1996 LT4 stock 81k

This problem is a little strange to me and very specific. Start of the day with a cold start it runs perfect, up to temp it runs perfect, and drives perfect. Once I shut it off and let it sit for about 15 minutes and start it back up the idle is low, 500 rpm for about 10 seconds then corrects and runs perfectly fine. If I shut it off and start it up right away, it's fine. If I shut it off and wait 5 minutes it's fine. For some reason with it still being hot and off for about 15-20 minutes the issue shows itself.

When I got home I was able to plug in my scanner after waiting the magical 15 minutes and start it up to capture some data. Starts up reading like this:

Coolant: 208
IAT: 115
MAF: 1.6
MAP: 18.0
RPM: 576

Then the MAP/MAF values start to drop and the STFT jumps way up and overcompensates bringing RPM in the 800's. Then the MAP goes down in the 10's where I would expect it and the STFT sits at 0 and RPM in 700s where it should be. The values look normal to me 30 seconds in like this:

Coolant: 207
IAT: 115
MAF: 0.98
MAP: 10.9
RPM: 703

At this point I shut it off, waited a couple minutes and start it back up. Idle was fine and values looked to be in normal range. Over the winter I replaced the intake gasket so there is a number of things I could have messed up but there was no issue I noticed because I never started it up hot after waiting 15+ minutes. Kind of stumped where to start. Thought maybe it had to do with the sec air injection but I've checked the scanner it's always running on every start.

I can list all the things that have been replaced last year when it did not have the issue. Fuel pump, fuel pressure regulator, fuel injectors. Optispark, plugs and wires are only a few years old.

 

Here is my guess: Coolant sensor

Since it runs great when cold, hot and (damn near) immediately after an off/start cycle however runs bajigitty after the engine has had a chance to cool for 20 minutes but snaps back to normal after a few seconds... Id say your coolant sensor is on the way out.

Now, ten seconds to "activate it" is real normal. I'd leave it and spend the money on beer.

 

I've read more about it and seems like it can wreak some real havok on the car. From what I've read the value on the digital dash does not come from this sensor (as in the sensor the ECM cares about) and I'm guess it's the same for my scan tool?

 

I'm not sure about your scan tool, but your ECM lives and dies by the coolant sensor as it is pretty much the first domino in your engine system.

 

If your scan tool and digital temp display on the car show the same, or just about, coolant sensor is good. For what it's worth, in my 93 FSM, it states that with a warm engine in drive,(auto trans) and no accessories on, it should idle at 550 rpm.

I had a problem where it idle stupidly low. And at anytime the AC was turned on, my car would stall. It was the IAC valve.