1.6 16v SE 2000 Golf - losing revs when idle, lots of shuddering

3.0L TD.
the real name is
3.0 L 4JX1 TD I4

diagnosing TD on fixya is like asking , in grade school NASA like questions. (direct
LOL
lost power.
can be lots of causes.
check fuses first
then scan it next. ever consider scanning it?/?????
first?
filters clean?
dash error lights glowing? ignored/?

"the 4JX1 is a 3.0L direct injection diesel engine, turbo, some are intercooled,"

low power can be , lost compression.
weak or dead turbo.
or weak fueling.
or even cam timing off.
besides skipping service on filters , etc, (scans)

You need to have a scan of the engine done There are a number of problems that could cause those symptoms. Ask your repair shop how much they charge for the scan Our shop does them free, some shops charge. Get a copy of the scan results and have them clear the codes. Than ask for quotes on the repairs.

Does the engine slow down in drive, or is it still running fast and trying to pull away from a stop ?
The problem could be a vac leak, or a sticking idle control valve.
The computer controls idle speed by how much air enters thru the control valve.

voltage sounds kinda high for while its running. the voltage regulator in the alternator sounds like it is bad. i would replace the alternator.

Hallo
Replace thermostat and outside housing (part betwin thermostat and wather hose that goes on front cooler).
Marko

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idle air speed control valve(iac),too low idle or driveline problem like play in the drive shafts. if you put it in park and give it to say2500 rpm and let go of the throttle,does it return to 900rpm?if it dips down to 5 or 600 then to 900 the iac is bad.

you need a new oxygen sensor

have your idle air control solenoid cleaned out!

have the fault codes read to get the problem area