850T - Intermittent stumble/very rough running

[foggydogg]

I had a similar issue with the 850 T5 Reluctant Daily driver. The fuel pump relay looked OK inside, but I swapped it for a known good spare out of the donation car, and a few hundred miles later the intermittent rough/stumble hasn't returned. Maybe that could be diagnostic.

[Roger_850T]

Roger..one of the most bizarre episodes of vacuum leak troubleshooting happened to me on my T5

Cause...completely rotted purge vacuum hose under the intake manifold, which splits to a T ( one side to snorkel for vacuum under boost, the other side to purge valve, the rotted part provides purge vacuum under normal running)

Symptom..severe chugging, running on 3 cylinders during first 60 seconds cold running. Suddenly clears amd then car ran well.

Documented here

viewtopic.php?f=1&t=84552&start=20

There is no substitute fir removing and replacing every vacuum hose on high mile/age cars

+1000 points for innovative use of the slash!

Hi abscate,

I typically keep an eye on all the rubber hoses and elbows, and to my knowledge, they're all good. One of the first issues I had with this car (at ~100k miles) was rough running due to a perished vacuum elbow under the intake manifold. It took me a while to find it then, now I'm always looking at the hoses and elbows for a bad one, every time I open the hood! Still, it's a good idea to double check, and I'll definitely do that. Thanks!

Roger