For about three years, when my 98 AHU Jetta had been sitting in cold temperatures (30F or lower) for several hours, I had starting problems. After the glow plug light went out, I cranked the engine and it started immediately. It ran for one to five seconds or so, then died. After that, it was very difficult to start.
I replaced many things, including the MAF, injection pump, coolant temperature sensor, intake air sensor, ECU, and probably some more stuff I can't remember.
The problem seemed related to the ECU switching from "start" to "run" mode, if there is such a thing. I suspect the controller uses some default settings at startup, then adapts to the running conditions that it senses.
Then I discovered G28 - crank position sensor. I just figured since the injection pump is timed to the crank, it could inject based on its own position. Not so, it uses the signal from G28. When I looked at the G28 signal with an oscilloscope when cold, it looked ok, the peak-to-peak voltage was about 10 volts. It goes up higher with engine rpm to about 50 volts. I replaced it anyway, thinking maybe when it was cold something weird was happening.
The problem went away. It starts fine in 15F weather without stalling. It hasn't been colder than that since I replaced the sensor. The sensor was relatively inexpensive, but I had to remove the front engine mount (and starter) to get to it.
I hope this helps someone at his wit's end.
I replaced many things, including the MAF, injection pump, coolant temperature sensor, intake air sensor, ECU, and probably some more stuff I can't remember.
The problem seemed related to the ECU switching from "start" to "run" mode, if there is such a thing. I suspect the controller uses some default settings at startup, then adapts to the running conditions that it senses.
Then I discovered G28 - crank position sensor. I just figured since the injection pump is timed to the crank, it could inject based on its own position. Not so, it uses the signal from G28. When I looked at the G28 signal with an oscilloscope when cold, it looked ok, the peak-to-peak voltage was about 10 volts. It goes up higher with engine rpm to about 50 volts. I replaced it anyway, thinking maybe when it was cold something weird was happening.
The problem went away. It starts fine in 15F weather without stalling. It hasn't been colder than that since I replaced the sensor. The sensor was relatively inexpensive, but I had to remove the front engine mount (and starter) to get to it.
I hope this helps someone at his wit's end.