I found your site (www.masterautotech.com) while searching for information on a problem with a 1991 Olds Cutlass Supreme (3.1 L engine).
The car works fine around town, when driven above 40 or 50 mph the Service Engine Soon light comes on. The light stays on until the engine is shut off and doesn't come on again until the car is again driven above 40 or 50 mph. Since the light comes on, the car is rarely driven on faster roads, but when it is I haven't noticed any difference in the way it runs or otherwise performs. No other warning lights come on.
I can start with a completely cold engine drive under a mile to a 50 mph road and by the time I reach the speed limit the light is on.
Take a paper clip and make in the shape of a "U" then place it in the (ALDL) connector under the steering wheel. (That is the connector you plug a scanner into.)
Turn the key on and count the (check engine) that is flashing. It will flash one long flash then two short flashes this is a code 12. Ignore that one. I suspect it will flash a 32.
If it does flash a code 32 then the EGR valve may be defective, unplugged, or the passageway may be clogged
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