check engine light but no code

[dl]

88' Corsica 2.8v6 auto.

First 20 deg weather in awhile and after she warms and driven ok. this....
When I'm idling for a bit I here the idle come up a bit,
and then the Check engine light comes on, I shut it down
check for codes (by jumping the diag connector) and nothing (only the 12).Do I need a scan tool? or will it eventually set a hard code.
TPS maybe getting flaky on the beginning of its travel.

Note: the Hypertech converts it speed density.

Thanks,
Dave
Brooklyn, NY

[geoffinbc]

When its started back up is the light gone?

Run the engine longer with the light on.

[customandsound]

code 12 is ok ... it states every thing is ok

[geoffinbc]

We know that. Code 12 means diagnostic mode has been set.

[dl]

When its started back up is the light gone?

yup.

Run the engine longer with the light on.

I tried, so far nothing, comes on at idle maybe half the time,
as soon as I start moving, it goes off.
I put on 200miles this weekend and it ran fine,
I'll just keep on checking it, and keep my eyes and ears open
for anything out of the ordinary.
Dave

[dl]

check engine light with no code.

so here is an update to all.

original problem....
check engine light coming on at warm idle in park/neutral after the idle goes a bit high with no code present using the jumper method.

After swapping the IAC with a known good one and the same problem,
I did some more thinking and realized the problem came up
a week after NY went to the Ethanol mix (up to 10%!!).

I reset the ECM and swapped out the KEM aftermarket 02 sensor for the used AcDelco in my toolbox -so I'm closer to factory spec,
did the idle relearn and all is well so far.

Oh I lost mileage too with the new mix, time will give me the hard
numbers.

Supposedly better emissions but more fuel used/refined etc.
EPA, politicians..........

Dave
Brooklyn, NY