low idle diagnostic help

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Jun 28, 2017 | 11:06 PM

ryank9398

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Instructor

Joined: Nov 2016

Posts: 138

Likes: 12

From: Watertown South Dakota

low idle diagnostic help

I have a 1990 auto that just started a very low idle. It takes a little blip to get started, idles 550 or so when cold. IAC counts increase steadily to 160 and stay there, within a few seconds. Commanded idle is 800 cold. The cold idle is steady and somewhat smooth. The IAC count does react to driving, but returns to 160 at idle. Hot idle is 500-525rpm and steady and it can restart without throttle. Commanded idle when warm is 700 or so. I didn't give it a very consistent test ride but it drives normal i think. The PCV is connected and working. I didn't check the plugs or fuel pressure yet. No codes. O2 sensor is crossing 450v as expected, no extra fuel smell. TPS 0.74v-4.3v and very small change in throttle position does change the engine speed.

I think I should try a new IAC. 160 counts means it thinks its open, right? Trying to increase idle but engine is not responding.

I did disconnect the battery recently while doing a stereo replacement, but i don't think this idle problem happened suddenly after.

Jun 28, 2017 | 11:09 PM

ryank9398

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Instructor

Joined: Nov 2016

Posts: 138

Likes: 12

From: Watertown South Dakota

Also...didn't check timing with the EST or ESC or whatever that wire is called pulled. I should do that too. Earlier this year everything was within spec, (fuel pressure, TPS, compression, base timing).

Jun 29, 2017 | 08:32 AM

aklim

Team Owner

Joined: Jul 2004

Posts: 28,475

Likes: 3,291

From: Hartford WI

Quote:

Originally Posted by ryank9398

I think I should try a new IAC. 160 counts means it thinks its open, right? Trying to increase idle but engine is not responding.

Quite the opposite. 160 means it thinks it is closed so it needs to open up as much as possible and if 160 is the max and it needs more, you are SOL.

Lets try this. Assuming your timing is on and your other sensors are right and there is no air leak, take the TB off. Remove the top plate and the IAC and the IAC housing. (This is a good time to do the coolant bypass). Clean all passages and reassemble with fresh gaskets. Run the motor till it is hot and check the IAC counts. Ideally, you want about 20-30. If it is higher, turn the adjustment screw in, rev and recheck after it settles down.

Jun 29, 2017 | 10:59 PM

ryank9398

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Instructor

Joined: Nov 2016

Posts: 138

Likes: 12

From: Watertown South Dakota

Thanks. On the way home from work it magically fixed itself. Supports your theory of some obstruction or dirty parts. I have a few things to check before heading to the Black Hills Corvette Rally in a couple weeks. Western South Dakota is barren, you do not want to be on the side of the road out here!

Jun 29, 2017 | 11:47 PM

aklim

Team Owner

Joined: Jul 2004

Posts: 28,475

Likes: 3,291

From: Hartford WI

Quote:

Originally Posted by ryank9398

I don't think it can heal itself. I think what it means is that the conditions are sufficient somehow for the ECM to adjust. When the conditions change again, well....

Jun 30, 2017 | 12:51 AM

Cliff Harris

Race Director

Joined: Apr 2002

Posts: 10,036

Likes: 346

From: Anaheim CA

Quote:

Originally Posted by ryank9398

I think I should try a new IAC. 160 counts means it thinks its open, right? Trying to increase idle but engine is not responding.

My recollection is that the ECM limits the IAC counts to 160. That means that it's maxed out, trying to get more air into the engine, which would increase the idle speed.