Super Rough Cold Starting

[Kmadras]

Hello this is my first time posting on this forum since about 2008. I'm the original owner of a 2000 VW Golf TDI ALH with 290k; he even has a name, Dieter. I've always taken excellent care of Dieter his whole life - he's probably only had about 5-6 mechanics work on him in his lifetime. For about the last year Dieter's been experiencing a really hard "cold" starting up in the AM, even at 40° unless I plug in my block heater. It is really bad sometimes when I start him up after being at work in the late afternoon/evening too.
So, in the last month I had a lot of work done to try to alleviate this. So far nothing is working and I've spent a lot of $$ w/o a good result - yet.
I have to say that this has been a great car, I've had Dieter 21 years now and we still get great mileage averaging about 42 city/46 hwy. The mechanic who works on Dieter says that he drives beautifully, except for the super hard start thing.
I think the issue is the engine temperature sensor > communicating with the glow plugs. I've had the temperature sensor & relay wires replaced, and a used but well-functioning ECM was installed. The glow plugs all test out just fine.
At the suggestion of my mechanic, last week I took Dieter to the local VW dealership to have the "new"used ECM coded. ( For reference, the ECM was from the exact same year/model as Dieter. ) This is what was noted on the paperwork when I got Dieter back:
"check engine light was on, 1 code p0380 glow plug A. they programmed the ECM to Value to note check engine light remains on the fault p0380 would not erase."
The next morning when I started Dieter up (w/o the assistance of the block heater - it was 42 degrees out) he started up really rough, smoke etc. Sigh. . I'm sort of at my wit's end. I'm afraid to travel far from home where I can't plug my car in to warm the engine block when starting after it's been sitting for more than a couple of hours. Does anyone have any suggestions? thank you so much! I am in Central Oregon, about 50 miles N of Bend, 110 mi from PDX

 

[IndigoBlueWagon]

Not sure why you swapped out the ECU, that wouldn't be the first thing I'd check. You say you've had a lot of work done to fix the problem, did it include glow plugs, relay, or temp sensor? That's where I'd start.

When you turn the key for a cold start and it's 40F or below, how long does the glow plug light stay on? If it comes on and goes right out again, the coolant temp sensor, wiring to the sensor, or the glow plug relay may be bad. If it says on for a few seconds or more, and the car is still hard to start, your glow plugs may have failed. The ECU measures differences in resistance between the plugs, and if they're all equally bad they may not throw a code.

The code you got is for a glow plug, but your system won't tell you which one. You'd have to check individual plug resistance to be sure. But at this point unless you've replaced them recently I'd just put in a new fresh set.

 

[Kmadras]

In my post I wrote: I've had the temperature sensor & relay wires replaced, and a used but well-functioning ECM was installed. The glow plugs all test out just fine.
The glow plug light only stays on for about 1-2 seconds irregardless of outside cold temp..
From what I have been reading on this forum so far I think it may be the GP's despite what 2 mechanics have said that they "test fine". The ECU was swapped out as one of the 2 mechanics thought it was the faulty link. Turns out probably not.

 

[Stupendous60]

Does it have a slow crank or normal cranking with no start/eventual start? My 01 NB still has the original starter and cranks slow on cold (here) days but above say 50 degrees no issue. I am planning to replace the starter soon. On one of my others, a new/reman starter cured cold slow crank issue.

 

[Ol'Rattler]

Coolant temp sensor. If you unplug the sensor IIRC the ECU gets tricked into thinking that the ambient temp is something like -40 and should make the glow plugs stay on longer.