I’ve read many articles about this issue, and before I take the car to the mechanic (for other work), I want to understand the problem better so we can zero in on one thing. I trust my mechanic, but they’re human—and despite their experience, they’ve sometimes missed the culprit on the first try.
Here’s the long-standing issue, which got worse after I replaced the entire exhaust system (new Sport Dansk muffler, catalytic pipe, heat exchangers, injectors, spark plugs, and the 02 wire that was nearly crumbling).
Symptom: Idle hunting, from 900rpm instantly goes down to 500-600rpm fluctuating (see video) after restarting the car once it’s warm. after few seconds seconds goes back to normal, and then after another few seconds it start the same flutuations, in cycles steady>fluctuation> steady> etc
Video: https://youtu.be/lmDKJHmeteM
When it occurs: Mostly in slow-traffic areas (aka poor cooling conditions); rarely after sustained highway driving (~60 mph).
Pattern: The longer I drive on slow roads, the worse it gets.
Duration: the entire cycles of steady idles/fluctuations lasts 1–5 minutes before returning to normal
Cold starts: Never a problem—only when the engine is warm.
Driving: besides the Idle the car runs great, super responsive, great acceleration. only the idle is annoying.
It feels like something “digital” or sensor-related is telling the engine to go richer/leaner—some posts mentioned the WUR, for example. I’m inclined to rule out an air leak or failing gasket, because those would cause rough running anytime the engine was warm; in my case, it always returns to a smooth idle.
That it only happens “after a while” makes me wonder if a temperature sensor is at fault—if it were purely a temp issue, I wouldn’t expect the reading to normalize so quickly. Bottom line: my prime suspects are the CDI box itself, the WUR, or the O₂ sensor.
ps. maybe my idle in the video s lower then the desired 950, i can def check with a timing light, and i can adjust the idle screw to make it higher and see if it "solve/patch" the problem for now