When the outside temperature is cold, say under the freezing point or lower, the engine starts, sometimes with difficulty, idles roughly but dies on me as soon as I try to accelerate. Re-start requires a lot of cranking, again idles very rough like it has been flooded.
After at least 5 to 10 minutes of idling and warming up, I can accelarate more or less normally. No problem whatsoever during summer time.
I had the engine checked by a independent mechanic, I did some checking myself. No luck. It seems that all sensors are OK.
Any ideas, suggestions, comments would be greatly appreciated.
"it seems that all sensors are OK"
What makes you say that? The symptoms are classic ones associated with a faulty CTS sensor. This tricks the ECU into thinking the engine is warm when it isn't. With coolant temp below a certain value the ECU does two things: It enriches the mix and it ignores the O2 sensor (because the O2 sensor is too cold to be reliable at that point).
If your CTS sensor is bad the ECU will be forced to either go into open loop mode or else pay attention to the O2 sensor. Either of these situations will cause hard running when cold.
Yes. Even at warmer outside temps a faulty CTS would likely create problems, but the colder it is the worse they will be. One easy check is simply to disconnect the O2 sensor. If the problem is immediately better then your CTS is suspect. If nothing changes it doesn't mean your CTS is OK, just means that test didn't fail. I don't know about the vans, but VW makes it really easy in pass cars to change the CTS. Fifteen bucks and 15 minutes buy a lot of peace of mind.
Have you considered water in the fuel? A bottle of fuel treatment designed to remove water might help.
This could also be MAF.
Another consideration is that you might have a problem with connections. I had a car (not VW) that did a similar thing. Turned out the wire to the + on the coil was a bit loose. At night condensation formed on the lug and then froze, causing the wire to make a poor connection. I found it by going out one really cold morning and pulling a plug wire and checking spark. Spark was really poor. So I figured it was coil related. But flaky coils usually have trouble when hot, not when cold. So I put a meter on the coil post and verified 12V with KOEO. I had a friend crank the engine and the voltage dropped to around 9V. So I cleaned the connections and that's when I noticed the lug wasn't tight.
In talking about this to a mechanic some time later I was asking how this could be, since it ran great when warm and connection problems usually get worse when warm. He was the one who told me it was likely condensation freezing on the lug and post.
This is very similar to a complaint we've had in another EV thread, about the same drivability issue, but occurring if you don't drive off immediately after starting the car. With mine and some others the van will start and run fine, but if you let it warm slowly is will suddenly stutter and slow idle, and will balk or stall if you then try and drive off. After a few more minutes it will again run fine.
I've also assumed it is related to CTS and/or OXS heating, as when you drive off immediately the exhaust gets hotter and helps warm the OXS faster so that it functions sooner. If the ECU tries to turn control over to the OXS before it is functional this is exactly the behavior I'd expect.
It could be tolerance or connectivity on the CTS as well. I'm going to change mine and also check the harness continuity back to the ECU from the CTS.