1991 Golf Cabriolet starts fine, runs for 4-5 seconds, then immediately stalls

[Legoguy]

Hey guys, it's been a while since I've visited the 'tex...
So yesterday I gave my girlfriend's '91 a full oil change and was happily driving it around - it's been working fine for the past few months. Last night it dropped to about -28C (-18.5F for you in the States), and this morning, when we tried to start it up, it cranked fine, fired right up, and then promptly stalled - the more times we tried, the longer it seemed to keep running before stalling. My initial idea was maybe the gas has gelled at that temperature and I neglected to put gas line antifreeze in it - and the gas tank was nearly empty, so I went and bought some gas and poured it in the tank. No change. Note that I do not have a block heater - if I did, it would be plugged in - I have one ready to install - that's my next project for when I have access to a heated garage.
It seems like fuel delivery is working properly, since it has enough to run for those few seconds that it does and every time it's restarted it gets another dose of fuel. I can confirm the pumps are running because I can hear them still running for a moment after the car stalls.

Engine is a 1.8L "RV" code (replacement for 2H by previous owner) with Digifant II

My ideas are:
fuel pump weak? doesn't make sense that it would happen so abruptly, it worked wonderfully yesterday.

blue CTS faulty? I unplugged it and tried to start it, it just started with less "pep" (I assume this disabled the idle stabilizer valve). Not sure what the proper resistance value across the terminals is for this temperature! the Bentley drops off at 0C

oxygen sensor? This is still the one that came with the car when we bought it a few months ago, so I cannnot vouch for its reliability...

Maybe it's the ECU or grounding? I haven't been able to thoroughly check this yet due to the weather and other plans but it's on the list...

Any ideas or experience would be wonderful.

[snowyroads]

Another shot in the dark here but:

fuel filter new???

Starts fine and dies soon after sounds like the ecu is getting the wrong signal and decides to kill the engine before its damaging its heart (the engine)


Edit: in reply to next post. (didn't want to re post)

Its not after its revved right?? I know from reading about timing that The ECU is reset after 2k rpm i believe. I would check all of the known sensors to make sure they are okay. And as brian would say, clean off your grounds (something i still have yet to get on )

[Legoguy]

I do not know if the fuel filter has ever been changed - certainly something that should be done, but I'm sure I would notice a more gradual failure than working one day and not the next if it were the fuel filter. Unless water is clogging it - but then I would expect the fuel delivery to be negatively impacted at all times rather than just 5-6 seconds after starting.

[SCOTTB87]

My 87 with CIS acted similarly when the in-tank pump failed. It would run for a couple of seconds and quit. I figure it was running off the cold start valve. I don't know if Digifant could have similar issues or not.

[briano1234]

o2 sensors can be unplugged, the car will run rich, but run.

unplugging the CTS will cause the car to run crappily. The CTS can cause hot start cold start and crappily running issues. I don't measure I just replace them as it is a under 10 dollar part, and they go bad with regularity.

Grounds, Replace them.

Cold weather will cause iffy vacuum lines to leak until the engine warms up. (sounds like a vac leak).

When is the last time you changed plugs or wires. If you look at the engine when it is dark with no lights on under the hood when running do you see sparking fairies dancing about?. If you do then you need to eliminate them.

To reset the ECU you need to unplug the CTS sensor, and Rev the engine above 3K 3 times.

[Legoguy]

I had initially considered a situation similar to the one you describe (with regards to the cold start valve) - but I stopped and thought a second -- DigiFant doesn't have one.

I had a similar problem with starting on my 16V: it wouldn't actually start at all, but it would try to, only when the CSV was activated by the ECU. Turns out the other four injectors were, in fact, frozen - after a chance glance at the end of the the block heater cord, I noticed it was severed and not actually providing power to the heater. Once I replaced that and heated the block overnight, the next morning it fired up fine - the same morning, in fact, that this stalling happened to her car!