Hello all!
I've noticed that I'm really struggling to get about diagnosing very slight idle problems. I've been watching the vids on premium/YT concerning how to find a vacuum leak/faulty MAF by using fuel trim numbers to verify what type of lean condition there is, o2 sensors, propane, smoke etc (e.g. Ford Lean case study, finding vacuum leaks with water etc.) and I feel pretty confident in narrowing down types of lean conditions.
However, all of the videos I've seen have to do with really severe vacuum leaks (torn gaskets, brake boosters, dirty MAFS) that cause very sluggish, up/down type idles. The type that you and everyone around you can hear/feel that throw codes or show higher than usual trim numbers.
What I struggle to diagnose are those really slight, barely noticable idle wobbles. You know, the type that everyone else would miss but we, as car guys, wouldn't.
Just yesterday I got into my mum's TT first gen only to notice that the idle was a bit weird (she obviously feels nothing). Slight needle fluctuating, definitely a bit shakier engine than usual. Every once in a while you'll even get a skip in rpms. It's the best I can describe it, it's as if a heart would "skip a beat" during the idle wobbling. It's barely noticable, you can only really feel it from sitting in the seat.
This happens regardless of engine temp.
Obviously, I pull up what I've seen on the videos. Fuel trim, (unfortunately, couldn't get any from my scanner but since it isn;t a very shaky idle I doubt I would see anything beyond ordinary values), o2 sensors etc. I don't see ANY abnormalities whatsoever. I was hoping that close monitoring of the MAF, TPS, MAP would reveal a slight dip in a graph/value corresponding with the slight dip in rpms. But, no, none. Propane/smoke test showed nothing. Obviously no codes.
I also hooked up the amp clamp from the scope to the injectors and ignition coils. Nothing there either, no drop outs whatsoever (I was kind of hoping to see one missing ramp when the idle would wobble).
I was thinking that a dirty thottle plate could cause this but it's squeaky clean (front and back of the thottle plate).
I remember at the workshop I used to work at, this guy brought in a lovely Megane RS, and the complaint was just that. Car runs perfect, but for that very slight idle hunting and occansional VERY VERY slight shake. In fact, it's not a shake it's more of a wobble.
So, if I haven't made anyone overly confused with my erratic description, has anyone else come across a method of finding the culprit? I mean, something IS causing this.
I'm not looking for a easy way out, I'm genuinly interested in learning what can cause this and, more importantly, how to close in on the fault? If someone could maybe point me in the right direction I'd be very much obliged.
So, to resum everything, are there any guides/videos on how to smoothen out a shaky (not jumping) idle and what might be causing that every-so-often wobble?
Thank you!
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