unstable idlling-stalling im going nuts

[GRH]

hi guys,
this one has been giving me lots of dramas as i cannot pinpoint why it happens.

sometimes engine either idles too low or too high, so i set it correct again , then it is fine for an unknown amount of time, maybe a day maybe a week, then suddenly whatever i set it to changes again.. say it was too low and i increased it , few days later now it is too high...
or it was too high and i decreased it, few days later it is too low..
in above cases engine works normal but idling varies for unknown reasons.

then less occasionally engine idling goes erratic.. tachometer moves up and down erratically then engine stalls and dies.. at this stage engine also blows black smoke. after sometime and not necessarily by engine getting hot, it stables on it's own.

according to receipts from previous owner car had the same problem and attempts to fix it were
replace faulty idle speed motor, check engine operation
which didn't fix the issue then later on
-found idle speed motor o-ring distorted, replaced
-clleaned idle air bypass back passage
-cleaned throttle body
and invoice mentions oil burning due to valve stem seals.. but i see no blue smoke at all.

again it didn't fix the problem.
when i bought the car and faced the same issue and plus at times car just didn't want to accelerate i did
-used 6 packs of injector cleaner, starting with the expensive full fuel system cleaner then few bottles of normal injector cleaner.
-cleaned maf sensor
-replaced front spark plugs
-replaced a dodgy spark plug wire
-changed all belts, timing belt, tensioner, etc water pump
-found a small vacuum leak and fixed it
-replaced fuel filter

these relevant or irrelevant measures didn't entirely fix the issue but greatly reduced it's occurance.
yesterday, i did some work inside the car installling stereo system, and kept the engine idling to get aircon.. over an hour of idling and no issue at all.. then i t urn off the engine do some work at home, and decide to go out for shopping eetc... and engine goes crazy again.. for over 20 minutes struggled idling, stalling even brakes not working at times then it stabilized again.. today i drove the car no issues at all..

my mechanic who did the belts and pump, didn't notice any abnormalities with the engine...

here is a video from yesterday
https://youtu.be/fMeaUgZrXDQ

i don't know what else to do.. rear spark plugs maybe due replace but i can't get there yet. i thought fuel system and injectors might be dirty.. hence used the cleaners.

[GRH]

also replaced oxygen sensor

[gutza]

Hi GRH.

I have about a dozen thoughts going on about your problems.

1. Any CEL pop up ?

2. I would get your battery / alternator checked first. They can cause all sorts of problems.

3. If that doesn't work out, get back on here, and I'm sure we can help.

Good luck

[GRH]

i also popped out the distributor rotor and checked tips on both cap too.. theyre ok.

[GRH]

hi Gutza

nothing on the cluster no lights
battery is good, alternator is brand new bosch two years ago. voltages are stable.

[gutza]

OK.....
Make sure you have your PIN for the Radio.
This probably won't help, but it costs nothing.
Disconnect -ve on the battery.
Wait about 10 minutes, reconnect. Turn on engine. Let idle for about 20-30 minutes

[Ensoniq5]

Resetting the idle speed by fiddling with the air screw won't fix any idle problems in the long term, it will only mask them for a while until the ISC resets its position. Similarly, disconnecting the battery will reset the ISC and the engine may idle fast-ish for a while but within a few days it will move back to where it was. You need to diagnose and fix the underlying condition. The first thing I'd be checking is the TPS to make sure it is consistently switching the ECU to idle mode with a closed throttle. If not, it may be out of adjustment or there's something dodgy with the throttle valve, shaft or cable preventing it from closing fully with your foot off the throttle.

[GRH]

one other thing to mention.
i remember hearing that if the air screw is fully tightened with throttle adjust screw fully loosened the engine should stall??
doing this on my car doesn't stall the engine.. air screw fully closed engine still idles around 500-600 rpm.
not sure if it means anything

[GRH]

i've removed the battery waited etc connected..
this seems to have improved idling. car sets the idle speed better when ac on ac off on gear etc.. yet this has been on and off before so i don't think it will be a solution but yet to be seen.

ensoniq5 thank you for the TPS suggestion, i've learnt something new.
i googled around and learnt how to test it, so using a voltmeter i checked the voltages. at this stage i noticed the throttle position screw (not sure what it's called) i tightened before to prevent the engine stalling was slightly high and caused TPS to fluctuate voltage when tapped on it. the voltage became stable once i loosened the screw at 0.62 which seems slightly low if what i googled is accurate. i've read people suggesting 17% of default 5volt line i guess comes around 0.8 volts or near.
apart from that TPS seems to operate smoothly up and down and doesn't fluctuate when rattled or tapped on.
while at it i also cleaned the connector plug.

it looks good now, but as mentioned before car does this on and off for no apparent reason. i will monitor and report back. in the meantime any other information about possible causes is most appreciated.

[GRH]

workshop manual suggests normal TPS voltage is 580-690 millivolts so that'd be 0.58-0.69 volts so it looks like my 0.62 is normal