Starter for 10...If a CEL is thrown but then disappears and everything returns to normal, is the error code stored? and for how long?

The Issue:
Driving all weekend with no problems. I pull up to park and realise in neutral that the engine revs up to 2000rpm then drops to 1500rpm all on its own in a nice rhythmic way, up for 2-3 secs, down for 2-3 secs repeat. CEL comes on. I am out and stuck so I just drive gingerly to next destination for 20 minutes. CEL disappears, no issues driving, attempt a bit of boost, fine, pull up, revs fine on tick over. Problem gone for now.

So if there is no CEL am I going to find a code? (Will check tonight) and has anyone had this precise issue before? Sure there have been many like it, but the rhythmic and regular fast idling makes me think this should be something specific.

Otherwise a good weekend of driving, learnt that 250kg less weight (on my car) can make up for a big gap in bhp!

 

You will likely find that the CEL code will be 24 for the idle control valve. It got stuck open which is why you got that cyclic idle. Do a search on here for the various ways of cleaning it.

 

it will be stored in the historic codes

 

Great, Google has willingly supplied the rest! Now to track down a gasket, its a 94 STi V1 if anyone knows the answer!