[MaupinMan16]

I have a 80,000 miles and when i start it up when cold it idles at about 1,200 rpm's then after 30sec it idles back down then when i shut it off after warmed up and turn it back on it idles normal at 650.....but if i let it sit for 40minutes or so it does its high idle again and the quits.....kind of sounds like a supercharger winding up....


what the heck is going on

 

[Submariner409]

Normal. Just like the old carburetor "fast idle" cam settings.

When cold or below xxx degrees coolant temperature the PCM commands fast idle, different injector timing and maybe secondary A.I.R. (Air Injection Reactor - old term) compressor run to light off the cat quickly, minimizing emissions. The idle drops to about 850, the compressor stops, then to 650 normal.

Normal.

 

[SwampeastMike]

The faster idle is about getting temps to the proper, most economical and least noxious exhaust producing conditions. Intake air is heated as necessary to a stable, warm temperature while additional oxygen is pumped into the exhaust catalyst bed to speed its operation thus generating additional heat. As these and likely other conditions are met and air injection into the catalyst is ceased, the idle speed will slow to normal.

Like Submariner said, it's similar to the high idle cam on carbuerated engines where it existed before emissions control to not only speed heatup to a stable, efficient temperature but to keep the engine running as without advanced computer control a cold engine would often sputter and die at normal (low) idle with a cold engine--particularly in cold or damp weather.

With modern control, engines run flawlessly compared to those of my youth in the 70s when emission controls--early air injection into the catalyst was wasteful--were introduced. Once engine controls achieved sufficient feedback loops to self-diagnose emission levels while adapting to normal variability over time, they tend to either run perfectly or not run at all.

 

[Submariner409]

Don't forget that the Northstar doesn't use GM's "Thermactor" system (heated intake air). In pre-2000 engines the throttlebody itself is heated by engine coolant passages, so the bore perimeter is always at engine running temperature. Later models attach the throttlebody to the coolant crossover, so there's heating there, too, but not as much as earlier. Fuel injector timing and pulse length take care of the older heated intake air warmup cycle.

The directly heated throttlebody was primarily designed to combat butterfly area bore icing: as air rushes past a slight (part-throttle) butterfly opening it loses pressure rapidly just downstream, causing vaporization and supercooling of the moisture content due to relative humidity, thus theoretically causing ice buildup under certain temperature and humidity conditions. It was later determined that this was not so much a problem as originally thought, and the later indirectly heated arrangement has proved satisfactory. The OBD-II comforming more sophisticated PCM fuel/air programs also helped.

If you think the throttlebody casting on a pre- or post-2000 Northstar is nice and cool from your so-called CAI pipe, come in from an in-town rush hour commute, shut down, pop the hood, and wrap your hand around the TB. You'll very quickly wish you hadn't.

 

[Jamler]

My STS does this as well... Just thought it was like and auto choke...
Does it normally only on first/cold start of the day.

 

[C0RSA1R]

I think almost every car I've ever owned has done this to some degree: 87 Olds Cutlass Cruiser 2.8L, 89 Buick Century Custom 3.3L, 97 Olds Cutty Supreme 3.1L, and now my 98 ETC Starship 4.6L.

Maybe a Honda doesn't, but I'd never drive one unless it was into a tree, so . . . yeah.