NefMoto

Yo guys. Question about a little "floating" idle. Im out of ideas and out of patience.

Since I got my 1.8t AUM, I always see a little floating idle needle with rpm constanly between 720-760 (VCDS), never standing still (both PB and LPG, but on LPG is a bit worse), occasionally missfiring (on LPG mostly) and a little shaking engine. Every other car I've driven (either gas and diesel) had very stable and stiff idle rpm. As I'm very pedantic, I was doing maximum effort to bring my idle to such state, but without luck. It's like my point of honor to make it standing still and stiff.

For now what I did:
- new PVC set
- new coolant temp sensor
- new oil pressure sensor
- new timing set also with tensioner and chain
- new fuel pump
- new suction jet pump
- leak test performed, all leaks eliminated, but there were only very minor ones and neglectible
- throttle body adaptation done
- N75 and DV swapped
- N249, SAI, cat and rear lambda deleted
- alternator replaced
- spark plugs and coils replaced (two different sets) with whole new wiring
- battery replaced
- different turbo (k03s)

MAF readings are between 2.3-4 g/s with 0.9l/h fuel consumption, IDLE ignition angle fluctuates between 3-9 BTDC. Idle fluctuates both on stock 150hp ecu file and AUQ 180hp too. Rising idle rpm seems to mask the problem a little but not resolving it.

Last thing I can think of is replacing throttle body (but It's clean and not showing any signs of failure) and MAF sensor (similar to throttle, it does not suggest any failure, engine runs similar with MAF unplugged).

All this and like.. no effect. Like none, nothing, zero changes, null. Still floating at it was from beginning. It does not affect driving or anything but It's just that I see it and every so often the engine chokes and drops rpm a little bit more. Every guy who works on 1.8t and every owner says that all these engines happen to work that way and I should accept it as it is, because thats normal.
Any other ideas or advice? Its just scraping my brain so much

So instead of diagnosing the problem you just replaced a bunch of parts? I don't see any mention of smoke or pressure test...

No smoke but pressure test only. Every leak sealed. There were only minor leaks.
Compression test 12-12.5b all cylinders.

Many of these parts were replaced because it was necessary - PVC was thrash and was falling apart from touch, oil leaks, failed tensioner and loose chain (-8CF on 093 group), timing set interval reached, fuel pump was very weak under load, mine old turbo was trash. So I expected some improvements anyway. But no luck.

Lambda factor floats between 0.97 to 1.04 on idle, STFT -3:+3%. I dont know what else is to be diagnosed :( Any advice?

Try to put req lambda at 0.9 at idle an report

Also make log with ME7logger

If there is no leaks and engine work fine when it is rich I would say that valves in head don't seal good or do not open full

Got it. Will try do test it till tomorrow afternoon.

Here are some idle logs. One from my usual ecu file (lambda 1), second as you adviced with lambda 0.9.
On 0.9 target, rpm still wiggles a bit but much less and engine works much smoother. On PB rpm idle is almost perfect (with occasional bigger drops) with LPG improved too. It seems like lambda 0.9 is much easier for my engine to keep. On lambda 1 it struggles much more.

Both logs were on PB, with lambda trims cleared before logging.

One strange thing was that there were two times I connected via logger and all parameters were observed, but lambda trims were off for about 5min. After that time they suddenly started working again. Dunno why that happened.

A little bump  :D

you didn't log nmot_w and this is crucial

nmot_w is selected in logger, logged and visible in both .csv logs as a 13th parameter :o

EDIT:
I dunno how but I clicked this and that, run ECUxPlot with Win7 compatibility and now I can see data. I attach some sample from rpm readings with unified 700-800 rpm axis range (about 10 last seconds). Lambda 1 target on left, lambda 0.9 on right.

We can see occasional spikes and drops on both targets, but overall, on lambda 0.9 rpm regulation is noticeably tighter, faster and smoother, which reflects perfectly my impressions from inside of my car while logging.

(https://i.postimg.cc/TwT4tNZh/Nowy-Obraz-mapa-bitowa-3.png) (https://postimg.cc/2qXG6Gjf)

Wouldn't even worry about that. Its 20 rpm range, man come on. That down and upspikes are caused by throttle pid doing its job - as you can see load is increasing or decreasing at this point. Probably something electrical kicks in because it has repeatable pattern.

Tbh I'd check engine mounts

I know that it's only 20rpm range, but you know.. for me it's more about curiosity and being pedantic than making it a huge deal. I can see and feel that my engine is struggling on idle (not much but noticeably) and if I can dig into this problem I will try :) I like doing such stuff. Many 1.8t owners have similar idling issues and no one is willing to take a closer look at it because everybody else is saying "leave it, dont bother, we all have the same". And I'm willing to change it :D So please do not quench my enthusiasm :)

It was BlackT idea to try run rich on idle, which could suggest it's a valve tightness issues so I'm waiting for his opinion :D And clearly, setting lambda to 0.9 not only let engine run still fine but also improved it's behaviour, so there must be something to it. I will try to play with lambda target a little more and see what happens. no one can convince me it's not worth it :D

Just because you're fighting something non-existent. Its not a perfect world, you wouldn't get perfect 760 rpm reading. Logging frequency doesn't match engine speed frequency.

It's definitely existent. And I can see it both in experience and in readings before and after lambda tuning :D

I'm not sayin it will be perfect, because i over 20yrs old engine. I'm just sayin it is possible to affect it a little, maybe even cure it :D Or at least discover source of this problem :)

Two problems:
Most likely injectors are fucked, so not spraying properly at low pulsewidth...
You probably also destroyed your valve seats with LPG usage... Also it could be that your LPG controller has some issues.

Yeah, that could be the case. Everything seems to indicate that this is the cause. Nevertheless, I probably won't take the gasket off since car runs fine and repairing gasket because this is a rather unprofitable operation in relation to the value of the car. I would rather try to tune fueling.

Well good luck running 0.9 on idle, I guess you dont care about consumption at all. Yes its small amount to change but it will have an effect.
The fluctuation  is really miniscule.

The bigger issue than consumption is fouling the cat and getting soot on the spark plugs.
It can also never pass a MOT sniffer test.

Man, setting lambda 0.9 is only for diagnostic and testing purpose and only in idle rpm range. I am not going to drive like that on a daily basic :D I went even further and set to 0.8 and engine works even little bit better, almost without rpm drops/spikes which are correlated with engine load changes (rpm spike when load drops, rpm drop when load spikes).


No cat here :D but situation was the same with and without cat.

Problem is now that with lambda under 1 engine runs open loop.