High idle, '88 5.7

[someotherguy]

'88 shouldn't have a mechanical speedometer; GMT400's are electronic. Is your truck a squarebody? You referred to it as a K1500 but I know some people are a little loose with the terminology. A K1500 would be in the GMT400 family. (Regardless, under 60K is still amazingly low miles for any vehicle of this age!)

Sounds like you're onto something with the idle screw as this is an area that the overwhelming majority of people should have never needed to adjust, and it's easy to get it wrong if you go in there turning it blindly.

Richard

 

[Schurkey]

I was wrong...
...someone had taken the tamper plug off the idle adjustment screw. The black T-15 screw head has some scratches on it. I think the previous owner had a low idle and just dialed up the screw to fix it rather than finding the cause (probably the IAC). I've dialed it back down and going to see how it goes. Tonight I'll put a tach on it and make sure its 750 in park and 500 in gear when at temp. Maybe I'll ask a friend put his smoker on it to make sure I'm not missing a leak.

Thank you Schurkey!

you're onto something with the idle screw as this is an area that the overwhelming majority of people should have never needed to adjust, and it's easy to get it wrong if you go in there turning it blindly.

I suppose folks could mess with the screw, and somehow it improves the situation. And then they'd never post about their success.

Seems like every time that screw gets dicked-with, it causes a problem and then there's a forum post about it. It's just uncanny how often idle problems end up being a misadjusted minimum-idle screw.

Point is, don't just turn the screw to where you think the curb idle is right. Follow the service manual instructions for setting the minimum-idle, and then let the IAC do the rest.

 

[PlayingWithTBI]

I suppose folks could mess with the screw, and somehow it improves the situation. And then they'd never post about their success.

Yeah, on a modified engine you need to but, on a stock one, you shouldn't have to once it's set properly.

Follow the service manual instructions for setting the minimum-idle, and then let the IAC do the rest.

^^^This, although there is another way to adjust it - get the engine up to operating temperature, with a scanner look at IAC counts and adjust the minimum idle screw until the scanner shows ~10 counts (up to 30 is OK). That'll ensure your idle is being adjusted by the ECM/PCM and not a hard stop on the throttle.

 

[PlayingWithTBI]

Here's an example of mine when I had the Comp Cam 08-430-8 roller cam in my 88. I was running in closed loop with the WBO2.

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[NMNorsse]

Reset the minimum idle screw, but it isn't as easy as just setting it. You have to calibrate the IAC. In fact, you probably have to do this whenever you replace the IAC.

I don't have a scanner so I watched a couple of you tube videos, read a couple of websits and did this:

Once the engine is at operating temp (I took it for a 15 minute drive and made sure it went above 195 and then dropped back down once the thermostat opened up). First I hooked up a tach to the white wire terminal off the coil wire loom. Then:

1. With the key out, jumper A and B on the ALDL (OBD1) under the dash using a paper clip.
2. Turn the key to power up the accessories but DO NOT START IT.
3. Wait about 10 seconds. This will make the IAC return to the home position.
4. Unplug the IAC
5. Turn the key off & remove the jumper.
6. Start the car- You may need to give it some gas to get it started and keep it going.
7. Adjust minimum idle screw until it is running at about 575, which is 125 less than what it should be idling at in park at full temp: 700.
8. Turn it off.
9. Plug in the IAC
10. Start the car and run it for 5 seconds.
11. Turn it off and wait at least 30 seconds.

done.

This solved my problem. Not sure if it was the idle screw or that I had replaced the IAC without calibrating it. Either way, its fixed!

Thank you everyone for your help!