I know bupkis about W140's but the W126's that use the Diesel fuel heater have endless problems with them leaking, maybe by pass it to see if the bubbles stop appearing .

I'm betting your intermittent miss is the air bubbles you're seeing .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better