS
  • Experienced Mechanic
  • 110,240 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.
Sep 16, 2021 at 12:29 PM
Avatar
JACOBANDNICKOLAS
  • Experienced Mechanic
  • 110,240 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
Sep 16, 2021 at 12:29 PM
Join 2CarPros for free, one of the most advanced car repair communities on the internet. From first-time drivers to seasoned mechanics, everyone is welcome to reply, share details, and get better answers. You can add a temporary image or video first; uploads are limited to 20 MB.
Ask a Free Question
Repair Safety Notice: This information is for general instructional purposes only. Vehicle repair can be dangerous. Verify all information, follow manufacturer service procedures, use proper tools and safety equipment, and consult a qualified repair shop when needed.