Idle drops when car gets warm

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May 1, 2021 | 04:39 PM

NOMOREWORK

Thread Starter

Racer

Joined: Dec 2006

Posts: 459

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From: Chelsea Mass

St. Jude Donor '13-'14-'15

Idle drops when car gets warm

Car starts and runs fine until it warms up, idle drops to 500 rpms and feels like it's trying to stall. When I step on the gas the car drives fine again until I come to a stop, then it drops slowly down to 500 rpms and stays there. Any thoughts?

May 1, 2021 | 05:44 PM

s carter

Melting Slicks

Joined: May 2014

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From: New Port Richey FL

Have you cleaned throttle plates and checked min. Idle/TPS if not it's a good place to start

Last edited by s carter; May 1, 2021 at 05:44 PM.

May 2, 2021 | 04:14 PM

NOMOREWORK

Thread Starter

Racer

Joined: Dec 2006

Posts: 459

Likes: 95

From: Chelsea Mass

St. Jude Donor '13-'14-'15

I cleaned throttle body, unsure what you mead about min.

May 2, 2021 | 05:04 PM

Nomake Wan

Drifting

Joined: Apr 2020

Posts: 1,925

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From: Orange, CA

Quote:

Originally Posted by NOMOREWORK

I cleaned throttle body, unsure what you mead about min.

He means checking the "minimum idle" setting as well as checking the throttle position sensor.

May 2, 2021 | 05:45 PM

NOMOREWORK

Thread Starter

Racer

Joined: Dec 2006

Posts: 459

Likes: 95

From: Chelsea Mass

St. Jude Donor '13-'14-'15

Thank you

May 2, 2021 | 06:42 PM

ToniJ1960

Burning Brakes

Joined: Jul 2013

Posts: 1,027

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From: Granite City Illinois

What year is your car? Have you checked for vacuum leaks yet?

May 2, 2021 | 07:49 PM

NOMOREWORK

Thread Starter

Racer

Joined: Dec 2006

Posts: 459

Likes: 95

From: Chelsea Mass

St. Jude Donor '13-'14-'15

It's an 89 and that is the next step I intend to take before a start throwing parts at it.

May 2, 2021 | 10:59 PM

aklim

Team Owner

Joined: Jul 2004

Posts: 28,472

Likes: 3,290

From: Hartford WI

Quote:

Originally Posted by NOMOREWORK

It's an 89 and that is the next step I intend to take before a start throwing parts at it.

Easiest way to check min idle rate is to see what the IAC counts are with a scanner. If you don't have a scanner, get one. It should be reasonable. HERE is one. Easy to use and does the bluetooth thing to your Android cell. Not sure what to use for fruit phones since I didn't, don't and never will have one. It enables you to see what the ECM is saying. Before I do that, I want to make sure that your timing is set right AND that your Harmonic Balancer is not walking around.

If you get 30 counts, give or take and the timing is right, turn the adjustment screw on the TB in, rev and see what happens. Keep adjusting but each time, I would rev the motor and let it settle down. Remember to make sure that everything but the motor is off.

Last edited by aklim; May 2, 2021 at 11:00 PM.

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May 4, 2021 | 05:58 AM

tequilaboy

Melting Slicks

Joined: Aug 2003

Posts: 3,399

Likes: 395

From: Lakeville MI

Like it or not, 550 rpm is the normal desired idle speed for warm/hot coolant temperature for auto trans L98 cars in gear. Due to the PID control behavior (deadband), it is typical for the average idle rpm to run a bit above or below the target. I will dig up an example log.

https://datazap.me/u/elijah/fixed-ti...5455-5432-5399

5 x 100 rpm according to the digi-dash is completely normal for a warm/hot idle in gear. 6 speed and 4+3 cars will target 700 rpm, but will also tend to run a bit above or below the target.

TABLE: Idle RPM vs Coolant Temperature (example from AANM 88 auto trans bin file)

Deg F RPM

304 550.00
284 550.00
262 550.00
241 550.00
219 550.00
198 550.00
176 550.00
154 600.00
132 600.00
111 750.00
90 900.00
68 950.00
46 1050.00
24 1050.00
3 1050.00
-18 1050.00
-40 1050.00

Last edited by tequilaboy; May 4, 2021 at 04:54 PM.

May 4, 2021 | 10:33 AM

#10

Phobos84

Melting Slicks

Joined: Mar 2002

Posts: 2,876

Likes: 764

From: Sligo PA

If the car was idling properly before and now it's not then I would clean the throttle body like previously mentioned. Then I would replace the IAC. If it's the original don't bother trying to clean it or mess with it, just get a new one. Probably not a bad idea to replace the TPS while you're at it. Also as far as a vacuum leak goes most of the time they'll make the rpm go up not down.

Jun 18, 2021 | 12:24 PM

#11

NOMOREWORK

Thread Starter

Racer

Joined: Dec 2006

Posts: 459

Likes: 95

From: Chelsea Mass

St. Jude Donor '13-'14-'15

Well finally got around to addressing my low idle problem. Cleaned throttle body, adjusted the idle and checked TPS voltage. Car now idles at 600 RPM as it did before ! Thanks for all the responses !

Jun 18, 2021 | 12:54 PM

#12

aklim

Team Owner

Joined: Jul 2004

Posts: 28,472

Likes: 3,290

From: Hartford WI

Quote:

Originally Posted by NOMOREWORK

Well finally got around to addressing my low idle problem. Cleaned throttle body, adjusted the idle and checked TPS voltage. Car now idles at 600 RPM as it did before ! Thanks for all the responses !

I would set the minimum idle with a scanner to see what is going on.