[s-cars] Help - Stumble, stall, and no start condition

[Dave Ellis]

I have on odd set of symptoms I'm hoping the group wisdom can assist with.

They just started yesterday and I'm supposed to start a week vacation 
tomorrow driving with our kayaks on the roof racks.  Needless to say my 
wife is none too pleased.

The last couple of months the car would intermittently drop a cylinder, 
sometimes for a few seconds, sometimes for minutes.  This behaviour 
seems to get worse as the ambient temperature rose.  Higher octane fuel 
seemed to help (97 as opposed to 91/92), but may have been co-incidental 
or placebo-like.

As of yesterday, when driving with the engine cold everything is good. 
As soon as the engine warms up and the engine bay gets heat soaked the 
stumble comes back *bad*, the car feels like it wants to stall, and if 
left to go to idle will actually stall.  Once stalled it will not start 
unless it's let sit for several minutes, and then will usually start, 
only to stall once more as soon as I remove my foot from the gas.

To add to this set of symptoms, the Engine Controller reports "No 
Response from Controller" after stalling when the engine's hot.  Other 
controllers I can talk to fine, so it's not the laptop or cable.

When I hooked the VAG-COM up right before my test drive tonight (engine 
cold) it connected just fine and was able to scan for fault codes (none 
found).  I'm going to try connecting it after things cool down tomorrow 
morning again and see what happens.

Any ideas?

I've replaced the spark plugs and gapped them small (0.030) just in case 
the ignition coils are getting weak (original with 240,000kms).  The MAF 
looks ok though I haven't yet checked the resistance on it (tomorrow 
morning).  I just replaced the N75 valve two days ago and have checked 
all hoses for leaks (haven't pressure tested though).

Once more thing to add, I had swapped in the 1.8bar wastegate spring 
which lead to some bucking at WOT and threw maximum boost exceeded 
errors so I swapped back the original spring.  It was while driving with 
the new spring that the car stalled for the first time and things got 
really bad.  Co-incidental?  The car is not chipped, just stock.

Any insight or suggestions welcomed!
Dave